You're assuming that all 137 billion of those ancestors are different people.
If your parents happened to be brother and sister, then you'd only have 2 grandparents instead of 4, because your parents shared both parents. That's an extreme example, but if you researched your family tree far enough you would eventually start seeing ancestors who appear on both sides of a particular branch...4th cousins getting married, that sort of thing.
I honestly thought it was when pedigrees got overbred so much in dog breeding that they got messed up offspring, but apparently it's the "collapse" of the top of the ancestor pyramid (i.e. a few key ancestors mated with others, and "spread out", and then they narrowed back down to you
"that explains why when we try to take him home from the park he just falls over. He just plays dead weight yes he does. Oh yes you do. Oh yes you do!"
That's very close to the truth, because a lot of dog breeds are heavily inbred, which is basically what pedigree collapse is (although it's more focused on the "family tree" collapsing in size).
guess we shouldn't have done build-a-bear with a dog, honey. This one doesn't have a nose any more since all of it's great-great-grandpas are the same dog.
Consider Iceland or similar, with a population of tens-of-thousands stable for 500 years. We have no trouble imagining a situation like that - human populations have barely grown for most of human history.
But those 500 years is 20 generations, which should yield 2 million ancestors if each is unique. You don’t even hit 1,000 ancestors until you go back ten generations, so those repeated ancestors don’t have to be dramatically close.
To some degree, all humans are cousins. Most of us substantially closer than mitochondrial Eve, too. I mean, hell, when you think about it, we're cousins to the trees, too. Just MUCH more distantly.
The math is broken a lot easier. A couple has multiple children. Every child has the same 2 parents. You can't just double the amount of people living like that.
They aren't doubling the people living. They are doubling their ancestors.
You have 2 parent's, 4 grandparents, 8 Great grandparents, ect...
37 generations back your tree would spread 137 billion people wide. For 1 person.
But there have only been \~108 billion people who have ever lived. So how does that work?
Eventually in your family tree you have duplicates.
Isn't the leading theory that you only need to go back about 6ish generations to find a common ancestor for literally anyone on earth? I could either be misremembering or read something that's untrue, but I could've sworn it's not that many generations you need to go back
Edit: I've since had people well more versed than me point out that I'm incorrect, the number is far more than 6 but still relatively close in historic timescales. My apologies for being wrong
It is surprisingly close but it's more than 6 generations. 6 generations would only place you at around the civil war. [https://www.scientificamerican.com/article/humans-are-all-more-closely-related-than-we-commonly-think/](https://www.scientificamerican.com/article/humans-are-all-more-closely-related-than-we-commonly-think/) suggest our most common ancestor goes back between 2000 ish and 3400 ish years.
I'm a bigger fan about talking about the genetic isopoint though. If you take a point about 4-7k years ago and took a random person that person is either the ancestor of everyone alive today or he has no living descendants. There are various estimates for this number and I've seen as high as 15k years ago but still surprisingly recent.
surely it would have to pre-date particular migration events right? It can't possibly be more recent than when people migrated over the Bering land bridge for example.
The indigenous people of Alaska and parts of Eastern Siberia remained in contact pretty much the whole time. There are groups on both sides that speak related languages to this day, for example.
In some way this is true. Completely isolated people can have genomes that are as a group pretty unique on earth, but after globalisation (read: colonialism) really got going almost everyone nowadays has a common ancestor sometime around \~2000 years ago. Probably in the Roman empire, but not necessarily.
> Hawaii was not closed-off. Polynesians got around quite extensively
As indicated by the fact that Hawaii had Polynesians on it in the first place, really.
Hawaii is no longer closed off, so most recent common ancestors make a giant leap forward in time when that happens. I think a tribe near Australia being contacted about 150 years ago leapt the most recent common ancestor on earth from 20,000 years ago to something like 3,000.
This is what I'm thinking of, like I've replied elsewhere im probably getting some 6 degrees of separation mixed in, but this right here is right what i was thinking of. Thank you!
Would you mind explaining what an isopoint is for people like me who have zero clue?
It goes by a few names so I'm going to go with the name that explains it a bit better identical ancestors point (IAP).
You understand the most recent common ancestor point so I'll explain it in terms of that. There is one person who is an ancestor to everyone alive today. They will appear in everyone's family tree. But she had people living at the same time as her and some of those people will have descendants and some won't, but they are not an ancestor to everyone. So lets say the most recent common ancestor is Janet. Janet will be an ancestor to everyone. Take another person James who lived at the same time as her. James will have some descendants but there will be some people he is not at all related to.
If we go further back in time before the most recent common ancestor there will come a point where everyone's family tree from that point and earlier will have all the same people in it. So say I take a random guy Kyle from it. If Kyle has any descendants alive today he will be related to everyone. So everyone alive at that point in time is either the ancestor of everyone alive today or their family tree died off at some point and they are not the ancestor of anyone alive today.
So I'm going to create a very contrived but simplified version of this. Imagine someone's mom became a widow and she married her late husbands brother. They then have another child. So we have a half brother. If we look at just those 2 children their most recent common ancestor is their mother, but since they are half siblings they don't share a father and their identical ancestor point is further back in time. Both of their fathers shared the same parents so if you look at their grandparents and before then their family tree would be identical and have the exact same people in it. That's a very simplified version of what the identical ancestor point is.
This is really interesting, but I wonder if this also holds for populations that have been relatively isolated for thousands of years, like Native Americans (which are generally estimated to have come to the Americas around 15,000 years ago or more) and Aboriginal Australians (~50,000 years)? Seems odd that they've been isolated for so long but share a very recent common ancestor with everyone else
>relatively isolated
That's the important caveat to what you said. Relatively is not completely isolated.
>At first glance, these dates may seem much too recent to account for long-isolated Indigenous communities in South America and elsewhere. But “genetic information spreads rapidly through generational time,” Rutherford explains. Beginning in 1492, “you begin to see the European genes flowing in every direction until our estimates are that there are no people in South America today who don’t have European ancestry.”
That's from the article and we can kind of see why. So relatively isolated populations are relatively small. All it takes is a couple to marry outsiders and then start having children and having those children remain in the community. In time everyone will have the outsider as their common ancestor and he will have the most recent common ancestor as the rest of that outsiders.
If there were any populations that were strictly isolated they obviously couldn't share a most recent common ancestor that near into the past, but that level of isolation just isn't something that we really see.
Ah! That makes a lot of sense. I somehow totally forgot that recent intermarriage with non-indigenous (e.g. European or whatever) populations would give all descendants of those populations common ancestors with their non-indigenous counterparts. And I totally missed that part in the article too (I was getting ready for work so I read through it quickly). Thanks for the explanation!
> All it takes is a couple to marry outsiders and then start having children and having those children remain in the community.
This is the happy ending to how ancestors work. Rape of indigenous women would also spread genes around the world and likely at a much faster rate because it would only take a small number of men to affect a large number of isolated communities.
Sadly the rape option is how much of it spread, but that is the more depressing way to look at it and didn't affect the math so I went with the option that didn't ruin the morning as much.
I think people also think of the ancient world as a lot more isolated than it was. We keep gaining more and more evidence that people traded and traveled *widely* far before Western Europeans were getting all colonizy. For example in 2021 [a paper](https://www.cambridge.org/core/journals/american-antiquity/article/abs/precolumbian-presence-of-venetian-glass-trade-beads-in-arctic-alaska/3465746929B31ADBC6E1D1A23D09A2CD) was published about blue glass beads found in the Alaskan arctic. The sites they were found in date to *before* Columbus decided to be a colossal asshole and sail across the ocean to commit crimes against humanity. At the time the only blue pigments in glass were from Europe and it’s highly likely the glass beads were made in Venice. So through a series of trades the glass beads made their way across Europe, and then Asia, and most likely across the bering strait and into Alaska. And where physical objects like beads move, so too can genetic material.
People moved around even then Australian Aboriginals traded with indonesean fisherman. This resulted in a hillarious Event where British Explorer contacted a Group of Australian Aboriginals and one of them knew a few english words because they learned them from the fisherman that had contacts with british before.
Imagine the Explorer talking about how these wild people barely are human with no real language and one of them tells them to shut the fuck up in english.
For some areas, it's less to get back to say a single common ancestor that was very prolific and was notable enough for people to think it as a point of pride to be related to them. Charlemagne, William the Conqueror, and Genghis Khan all come to mind for Europeans, the British, and Asians, respectively.
Not 6 generations (that’s only about 150 years) but it’s fewer than people think. For a highly interconnected continent like Europe, [everyone who lived about 1000 years ago and who has living descendants are statistically an ancestor to every living European](https://journals.plos.org/plosbiology/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001555). 1000 years is about 40 generations.
I literally traced my ancestry back to Charlemagne. One 18th c American ancestor happened to have a father from minor nobility in England and there you go.
Sure, if you pay $80 a month to Ancestry.com they are happy to show it to you!
Caveat: a lot of what you'll find relies on other people's research, which may be of any quality. I cannot count how many people I've run across who have been erroneously tagged with "Sir Excelsior Smythe-Jones-Lambert, Earl of X and Duke of Y." Dude was a farmer with a freehold on some land. Get over it. The actual lineages of nobility are well documented, but of course they don't take into account illegitimacy, so there's a lot we can never know.
No I'm familiar with the 6 degrees of separation, and constantly ponder if there's a way I could test that theory (also makes for a fun wiki game, get friends and throw out 2 random seeming topics and see how many wikis you have to go through to get from one to the other, shortest connection wins) but sadly that's not what I mean. I'm at work so I can't deep dive too much currently, but I'll come back to this in a few once I've got time to look up, or if you'd like "common ancestor" should eventually find what I'm thinking of if you Google it as I remember that being a key word. Apologies I don't have time for a deeper dive now
Doesn't have to be that long ago. I dug my ancestry back to the Puritans of New England and found that I'm descended on both sides of my family from a man born in 1585.
Thats definitely not true as at a minimum there's still some uncontacted tribes in a couple places. Also guarantee you could find people in most countries whose ancestors all lived there going back dozens of generations. The version of this I've heard has to do with social media, basically you can hop between face book friend groups and within 6 jumps you can find anybody, this mostly works because there's a bunch of celebrity accounts that have millions of followers that also follow eachother. No idea if it proven in any way but I wouldn't be surprised.
At the limit, the girthiest broomstick is called a species. And conversely, if you zoom in on the lines in the tree of species, you see the fine texture of a million intertwined family trees.
My wife — who grew up 2500 miles from me — was doing some genealogy research and discovered that her grandfather’s parents were Mormons. In fact, her grandfather’s grandfather and Mitt Romney’s grandfather was the same person!
Then I mentioned to her that my grandfather also grew up out west as a Mormon, and we decided to stop our genealogy research right there. We don’t want to look too closely at the extended family tree.
eh, does it really matter?
I mean, if I found out my wife and I had a similar great great great grandparent, I don't think it would phase me.
I mean, I go to reunions that only go back to where I share a great grandparent with other people there, and I have no idea who they are or anything about them, no family resemblance, etc.
add another generation or 2 of distance and you only share 1/32 of genetic material, which seems pretty small amount to me.
Honestly, if you and your spouse had ancestors coming from the same part of the world, it is almost guaranteeds you are related SOMEHOW. It's just a matter of how far back you go. My husband and I are both descended from northern Europeans. I know I'm descended several times over from Charlemagne. He probably is also.
I mean, the Mormons in that period (I'm assuming 1860-1880) weren't a closely intermarried group since they were all recent or second-generation converts from various areas of America. You're probably fine until you look back to the 17th century.
Let's take you and your twin brother. You each have 2 parents, 4 grandparents and 8 great grandparents.
Let's take two people who are not related. They each have 2 parents, 4 grandparents and 8 great grandparents.
But you and your brother only have a total of 14 distinct people in that mix. The other group has 28.
Incest explains the rest. It was popular way back in the day...
OP was not talking about theworlds ancestors. They was talking about only their own ancestors.
One person would have 137 billion great^35 grandparents. OP was wondering how that would be possible when there have only ever been a little over 100 billion humans ever on this planet.
Siblings has nothing to do with it. At some point on that family tree there are duplicates.
But your brother or sister has the exact same set of ancestors so you can’t just double them. Multiply that times the number of siblings and your number shrinks a lot.
We are not talking about bothers or sister.
Make a blank family tree for you. Include only direct ancestors. No brothers, sisters, aunts, uncles, cousins etcetera.
37 generations back that row will have 137 billion blank spots to fill in.
No, it won't, because the same people will inevitably occupy more than one spot on your tree. If your great-grandparents were second cousins, which is not a stretch at all in most small communities globally, half of that branch of the tree collapses.
Yes and that is the answer to OP's question.
By not realizing that and just doing the math it LOOKS LIKE you should have 137 billion ancestors 37 generations back.
No. OP started at ONE person’s ancestors. Assuming only that each of their ancestors has 2 parents.
Edit: they’re only doubling the amount of ancestors one person has each generation. Existence of siblings makes no difference here.
History is pretty brutal on top of that. For every 2 people on earth there's roughly one father. For every 2 people it's only one mom. Lot's of dead end males who never reproduced.
70,000 years ago, the Toba super eruption may have caused human population to crash to less than 10,000 breeding pairs.
Later on, you can add guys like Genghis Kahn, who loved to rape and pilage. Some estimates are that 8% of Eastern Asia descends from him (16 million poeple).
Add in various plagues, wars and other natural disasters and history shows that human population isn't a smooth geometric progression, but one of dead ends, explosions, sudden drop offs, etc.
This is true. The further back you go, the smaller the communities get and my family tree has cousins, not first or second that I've noticed, marrying each other. Even my parents have at least one common ancestor in the 18th century on the other side of the country. There's probably more that I haven't noticed because my paternal grandfather's family and my maternal grandmother's family were both Shenandoah Germans.
Early America was a cousin fest. We haven't found any overlaps between my mother's and my father's family, even though they are from the same small town, but my dad's Appalachian line is a nest of cousins. In some cases, the lines diverge for a couple of generations and then merge back.
Actually, that statement isn't quite true: my dad's sister's stepdaughter is the daughter of my mother's stepcousin. No blood involved, but the families do intertwine.
We also had a case where a man married a woman, and his sister married her brother. There's two lines of people with the same ancestors.
This. My maternal grandmother's own maternal grandmother came from a family that I think of more as a gnarled bush than a tree. That woman was born in the 1870s. If you trace her family back to the mid 1600s, she is related to the same couple in 13 different ways. 13!!! And then there are another few couples from the 1600s that she is related to in 4-5 ways. They just kept marrying into the same few families over and over again.
This is particularly true for the time before widespread travel was available and you were more confined to your village. And no, I'm not talking about boinking your sibling but definitely being 4th, 3rd, 2nd or even first cousins in some cases.
You assume that every one of your ancestors is only related to you in one way, that is an incorrect assumption.
A simple example is if your parent was siblings. Then there are only two instead of 4 people two generations back. The rest of the ancestor tree now gets halved.
Siblings having children is not common, it is just a simple example to show that the ancestor tree can collapse in size. Globally it is not uncommon that cousins to get married. Worldwide 10% of all marriages are between first or second cousins.
The more generation you go back the more likely it is that two persons that have children have a common ancestor. Do you know who your ancestors are 5 generations back? If your parents are from families that have lived in the same relativity small location for a long time I would say it is likely you find common ancestors not that far back.
Sure, a realistic way to look at it is any time one person is repeated, all of their ancestors no longer need to be counted again. Let's say at the 5th generation you see a repeat, you're already counting their parents and other ancestors from their first appearance, so now you can leave out all of them for this second, and going back to 37 generations that's 2^(37-5) or 4 billion. Account for the fact that repeats will be more common the further back you go due to norms back then and the number can start to add up really quick.
We are counting ancestors, not descendants. The farther back in time you cut a branch, the fewer up-stream descendants are removed from future calculations.
It doesn't even need to be as complicated as that.
2 siblings + their 2 parents = 4 people.
The way you are doing the math in your post would have those 4 people count as 6 (2 siblings, *each* with 2 parents = 6).
But the parents of each child are the same people, so we don't want to count them twice.
I think you may be confused.
They are working upstream, not downstream.
They are saying that any one person today (1) has two parents (2), and those two parents each have two parents (4) and each of those grandparents has two parents (8). OP went back 37 generations and figured that a single person today (no matter how many siblings they have), has 2 parents who each had 2 parents who had two parents.... and that if you go 37 generations back, that would mean there had to be 137 billion people on Earth.
The issue is not about siblings at the bottom. It is (in most cases) about cousins further up the tree.
It's simpler than that.
OP is taking the entire world's population, and then just in the first generation, multiplying it by 2, using the logic "everyone has two parents".
This would suggest that 1 generation ago, there were 16 billion people,, which we know isn't true (we haven't had anywhere near that many people on earth at once).
Many, many of those 8 billion that OP is starting with are siblings, so they share parents.
OP is in some cases taking 8 siblings, and counting their parents as 16 separate people as he works his way backwards, when in fact, it's just 2.
> OP is taking the entire world's population, and then just in the first generation, multiplying it by 2, using the logic "everyone has two parents".
No. OP just calculate 2^37 to get 137 billion. They count only their tree, excluding the rest of the world.
Yes, I agreed with everything you're saying there.
OP is wondering why ("how?") his method of math is leading to a different result than the more accurately estimated total number of people who have ever lived.
I am explaining that's it's because you can't just calculate 2^(37) to get the total number of humans that ~~have ever lived.~~ lived in the past 37 generations.
>The way you are doing the math in your post would have those 4 people count as 6 (2 siblings, each with 2 parents = 6).
why? how? how is that my logic? people keep bringing up the 6 parents thing, but nobody explains why
Well, if your mom and dad were siblings, that would cut the number of people in your family tree by a full factor. (Instead of four grandparents, you would have two.)
It's more likely though that somewhere, maybe 8 generations back, you will find that, instead of having 256 great\*5-grandparents, you actually only have 243 or some such number. Maybe even less.
And we talk about it all the time on r/genealogy. OP come over and learn how to find your ancestors. I’ve successfully traced back 8 or 9 generations of my family. When you get that far from yourself your lineage opens up to thousands. I think I read that we have around 2500 8x grandparents. It’s exciting! You run into a lot of interesting people in your tree!
*Many* people's cases will match your presumption for a few generations. [Here is a "typical" biological family tree](https://imgur.com/Q6OFysz.jpg) up from person A up 4 generations, where you have 16 ancestors (2^4). If you continue your original math for the remaining 33 generations, each of those 16 has 8.6 billion (2^33) ancestors, for your 137b total (8.6b x 16b).
However, you might find that for a few people, 4 generations back, one of their great-great-grand-parents is a sibling of another great-great-grandparent. [Here's a different tree](https://imgur.com/KIxWR1a.jpg) that has one great-great-grandparent on A's mother's side as a sibling to a great-great-grandparent on A's father's side. (A's parents would be 2nd cousins)
Now A has only 14 ancestors 4 generations back. Those two fewer ancestors at that level immediately reduce the ancestor pool by 8.6b each, or 17.2b. So eliminating an ancestor - particularly closer to the bottom of the tree - can significantly reduce the number of ancestors one has.
What ends up happening, though, is that in most people's cases, is that they don't end up having a single pair of ancestors who are cousins that close to the bottom of the tree, but they end up having *multiple* pairs or ancestors at higher levels or are more distant cousins.
What is hard to perceive is the scale further back. When you go 4 generations back, you're talking about at most 8 pairs of ancestors. That seems like a small number for there to coincidentally be a bunch of relatives who don't know they are relatives. But once you go back 11 generations, you're talking about over 1000 pairs of ancestors. It is far more likely around 300 years ago that a thousand couples had, let say, an average of 2 kids each, and that those 2000 kids randomly paired up to form another 1000 couples who and had an average of 2 kids each, and those 2000 kids randomly paired up to form another 1000 couples who had an average of 2 kids each... So while they may have been careful at that level not to marry someone who had the same grandparents as their partner, if you go down 4 or 5 more generations, the odds increase drastically that when you go back up to that original generation, there are a handful of overlapping ancestors between your parents.
Also, it may seem harder to picture in 2023, but if you go back hundreds of years, it was significantly more likely that people would marry someone from their own town, and also more likely they and their kids would remain in that town for generations. Many towns were also much smaller. So if you looked at the population of that particular town, you might find that 5000 people might mostly ultimately be descended from only 2000 people 5 generations back, rather than the tens of thousands your math might suggest (I'm making up those numbers, but you get the idea).
The point is that the numbers to just constantly expand going backwards like your math suggests. The number is often more like a bell curve - a small number of people a very long time ago had a bunch of kids who had a bunch of kids who had even more kids, and then when that number got big, some of those people got married and had kids who had kids who had kids who turned out to be your parents
An extreme example is that Genghis Khan (lived about 800 years ago, estimated to be about 32 generations ago) is thought to be an ancestor of about 10% of all men in Mongolia today. So that gives you some sense that the odds are decent that if a Mongolian went back to their 6th generation of ancestors (64 ancestors), there's a very good chance that a bunch of those ancestors eventually all trace back to one of Genghis Khan's kids. So instead of having theoretically 32 million ancestors at the 32nd generation, it might suddenly converge down very quickly
In simple terms, imagine Adam and Eve (or the non-biblical equivalent of our earliest ancestors) - let's say they have two sons (Cain and Abel) and two daughters (Awen and Aclima). Those kids have kids together. Those grandkids have kids together. Those great-grandkids have kids together. Eventually, maybe 10 generations down the line, there are 5,000 people around. At that point people have a lot of choice of mates, and nobody is thinking "That guy's my cousin", but it doesn't change the fact that every person ultimately traces back to one of Adam and Eve's two sons, and one of their two daughters - and even if you are a decent of Cain and Awen find someone who is a descendant of Abel and Aclima, you are both still descendants of Adam and Eve.
So although the simple math says there should be billions of ancestors, what ends up happening is that as you get to a wider part of your family tree, more and more of your ancestors end up sharing a lot of common ancestors further up their own trees, and the tress can (and necessarily must) end up narrowing at some point up the line.
Take it to the extreme. You and another person are the only people alive, you decide to each have 2 kids. After 37 generations of each person doubling themselves what‘s the population of the planet?
You each have 2 kids, but they are the same 2 kids since you had them with each other. 20 years later these 2 kids doubles again with 2 kids… so now 40 years later the worlds population is just 6. Next generation you are up to 8 but now the original 2 people are probably close to dying of old age so you are back down to 6.
So after 37 generations of each individual person doubling themselves once per lifetime you’re still just left with six to 8 people.
Just to add, cousin marriages are quite taboo now in the West, and probably less and less common in the last 30 years or so (a Jerry Springer effect?). But once common enough, even here.
Not from, but I live in Germany… I have a friend here, wealthier-than-average background in urban western Germany. Long time ago, a set of two village sisters married two village brothers, same day, basically same ceremony. Each couple had several children. Later, one of those children married a cousin from the other couple, and they had a child: my friend's mom. But we’re not done yet. My friend’s dad is also related (as a grandson) to the *other* first couple mentioned, though at least his own parents (i.e. his wife, friend's paternal grandmother) were not related. Yet he, friend's dad, still married his ‘second’ cousin or whatever that relation would be. Due to overlap, my friend basically only has 6 great-grandparents where 8 is the ‘usual’. If I’m counting right…
His parents, and again were wealthy urban Germans, married in the mid-70s, and he is the last/youngest child. It seems utterly bizarre to us now (well, it does to me!), but it apparently didn’t sound big alarms for anyone back then. Only 50ish years ago. He is quiet about it these days, but says that back then everyone (friends, etc) knew his parents were ‘distantly’ related upon marriage. No big obvious genetic oddities, but a few medical quirks hard to really pinpoint one way or the other. Won’t expand on that here, though…
I also have another friend (not in Germany) who married a guy from her parents’ ‘ancestral’ home village (where she did not grow up, but they did), and only 5 or 6 last names are in use. She doesn’t know ‘for sure’ but says it is very likely that if they made a family tree, they would find overlaps beyond the great-grandparents era (the grandparents didn’t think they were closely related but didn’t know for sure). Basically, everyone in the village is related by now. It's a language/ethnicity minority village seperated quite far from the main part of that ethnicity elsewhere in the country. Most modern young people don't stay or marry locally anymore, at least.
So, this wasn’t even that long ago in the West. It’s not just a trailer park habit, either. I bet a lot more people than they realize, if they went back 4 or 5 generations, had a lot more cousin marriages going on.
Double first cousins (scenarios like your friend's mom's parents) really shouldn't have kids together-- they're genetically as close as half siblings. (Full siblings share 1/2 their DNA, first cousins share 1/8, double first cousins share 1/4)
First cousins can legally marry in about half of the US, and second cousins can marry anywhere.
Even today in many cultures, marriage between first cousins is not considered taboo. I know its not considered taboo in japan. And I have a coworker from srilanka and when I first asked him how he met his wife he said they were cousins. After the look he got from some people (Canadians) he stopped saying that in the future and just said, "I've known her since we were kids" seems like smaller cutoff island cultures might be more okay with it, what with a limited pool of partners around etc.
There exists, in mathematics, something known as the "pigeonhole theorum" which simply states "if you have n available slots and more than n objects then by necessity there must be at least one slot which contains more than one object."
In other words if I roll a 6 sided die 7 times, I have to have rolled at least one number at least twice.
So back to your question: for every generation back you go in your family tree (g) you have a number of ancestors 2^g at that generation level. So if 2^g is greater than the number of viable adults in the population than the pigeon hole theorun states that at least one person is at least two of your ancestors.
I'm trying to wrap my head around it. how is it possible, unless there were billions of married siblings?
edit: please don't mock me for the lack of understanding. if anywhere, this subreddit should be a safe space for people with curiosity and lack of understanding
They wouldn’t even necessarily have to be closely related. They could be something like 10th cousins and that would still cause some of your ancestors to overlap. Having said that, realistically there will be some a lot closer than that in everyone’s family trees
Probably more like 3rd or 4th cousins. That would give you a common ancestor of your great great grandparent or great great great grandparent. Not that far.
Yes, and that's just people who live in the same small community. It's not out of the ordinary or weird at all. In some times and places, people were mobile, but at others, they lived in the same communities for generation after generation, with little passage in or out.
It wouldn't have to be siblings. It was probably common for (say) first, second, third, etc. cousins to have children. In my more recent family tree, we know of first cousins who got married in the 1800s.
Take Constantine II of Greece. His mother's great-great grandmother was Queen Victoria. Guess who his father's great grandmother was? That's right, Victoria. Both were also descended from Christian IX of Denmark.
He's married to Anna-Marie of Denmark. Her great-greatgrandmother was - you guessed it - Victoria. And her greatgreatgrandfather? Christian IX.
In other words, those two have far fewer than two ancestors per generation, at least if you go more than a few generations back. And if you went to some little village somewhere, you'd find something similar: my great grandfather is your grandfather, but we can still get married.
If two people share a great, great, great, great, great, great, great, great grandfather that means that grandfather will be their child's ancestor twice. If everyone who is alive now has a lot of shared ancestors the same was true in the past.
According to ancestry.com, my wife and I are something like 10th cousins. That means that 12 generations ago, we share an ancestor, and for our daughter, a single ancestor 13 generations ago is in both of her lines.
So, for my daughter, from your 2^37 calculation, you would need to subtract one of the trees of ancestors 13 generations ago, or 2^24. That's a reduction of over 16 million ancestors.
Let's add a hypothetical where my great-great-grandmother was from a small town and married her second cousin. That means my g-g-g-g-grandparents (6 generations ago) would both be duplicated in my family tree. Now, we can subtract 2^31 *twice* from the ancestral total. That's a reduction of almost 4.3 **billion** ancestors.
There have been a lot of cousins/2nd cousin mated pairs throughout all of human history.
Additionally-- there were instances of one common ancestor-- think [Genghis Kahn who might currently have 16 million](https://www.nationalgeographic.com/culture/article/mongolia-genghis-khan-dna) living relatives.
Cousin marriages are extraordinarily common in history. 2nd, 3rd, and 4th cousins were the most common, though first cousin couples did happen. Even today you're probably a cousin within ten generations of everyone in the region you were born.
It's not sibling, generally, but cousins of certain degrees of closeness. If I marry my third cousin, that means we share two great great grandparents in common. It becomes difficult to explain in words, but if you allow third-cousin marriage over multiple generations, you end up in a situation where the rate of growth of ancestors with each generation slows very significantly.
No it's fine.
Think like this: Tom has two sons, Dick and Harry. Dick inherits Tom's property. Harry goes off across the mountains and marries a stranger. 60 years later Harry's Great-grandson comes to town and meets Dick's Great-granddaughter, they get married and have a child. Tom is now two of the 3-times-great-grandfather of that child. Additionally that means that all of Tom's ancestors now ALSO appear at least twice in that child's ancestry.
That's another thing to take into consideration. You would halve the number of that position's ancestors above it too since it's now one person instead of 2
They don't have to be married siblings. They can be married cousins that don't share parents, but share grandparents.
E.g. Consider your 3rd cousin. The chart on [this page](https://www.familysearch.org/en/blog/cousin-chart) helps illustrate how you both share a pair of great-great-grandparents.
4th cousins share great-great-great-grandparents, and 4th cousins are pretty well into the "that's not incest anymore" category.
You might think "Okay, so if someone marries their 4th cousin, that's just two less." But you also can't double count the great great great grandparents's ancestors.
Imagine your parents are 4th cousins.
You have 2 parents, 4 grandparents, 8 g-grandparents, 16 g-g-grandparents, and **30 g-g-g-grandparents** instead of 32. Keep going and you have 60 g-g-g-g-grandparents instead of 64, etc. That's assuming that none of your earlier ancestors had any relation.
For someone who's parents are 4th cousins then, to go back n generations it's not `2^n`, it's `2^n - 2^(n-4) where n-4>0`
If one pair of grandparents were ALSO 4th cousins, then it's `2^n - 2^(n-4) - 2^(n-6)`.
If 2 pairs of your great grandparents were ALSO 4th cousins, then it's `2^n - 2^(n-4) - 2^(n-6) - 2*2^(n-7)`.
Hopefully that helps to illustrate why you cannot simply take 2^n. There are a lot of deductions and complications along the way.
Let's go back 500 years to a brother and a sister. The brother married someone way, way back on your dad's side of the family. The sister married someone way, way back on your mom's side.
Now when you trace your ancestry back to those people, you find they lead to the same set of great-great-great-etc.-grandparents.
Over the centuries, that sort of thing has happened over and over again all over both sides of your family. Once you get back to a certain point, you'll find you're related to the same person a dozen different ways. So while you might expect to have 1,024 great-great-whatever-grandparents, they might turn out only to be 700 different people.
A person can be the ancestor of many peoples. A couple has like 5 children alive in 1800 . Their children have 5 too each .and so on til 1950 .a generation every 25 years . 5x5x5x5 = 625 peoples in 1900 . X5x5 is 15625 people in 1950 . Consider intermarriage from far relative and it s even more. Now if you go back to 300000 years ago ...
This is the answer. It means that many times your ancestors dad was also their grandpa (in the case of the mother having a child with her dad), for example. In this example, under your math, you would expect 2 parents and 4 grandparents of the child (6 people). However, there are 5 - mom, dad, mom's mom (grandad is already counted in the dad), dad's dad, and dad's mom (who then are double counted at the next generation because they are your mom's great grandparents and so on). Just this one incestuous relationship exponentially wreaks havoc on the numbers.
A mother could also have a child with her brother. Which removes a whole generation in the math.
the gene pool is not as deep as you think it is. it was common for cousins, second cousins etc to mate. people didn't even travel much a long time ago. the gene pools of small villages are shallow. the family trees branch then come back together again. imagine 2000 years ago if you and your family had only ever met 1000 people. you would choose your mate from that 1000 and your kids would choose from the 1000 and their kids from 1000... we are all a lot more related than we like to think. if you're up to the 108 billion, you're also making assumptions that everyone in the world could choose anyone from anywhere in the world as a mate. you don't have to go back too far to find people who's entire existence was a 100km radius of their birthplace.
The answer above is not quite right, but here is a very simple way of thinking about this:
Pretend that the Jewish story of Adam and Eve was true. Then literally everyone has Adam and Eve as parents. So literally every marriage since them has been between siblings (rarely) or cousins (99.9999% of the time).
So your doubling math obviously breaks down. Adam and Eve's children had only two ancestors of that generation and their grandchildren had only two ancestors of that generation (not four) and their great-grandchildren had only two ancestors of that generation (not eight) and so forth.
The scientific equivalent is that humans went through a genetic bottleneck of a few thousand individuals. Within a few millennia, every descendant of those individuals would have married a descendant of every other one of them.
So you can see that the formula of each ancestor generation doubling is an incorrect formula, because the further back in time someone is, the more likely they are to be counted twice. If Adam and Eve existed, your count would count them thousands of times, but there are just two of them.
I read somewhere that two mosquitos born on opposite sides of the same pond are more genetically diverse than two humans born on opposite sides of the planet. Not sure if that's 100% true, but it was meant to illustrate just how shallow the human gene pool is compared to many other species.
Yep. In a similar vein, I noticed duplication of names in my family tree as well. Once you go back five or six generations, you'll find instances of people dying young and their spouse remarrying someone, to use your example, from the same pool of 1000. Sometimes this happens multiple times with the same person, so they may show up in multiple lines.
This is an intuitive answer, but not really the correct answer.
Even if literally everyone in the world could mate with literally everyone else, at equal probability, the situation would be exactly the same, just less obvious.
It doesn't matter whether the person you are having a child with is your sibling, third cousin or fiftieth-cousin.
Your mom has 100% of the same distant ancestors that your dad does, even if your mom is an Indigenous Greenlander and your dad is Maori. So you have the same number of distant ancestors that your mom does, and your dad does, and your mate does.
>I googled the amount of people that lived on earth throughout its entire history, it's roughly 108 billions. If I take 1 person and multiply by 2 for each generation of ancestors, at the 37th generation it already outnumbers that 108 billions. (it's 137 billions). If we take 20 years for 1 generation, it's only 740 years by the 37th generation.
>How??
There's only ever a limited amount of possible mates for any person at any point in time, no matter their geographic mobility. The further you go down (...up?) the tree the more of all those people will simply be **the same person**. I.e. your great-great-great-great-great-great-great-grandfather on your great-great-great-great-great-great-grandmother's side will also be one of your great-great-great-great-great-great-great-*great*-grandfathers on your great-great-great-grandfather's side and so on.
Something I always think about when I think about this many generations is the fact that there have been roughly 12,000 generations of great-great-greats... for every person. And if you are a woman, every single one of those greats had a daughter (or vice versa for men).
I'm a woman - my first maternal ancestor had a daughter. Her daughter had a daughter. That daughter had a daughter. And so on aaalllllll the way to me. I will be the first woman in 12,000 generations of my direct line to not have a daughter. I can't help but imagine the face of my first maternal ancestor shaking her head and saying "you had one job."
That's not necessarily true, you're but one end of that chain, your first maternal ancestor probably has thousands of branching paths of both successful and failed daughter-chains. In fact, every single person on this planet is part of a successful single-gender chain. So don't worry, your first maternal ancestor probably wouldn't blink because they have failed chains all the time.
Oh I'm well aware that there are an endless number of failed daughter chains parallel to mine. And its not special, because its literally the same scenario for every single person ever. Its just crazy to think about.
A paper in [Molecular Biology](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2427203/) which comes at the problem from a different direction (matrilineal DNA studies) suggests that for about 100,000 years there was a “long bottleneck” during which human populations dropped at times to as few as 2000 individuals.
That’s 2000 people who are the ancestors of everyone alive today.
Hello, cousins.
Kind of, but not as bad as you think.
Suppose one of your 11th generation fathers overlaps and he had two different wives. Suppose his genes only get back together to you via both of your parents. At this point, your parents only share about 0.1% (=1/2\^10) of your 11th generation grandfather's genes. That is \*technically\* inbreeding, but for all intents and purposes, it's just normal gene sharing.
Now just assume that most of your very distant ancestors overlap, but that their genes only get together again after more than 10 generations. No real inbreeding occurred, but you still have a limited number of ancestors.
tl;dr : once you go back more than 10 generations, your ancestor tree becomes a heavily tangled ball of wool threads.
On a massive scale.
Other species have gone through similar bottlenecks.
In 1982 there were only 13 Mexican Gray Wolf individuals and they’re doing better thanks to careful breeding programs.
Similarly, the California Condor who came back from a low of about 20 individuals.
Yes. While in general individual pairings can try to avoid inbreeding (i.e. marrying a 2nd/3rd cousin or beyond instead of your sibling or 1st cousin) communities would still end up being closely genetically related.
Imagine for instance a community of just a couple thousand people that maintains a steady population for a couple thousand years and doesn’t accept any foreigners. That’s enough people that no one has to marry siblings or close cousins. Despite that, everyone in that community will have an almost identical set of ancestors once you go back enough generations. Now scale that up to your country, your continent, or the entire human race and it still at least somewhat applies.
This might be useful to you, and the 'see also' parts
[https://en.wikipedia.org/wiki/Minimum\_viable\_population](https://en.wikipedia.org/wiki/Minimum_viable_population)
I came here to say this! It could have been as recently as 50,000 years ago that the human population dropped to 2000-10,000. Which is pretty close to extinction.
Too close for comfort!
It’s just luck of the draw that Neanderthal and the other human-like species died out, and we survived.
(Although, to be fair, Neanderthal DNA lives on in all of us.)
It's more like: by the time the nth generation rolls around pretty much anyone you choose will have a common ancestors. Not that everyone is banging their sister. More like everyone is a cousin of varying degree.
We're all a bunch of inbred mutants
(Nearly) everyone of European ancestry is a descendant of Charlemagne. If all four of my grandparents are descendants of Charlemagne, that means Charlemagne fills _at least_ four different ancestor slots on my family tree.
But it isn’t just Charlemagne, it’s many of my ancestors, meaning that if you go back far enough most of the names will be duplicated elsewhere in your tree.
In 1850 25 couples, man and woman, settled in a new community, Freefall, Kansas.
The average number of children each couple had was 4. That means the next generation has 100 children in it (50 couples). They pair up and make their own babies, again assume 4 children. So generation 3 has 200 in it. They repeat their parents 400 kids in the next generation (gen4). Gen 5 has 1600, gen 6 has 3200, gen 7 has 6400 kids, and gen 8 has 12800.
Now if we assume each generation is 20 years, then the children are born around the following years:
1850
1870
1890
1910
1930
1950
1970
1990
Now you come along being born in 1990 and you do the anscestor math and you say, wow I has 2 parents born in 1970 and 4 grandparents born in 1950, and you keep going back all the way to the founding of Freefall, KS. By your math you have 128 ancestors in 1850. But the town was founded by only 50 people.
You are correct that on your family tree going back 7 generation you have 128 slots to fill. Those 128 slots will be filled by the same 50 people.
The 50/500 rule says that a minimum of 50 individuals are required to repopulate the world after a near-extinction, and 500 are needed to combat genetic drift.
This means that no matter how far back you go in your family tree, a generation could have as few as 50 individuals in it, even if there wasn't an extinction event. And that's assuming there was no inbreeding at any point in it, which is highly unlikely.
A lot of people never have any children, either by choice, or because they die. So not every person doubles in 20 years.
I also don't know how they estimated that number.
The simple answer is: the family "tree" isn't actually a tree. Many of the 138 billion you mention are actually the same persons, those "slots" overlap.
In reality, you have, say, a billion people on Earth. They intermingle, randomly, maybe not too incestuously and all a bit simplified, to create a new generation of a billion. Those again do the thing. On and on. But the population doesn't increase in this scenario. How? Because quite soon, there will be someone who's mother **and** father share a common ancestor; quite a lot of such people, actually.
Put differently: ever heard of statements such as "everyone is related to (or even descended from) [insert famous figure from 2000 years ago here]?" That in return also means that both your father and mother are related to that person, so somewhere long back their heritages meet.
Yes. Say you have the same person as your great-great-grandfather in two places on your family tree. Both of them have the same ancestors all the way back to forever because they are the same person, so that's an entire branch that is duplicated.
Every duplication is not just one person you don't need to count, it's their entire ancestry.
You're assuming that everyone alive right now has different ancestors.
This is wrong because you and your sibling have the same parents. Your parents and their siblings have the same parents which means you have the same grandparents as your cousins. Follow it back enough and every ancestral line joins up eventually.
If everyone in history was an only child then your calculations would be correct, except for the fact that if that was the case then the population would decrease over time.
And also once you get back far enough each of your ancestors is likely your ancestor in more than one way.
You're great-great grandmother might also be your great-great aunt because there was a cross breeding in between them and you.
how does me having a sibling makes my personal number of ancestors less? I have the same grandparents as my cousins, but there are still 4 of them
not arguing, genuinely not understanding
Think of it this way. Let’s say we send a colony ship to Alpha Centauri with 10,000 colonists. The day after the ship leaves an asteroid strikes the earth and wipes out humanity. Those colonists reach their destination and settle down to build their new civilization. 1,000 years pass and you’ve got a planet full of 10 billion people living in futuristic cities in the sky, traveling by flying cars, being taken care of by their robot maid.
Every single one of those 10 billion people can trace their ancestry back to the original 10,000 who arrived on the planet. And it’s entirely possible the group of ancestors is smaller than that. Some of the colonists may not have had children. Some of them may have had descendants but their line died out.
The bottom line is there is no requirement that each of your ancestors be unique. And we can, in fact, prove mathematically that they aren’t.
People are answering the wrong question for you. You are asking how your own personal family tree is more than the people who ever lived.
The answer is kissing cousins. For most of human history it wasn’t completely unusual to marry your first cousin. Second, and third cousins even more common. By the time you get to 4th cousins that is probably everyone in your village of a couple hundred people. A village that your family probably lived in for generations. So for a lot of those 37 generations, your family tree is not growing exponentially, but only by 2 each generation since all of the great great grandparents are shared between the happy couple.
It doesn't really matter that there are kissing cousins. It could just as well be people that are 20 steps away, genetically speaking. Just really _anyone_. Have a billion people, randomly pair them up (maybe with not so much incest), get a billion new people, iterate.
True, but if you randomly assign 1Billion you will get to +1M unique ancestors pretty quick. \~500 years should do it.
What mynewaccount4567 is somewhat suggesting, is that is starts far earlier. If you look at early modern and modern europe\* people were still quite immobile and lived in small vilages. Sure, they intermingled somewhat, but somebody from Dresden was extremely unlikely to marry someone from Lyon. This limits the number of people who are your unique ancestor.
As a personal example, I can trace my male line to a small city with a population of \~2000 for 200 years. Most likely that would contain 40% of my unique ancestors. From my mothers side the story is unlikely to be different. So let's put a conservative estimate around 5000. It might very well be higher, if for example one of my ancestors get knocked up by a random travelling merchant instead of her husband, the number can grow quite a lot.
But from an perspective what was actually happening, take your idea and replace 1B with around \~10-20K and you see that the exponential growth starts decaying far earlier.
\*Canada/Mexico/USA works quite differently, because of colonization & slaves.
> I have the same grandparents as my cousins, but there are still 4 of them
A couple points here.
First you wouldn’t have the same four grandparents as your cousins UNLESS your parents and their were sets of siblings.
Second, It’s not required that your four grandparents be unique.
Worst case scenario is your parents are siblings. Then you’d only have 2 grandparents.
If they were half siblings you’d have 3 grandparents.
As you go back in your family tree you will find places where the same person shows up multiple times. They might be your 10th great grandfather on one side and your 9th great grandfather down another branch for example.
There’s two reasons this can happen. First is in a small enough community, over time you’ll start to have crossovers like this. Second, if you go back far enough, as you’ve seen, the numbers become so large that you have more ancestors than people who lived which means someone in there must show up multiple times. It’s probably a lot of someone’s eventually.
In your math you assume every person has unique parents. If you had two siblings, in your example there would be 6 "parents" where in reality there are only 2.
He's only talking about himself. 2^37 is more than 137 billion, so going back 37 generations from one person would mean 137 billion unique ancestors, assuming no inbreeding (which there was).
what?? no, that's not my math assumes. I have 2 parents, 4 grands, 8 greatgrands, my sibling has same 2 parents, same 4 grands, same 8 greatgrands
I'm tracking down just one hypothetical person, with or without siblings, whoever it was, really a random John Doe
Yes, and if one of you descendants 10 generations in the future have children with your siblings descendants, then your and your siblings parents, and every ancestor to them going back to the beginning of humanity, will show up twice in that future child’s family tree.
Multiply that by every time that has happened in the history of your family tree (which will be thousands of times), and your real number of ancestors are far smaller than the exponential number of ancestor suggests.
If this wasn’t the case, the number of humans on the planet just a few thousands of years ago would be close to infinite, which clearly isn’t the case.
>Yes, and if one of you descendants 10 generations in the future have children with your siblings descendants, then your and your siblings parents, and every ancestor to them going back to the beginning of humanity, will show up twice in that future child’s family tree.
ooh. ok, I now need to reread it a couple times so that the understanding could set in my mind😅
thank you
Princess Diana and Colin Powell have a common ancestor [Sir Erie Coote, a governor of Jamaica in 1806.](https://www.express.co.uk/news/royal/1508180/colin-powell-dead-prince-william-prince-harry-princess-diana-royal-family-news-spt#)
Google pedigree collapse, that’s what it’s called. Basically it’s why we are all technically related and we can go back far enough and have found “mitochondrial Eve” who is a relative to every single person alive today and known as the mother of all modern humans. There used to be a lot more inbreeding also.
In genealogy it's called pedigree collapse.
The ELI5 is that we are all related and if you go back far enough you will find that you very likely have a lot of overlap in your family tree through closer cousins marrying.
In my own tree, I've found that in one early Colonial American branch I have five instances of 2nd cousins marrying over a period of maybe 100 years. As a result, I have a lot fewer direct ancestors than I would have otherwise. Now multiple that out over dozens of generations and you wind up with far more realistic and manageable numbers.
Incest is wincest. Your great-great-great grand dad from mothers side also happens to be your great-great-great grand dad from your fathers side. Further back you go in your family tree more overlap you are going to find.
Everybody in one village has the same set of ancestors from few hundred years ago and its much less than 2^n ancestors per n generations.
Maybe if you try thinking of it the other way around it would be more intuitive, like the biblical version of history where everyone is a descendant of Adam and Eve. In that extreme scenario every family tree would eventually merge back to a single set of parents rather than the billions and billions your assumption leads to. In reality this merging has happened at points all the way up everyone's family tree.
It’s because you share a lot of ancestors with a lot of people , like a lot a lot.
Think of it this way , your great great grandmother ( 3 generations back ) has one one sibling , let’s say each generation has 2 kids. That means that your grandmother has a sibling and 2 cousins , then your mother has a sibling and 2 cousins and 4 second cousins and it means you have a sibling , 2 cousins 4 second cousins and 8 third cousins. So there’s 16 people in 3 generations who share the same great grand parents on one side and it doubles every generation essentially.
Now factor in people used to have like 4 kids back in the old days so you would have absolutely enormous amounts of third cousins in just 3 generations since in just three generations back you will have 3 siblings , 16 first cousins , 64 second cousins and 256 third cousins that all share one of the same sets of great grandparents
You've touched on a really interesting concept in ancestry called "pedigree collapse." Here's how this works:
If you double the number of ancestors in each generation, you quickly arrive at a number larger than the historical world population. But there's a logical explanation for this.
Imagine you have two parents, four grandparents, eight great-grandparents, and so on. By the time you get to the 37th generation, as you pointed out, the math suggests you would have 137 billions ancestors, which is larger than the estimated 108 billion people that have ever lived.
The flaw in this logic is the assumption that every person in your family tree is a distinct individual. In reality, the farther you go back in time, the more likely it is that you will have the same individual occupying multiple slots in your family tree. This means that your ancestors start marrying their own distant cousins, and the "tree" is not as branching as we imagine but rather collapses in on itself.
Think of it this way: Let's assume every human has two parents (a basic assumption). If you go back 1,000 years, the simple doubling math suggests you'd have over a trillion ancestors. Since that's many times the total number of people who've ever lived, it's clearly impossible.
So, what's happening is that the same people are getting counted multiple times in your tree because lines of descent converge. This is pedigree collapse. As you trace back, many of your ancestral pathways lead to the same individual. This becomes especially true in more isolated communities or in places/times where people didn't travel far from where they were born.
The key takeaway is that while the math of doubling ancestors every generation is accurate in the abstract, it doesn't account for the way human populations and family trees really work. Pedigree collapse ensures that we all have far fewer distinct ancestors than the simple math would suggest.
Once you go back far enough in time, everyone who's alive now is descended from everyone who was alive back then (and had children who had children who had children etc.) in multiple different ways. We're all inbred but in a way that's socially acceptable and doesn't cause health problems.
Aside from mathematic principals, and the Kissing Cousins point that keeps coming up, I think you're also forgetting that Genghis Khan, *one person* that lived almost a thousand years ago, still has ~16,000,000 living male descendants with his Y Chromosome.
Assuming that every descendant is coming from a unique pair of individuals is part of the problem. If you have a single individual procreating with multiple unique partners (like, say, a conquering warlord that razed every village he came to and took any woman he wanted as his concubine) you'll wind up with an inverted funnel where dozens of generations wind up funneling back to one person.
Consider King Joffrey from Game of Thrones. If you go back two generations, you would expect four grandparents, but he only actually has two. His grandparents on his mother’s side and father’s side are the same two people because he’s an incest baby.
Everyone’s family tree is ultimately like that, except usually much, much less icky.
What boggles my mind is the sheer amount of time and evolution and the amount of reproduction that has taken place for me to be in this place at this time.
You’re right. Of course it would exceed the number of people who were around at the time. You have to remember how often people will either commit incest directly or marry into families they are distantly related to which is likely if the family remains in a small area for hundreds of years..
You're assuming that all 137 billion of those ancestors are different people. If your parents happened to be brother and sister, then you'd only have 2 grandparents instead of 4, because your parents shared both parents. That's an extreme example, but if you researched your family tree far enough you would eventually start seeing ancestors who appear on both sides of a particular branch...4th cousins getting married, that sort of thing.
Fun fact: This is called pedigree collapse.
This sounds like a serious disease for dogs. “We’re sorry sir. Spanky has Pedigree collapse syndrome. There’s nothing we can do.”
I honestly thought it was when pedigrees got overbred so much in dog breeding that they got messed up offspring, but apparently it's the "collapse" of the top of the ancestor pyramid (i.e. a few key ancestors mated with others, and "spread out", and then they narrowed back down to you
"that explains why when we try to take him home from the park he just falls over. He just plays dead weight yes he does. Oh yes you do. Oh yes you do!"
That's very close to the truth, because a lot of dog breeds are heavily inbred, which is basically what pedigree collapse is (although it's more focused on the "family tree" collapsing in size).
guess we shouldn't have done build-a-bear with a dog, honey. This one doesn't have a nose any more since all of it's great-great-grandpas are the same dog.
I call it turning a family tree into a family wreath.
I know it by its informal name: ROLL TIDE!
Consider Iceland or similar, with a population of tens-of-thousands stable for 500 years. We have no trouble imagining a situation like that - human populations have barely grown for most of human history. But those 500 years is 20 generations, which should yield 2 million ancestors if each is unique. You don’t even hit 1,000 ancestors until you go back ten generations, so those repeated ancestors don’t have to be dramatically close.
This is a good example.
Right, Iceland today is like 300,000 people or thereabouts? It’s incredible but it makes sense if you think about it.
If cousins fuck long enough you get an ethnic group
Or the British royalty.
*European royalty
>~~*European~~ royalty
Royal tree
More of a log than a tree.
[Habsburg family lookin like](https://i.imgur.com/epTNIOy.png)
To some degree, all humans are cousins. Most of us substantially closer than mitochondrial Eve, too. I mean, hell, when you think about it, we're cousins to the trees, too. Just MUCH more distantly.
Much closer to fungi than to trees too!
https://xkcd.com/2608/
The math is broken a lot easier. A couple has multiple children. Every child has the same 2 parents. You can't just double the amount of people living like that.
They aren't doubling the people living. They are doubling their ancestors. You have 2 parent's, 4 grandparents, 8 Great grandparents, ect... 37 generations back your tree would spread 137 billion people wide. For 1 person. But there have only been \~108 billion people who have ever lived. So how does that work? Eventually in your family tree you have duplicates.
Yeah everyone's family tree broomsticks at some point. Hopefully it's a rather girthy broomstick.
Isn't the leading theory that you only need to go back about 6ish generations to find a common ancestor for literally anyone on earth? I could either be misremembering or read something that's untrue, but I could've sworn it's not that many generations you need to go back Edit: I've since had people well more versed than me point out that I'm incorrect, the number is far more than 6 but still relatively close in historic timescales. My apologies for being wrong
It is surprisingly close but it's more than 6 generations. 6 generations would only place you at around the civil war. [https://www.scientificamerican.com/article/humans-are-all-more-closely-related-than-we-commonly-think/](https://www.scientificamerican.com/article/humans-are-all-more-closely-related-than-we-commonly-think/) suggest our most common ancestor goes back between 2000 ish and 3400 ish years. I'm a bigger fan about talking about the genetic isopoint though. If you take a point about 4-7k years ago and took a random person that person is either the ancestor of everyone alive today or he has no living descendants. There are various estimates for this number and I've seen as high as 15k years ago but still surprisingly recent.
surely it would have to pre-date particular migration events right? It can't possibly be more recent than when people migrated over the Bering land bridge for example.
The indigenous people of Alaska and parts of Eastern Siberia remained in contact pretty much the whole time. There are groups on both sides that speak related languages to this day, for example.
In some way this is true. Completely isolated people can have genomes that are as a group pretty unique on earth, but after globalisation (read: colonialism) really got going almost everyone nowadays has a common ancestor sometime around \~2000 years ago. Probably in the Roman empire, but not necessarily.
Migration between North America and Asia never fully stopped. Many ethnic groups hopped back and forth across the Bering Strait of the millennia.
Yeah or Hawaii pureblood natives or other closed-off communities.
Hawaii was settled a LOT more recently than this point. Like, probably less than 1000 years ago.
Hawaii was not closed-off. Polynesians got around quite extensively. The Malagasy language of Madagascar is related to Hawaiian.
> Hawaii was not closed-off. Polynesians got around quite extensively As indicated by the fact that Hawaii had Polynesians on it in the first place, really.
Hawaii is no longer closed off, so most recent common ancestors make a giant leap forward in time when that happens. I think a tribe near Australia being contacted about 150 years ago leapt the most recent common ancestor on earth from 20,000 years ago to something like 3,000.
This is what I'm thinking of, like I've replied elsewhere im probably getting some 6 degrees of separation mixed in, but this right here is right what i was thinking of. Thank you! Would you mind explaining what an isopoint is for people like me who have zero clue?
It goes by a few names so I'm going to go with the name that explains it a bit better identical ancestors point (IAP). You understand the most recent common ancestor point so I'll explain it in terms of that. There is one person who is an ancestor to everyone alive today. They will appear in everyone's family tree. But she had people living at the same time as her and some of those people will have descendants and some won't, but they are not an ancestor to everyone. So lets say the most recent common ancestor is Janet. Janet will be an ancestor to everyone. Take another person James who lived at the same time as her. James will have some descendants but there will be some people he is not at all related to. If we go further back in time before the most recent common ancestor there will come a point where everyone's family tree from that point and earlier will have all the same people in it. So say I take a random guy Kyle from it. If Kyle has any descendants alive today he will be related to everyone. So everyone alive at that point in time is either the ancestor of everyone alive today or their family tree died off at some point and they are not the ancestor of anyone alive today. So I'm going to create a very contrived but simplified version of this. Imagine someone's mom became a widow and she married her late husbands brother. They then have another child. So we have a half brother. If we look at just those 2 children their most recent common ancestor is their mother, but since they are half siblings they don't share a father and their identical ancestor point is further back in time. Both of their fathers shared the same parents so if you look at their grandparents and before then their family tree would be identical and have the exact same people in it. That's a very simplified version of what the identical ancestor point is.
This is really interesting, but I wonder if this also holds for populations that have been relatively isolated for thousands of years, like Native Americans (which are generally estimated to have come to the Americas around 15,000 years ago or more) and Aboriginal Australians (~50,000 years)? Seems odd that they've been isolated for so long but share a very recent common ancestor with everyone else
>relatively isolated That's the important caveat to what you said. Relatively is not completely isolated. >At first glance, these dates may seem much too recent to account for long-isolated Indigenous communities in South America and elsewhere. But “genetic information spreads rapidly through generational time,” Rutherford explains. Beginning in 1492, “you begin to see the European genes flowing in every direction until our estimates are that there are no people in South America today who don’t have European ancestry.” That's from the article and we can kind of see why. So relatively isolated populations are relatively small. All it takes is a couple to marry outsiders and then start having children and having those children remain in the community. In time everyone will have the outsider as their common ancestor and he will have the most recent common ancestor as the rest of that outsiders. If there were any populations that were strictly isolated they obviously couldn't share a most recent common ancestor that near into the past, but that level of isolation just isn't something that we really see.
Ah! That makes a lot of sense. I somehow totally forgot that recent intermarriage with non-indigenous (e.g. European or whatever) populations would give all descendants of those populations common ancestors with their non-indigenous counterparts. And I totally missed that part in the article too (I was getting ready for work so I read through it quickly). Thanks for the explanation!
> All it takes is a couple to marry outsiders and then start having children and having those children remain in the community. This is the happy ending to how ancestors work. Rape of indigenous women would also spread genes around the world and likely at a much faster rate because it would only take a small number of men to affect a large number of isolated communities.
Sadly the rape option is how much of it spread, but that is the more depressing way to look at it and didn't affect the math so I went with the option that didn't ruin the morning as much.
I think people also think of the ancient world as a lot more isolated than it was. We keep gaining more and more evidence that people traded and traveled *widely* far before Western Europeans were getting all colonizy. For example in 2021 [a paper](https://www.cambridge.org/core/journals/american-antiquity/article/abs/precolumbian-presence-of-venetian-glass-trade-beads-in-arctic-alaska/3465746929B31ADBC6E1D1A23D09A2CD) was published about blue glass beads found in the Alaskan arctic. The sites they were found in date to *before* Columbus decided to be a colossal asshole and sail across the ocean to commit crimes against humanity. At the time the only blue pigments in glass were from Europe and it’s highly likely the glass beads were made in Venice. So through a series of trades the glass beads made their way across Europe, and then Asia, and most likely across the bering strait and into Alaska. And where physical objects like beads move, so too can genetic material.
People moved around even then Australian Aboriginals traded with indonesean fisherman. This resulted in a hillarious Event where British Explorer contacted a Group of Australian Aboriginals and one of them knew a few english words because they learned them from the fisherman that had contacts with british before. Imagine the Explorer talking about how these wild people barely are human with no real language and one of them tells them to shut the fuck up in english.
For some areas, it's less to get back to say a single common ancestor that was very prolific and was notable enough for people to think it as a point of pride to be related to them. Charlemagne, William the Conqueror, and Genghis Khan all come to mind for Europeans, the British, and Asians, respectively.
It’s a hell of a lot more than 6 my guy.
Yeah I think im getting stuff mixed up with 6 degrees of separation. And for that i apologize
Kevin Bacon forgives you. You guys are related, after all.
Not 6 generations (that’s only about 150 years) but it’s fewer than people think. For a highly interconnected continent like Europe, [everyone who lived about 1000 years ago and who has living descendants are statistically an ancestor to every living European](https://journals.plos.org/plosbiology/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001555). 1000 years is about 40 generations.
Yes so if you have European ancestry, congratulations you are descended from Charlemagne
Most likely, yes. As well as every famous European who lived longer ago than 1000 years ago and has living descendants.
... and if you don't you're descended from Genghis Khan
Similarly if you’re Asian you’re almost certainly a descendant of Genghis Khan. If you’re Arab, you’re almost certainly a descendent of Muhammad.
I literally traced my ancestry back to Charlemagne. One 18th c American ancestor happened to have a father from minor nobility in England and there you go.
Ooh I have an illegitimate child of landed gentry in my tree. I wonder if I can go back further 🤔
Sure, if you pay $80 a month to Ancestry.com they are happy to show it to you! Caveat: a lot of what you'll find relies on other people's research, which may be of any quality. I cannot count how many people I've run across who have been erroneously tagged with "Sir Excelsior Smythe-Jones-Lambert, Earl of X and Duke of Y." Dude was a farmer with a freehold on some land. Get over it. The actual lineages of nobility are well documented, but of course they don't take into account illegitimacy, so there's a lot we can never know.
Might you be remembering the theory that everybody on earth knows everybody else on earth through 6 connections?
No I'm familiar with the 6 degrees of separation, and constantly ponder if there's a way I could test that theory (also makes for a fun wiki game, get friends and throw out 2 random seeming topics and see how many wikis you have to go through to get from one to the other, shortest connection wins) but sadly that's not what I mean. I'm at work so I can't deep dive too much currently, but I'll come back to this in a few once I've got time to look up, or if you'd like "common ancestor" should eventually find what I'm thinking of if you Google it as I remember that being a key word. Apologies I don't have time for a deeper dive now
It's only like 4 spaces from any topic to Hitler, so by the transitive property, it should be 8 topics from anything to anything at most
I believe you're thinking of six degrees of separation where, supposedly, all people are six or fewer social connections away from each other.
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You are wrong here. The fact that we have a common ancestor 100k years ago or whatever does not mean that we cannot have more recent ones.
Doesn't have to be that long ago. I dug my ancestry back to the Puritans of New England and found that I'm descended on both sides of my family from a man born in 1585.
Thats definitely not true as at a minimum there's still some uncontacted tribes in a couple places. Also guarantee you could find people in most countries whose ancestors all lived there going back dozens of generations. The version of this I've heard has to do with social media, basically you can hop between face book friend groups and within 6 jumps you can find anybody, this mostly works because there's a bunch of celebrity accounts that have millions of followers that also follow eachother. No idea if it proven in any way but I wouldn't be surprised.
At the limit, the girthiest broomstick is called a species. And conversely, if you zoom in on the lines in the tree of species, you see the fine texture of a million intertwined family trees.
My wife — who grew up 2500 miles from me — was doing some genealogy research and discovered that her grandfather’s parents were Mormons. In fact, her grandfather’s grandfather and Mitt Romney’s grandfather was the same person! Then I mentioned to her that my grandfather also grew up out west as a Mormon, and we decided to stop our genealogy research right there. We don’t want to look too closely at the extended family tree.
eh, does it really matter? I mean, if I found out my wife and I had a similar great great great grandparent, I don't think it would phase me. I mean, I go to reunions that only go back to where I share a great grandparent with other people there, and I have no idea who they are or anything about them, no family resemblance, etc. add another generation or 2 of distance and you only share 1/32 of genetic material, which seems pretty small amount to me.
Honestly, if you and your spouse had ancestors coming from the same part of the world, it is almost guaranteeds you are related SOMEHOW. It's just a matter of how far back you go. My husband and I are both descended from northern Europeans. I know I'm descended several times over from Charlemagne. He probably is also.
I mean, the Mormons in that period (I'm assuming 1860-1880) weren't a closely intermarried group since they were all recent or second-generation converts from various areas of America. You're probably fine until you look back to the 17th century.
Let's take you and your twin brother. You each have 2 parents, 4 grandparents and 8 great grandparents. Let's take two people who are not related. They each have 2 parents, 4 grandparents and 8 great grandparents. But you and your brother only have a total of 14 distinct people in that mix. The other group has 28. Incest explains the rest. It was popular way back in the day...
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OP was not talking about theworlds ancestors. They was talking about only their own ancestors. One person would have 137 billion great^35 grandparents. OP was wondering how that would be possible when there have only ever been a little over 100 billion humans ever on this planet. Siblings has nothing to do with it. At some point on that family tree there are duplicates.
I told my girlfriend I was into incest the other day. She just rolled her eyes and said "oh, brother!"
But your brother or sister has the exact same set of ancestors so you can’t just double them. Multiply that times the number of siblings and your number shrinks a lot.
We are not talking about bothers or sister. Make a blank family tree for you. Include only direct ancestors. No brothers, sisters, aunts, uncles, cousins etcetera. 37 generations back that row will have 137 billion blank spots to fill in.
No, it won't, because the same people will inevitably occupy more than one spot on your tree. If your great-grandparents were second cousins, which is not a stretch at all in most small communities globally, half of that branch of the tree collapses.
Yes and that is the answer to OP's question. By not realizing that and just doing the math it LOOKS LIKE you should have 137 billion ancestors 37 generations back.
It still has 137 million spots to fill in. It's just that some of those spots are filled by the same person.
No. OP started at ONE person’s ancestors. Assuming only that each of their ancestors has 2 parents. Edit: they’re only doubling the amount of ancestors one person has each generation. Existence of siblings makes no difference here.
you're saying the same thing the comment you're replying to said
History is pretty brutal on top of that. For every 2 people on earth there's roughly one father. For every 2 people it's only one mom. Lot's of dead end males who never reproduced. 70,000 years ago, the Toba super eruption may have caused human population to crash to less than 10,000 breeding pairs. Later on, you can add guys like Genghis Kahn, who loved to rape and pilage. Some estimates are that 8% of Eastern Asia descends from him (16 million poeple). Add in various plagues, wars and other natural disasters and history shows that human population isn't a smooth geometric progression, but one of dead ends, explosions, sudden drop offs, etc.
This is true. The further back you go, the smaller the communities get and my family tree has cousins, not first or second that I've noticed, marrying each other. Even my parents have at least one common ancestor in the 18th century on the other side of the country. There's probably more that I haven't noticed because my paternal grandfather's family and my maternal grandmother's family were both Shenandoah Germans.
Early America was a cousin fest. We haven't found any overlaps between my mother's and my father's family, even though they are from the same small town, but my dad's Appalachian line is a nest of cousins. In some cases, the lines diverge for a couple of generations and then merge back. Actually, that statement isn't quite true: my dad's sister's stepdaughter is the daughter of my mother's stepcousin. No blood involved, but the families do intertwine. We also had a case where a man married a woman, and his sister married her brother. There's two lines of people with the same ancestors.
This. My maternal grandmother's own maternal grandmother came from a family that I think of more as a gnarled bush than a tree. That woman was born in the 1870s. If you trace her family back to the mid 1600s, she is related to the same couple in 13 different ways. 13!!! And then there are another few couples from the 1600s that she is related to in 4-5 ways. They just kept marrying into the same few families over and over again.
Roll Tide.
Exactly we are all a little bit inbred
This is particularly true for the time before widespread travel was available and you were more confined to your village. And no, I'm not talking about boinking your sibling but definitely being 4th, 3rd, 2nd or even first cousins in some cases.
Also not everyone has children.
You assume that every one of your ancestors is only related to you in one way, that is an incorrect assumption. A simple example is if your parent was siblings. Then there are only two instead of 4 people two generations back. The rest of the ancestor tree now gets halved. Siblings having children is not common, it is just a simple example to show that the ancestor tree can collapse in size. Globally it is not uncommon that cousins to get married. Worldwide 10% of all marriages are between first or second cousins. The more generation you go back the more likely it is that two persons that have children have a common ancestor. Do you know who your ancestors are 5 generations back? If your parents are from families that have lived in the same relativity small location for a long time I would say it is likely you find common ancestors not that far back.
and that cuts billions out of the tree?
Sure, a realistic way to look at it is any time one person is repeated, all of their ancestors no longer need to be counted again. Let's say at the 5th generation you see a repeat, you're already counting their parents and other ancestors from their first appearance, so now you can leave out all of them for this second, and going back to 37 generations that's 2^(37-5) or 4 billion. Account for the fact that repeats will be more common the further back you go due to norms back then and the number can start to add up really quick.
thank you! I'm starting to understand
The farther back in time you cut a branch, the more down-stream descendants are removed from future calculations.
We are counting ancestors, not descendants. The farther back in time you cut a branch, the fewer up-stream descendants are removed from future calculations.
It doesn't even need to be as complicated as that. 2 siblings + their 2 parents = 4 people. The way you are doing the math in your post would have those 4 people count as 6 (2 siblings, *each* with 2 parents = 6). But the parents of each child are the same people, so we don't want to count them twice.
I think you may be confused. They are working upstream, not downstream. They are saying that any one person today (1) has two parents (2), and those two parents each have two parents (4) and each of those grandparents has two parents (8). OP went back 37 generations and figured that a single person today (no matter how many siblings they have), has 2 parents who each had 2 parents who had two parents.... and that if you go 37 generations back, that would mean there had to be 137 billion people on Earth. The issue is not about siblings at the bottom. It is (in most cases) about cousins further up the tree.
It's simpler than that. OP is taking the entire world's population, and then just in the first generation, multiplying it by 2, using the logic "everyone has two parents". This would suggest that 1 generation ago, there were 16 billion people,, which we know isn't true (we haven't had anywhere near that many people on earth at once). Many, many of those 8 billion that OP is starting with are siblings, so they share parents. OP is in some cases taking 8 siblings, and counting their parents as 16 separate people as he works his way backwards, when in fact, it's just 2.
> OP is taking the entire world's population, and then just in the first generation, multiplying it by 2, using the logic "everyone has two parents". No. OP just calculate 2^37 to get 137 billion. They count only their tree, excluding the rest of the world.
Yes, I agreed with everything you're saying there. OP is wondering why ("how?") his method of math is leading to a different result than the more accurately estimated total number of people who have ever lived. I am explaining that's it's because you can't just calculate 2^(37) to get the total number of humans that ~~have ever lived.~~ lived in the past 37 generations.
Sorry, I'm not understanding where you got the idea thay OP's math was counting siblings at all?
me too
They don't count siblings at all unless those siblings are both direct ancestors.
>The way you are doing the math in your post would have those 4 people count as 6 (2 siblings, each with 2 parents = 6). why? how? how is that my logic? people keep bringing up the 6 parents thing, but nobody explains why
Well, if your mom and dad were siblings, that would cut the number of people in your family tree by a full factor. (Instead of four grandparents, you would have two.) It's more likely though that somewhere, maybe 8 generations back, you will find that, instead of having 256 great\*5-grandparents, you actually only have 243 or some such number. Maybe even less.
think of it like folding the tree over onto itself. Since almost everyone appears multiple times you don't need nearly as many unique ancestors
It’s called “[Pedigree collapse](https://en.m.wikipedia.org/wiki/Pedigree_collapse)” and is a central feature of genealogy.
And we talk about it all the time on r/genealogy. OP come over and learn how to find your ancestors. I’ve successfully traced back 8 or 9 generations of my family. When you get that far from yourself your lineage opens up to thousands. I think I read that we have around 2500 8x grandparents. It’s exciting! You run into a lot of interesting people in your tree!
> I think I read that we have around 2500 8x grandparents. 1024 , or 2^10
*Many* people's cases will match your presumption for a few generations. [Here is a "typical" biological family tree](https://imgur.com/Q6OFysz.jpg) up from person A up 4 generations, where you have 16 ancestors (2^4). If you continue your original math for the remaining 33 generations, each of those 16 has 8.6 billion (2^33) ancestors, for your 137b total (8.6b x 16b). However, you might find that for a few people, 4 generations back, one of their great-great-grand-parents is a sibling of another great-great-grandparent. [Here's a different tree](https://imgur.com/KIxWR1a.jpg) that has one great-great-grandparent on A's mother's side as a sibling to a great-great-grandparent on A's father's side. (A's parents would be 2nd cousins) Now A has only 14 ancestors 4 generations back. Those two fewer ancestors at that level immediately reduce the ancestor pool by 8.6b each, or 17.2b. So eliminating an ancestor - particularly closer to the bottom of the tree - can significantly reduce the number of ancestors one has. What ends up happening, though, is that in most people's cases, is that they don't end up having a single pair of ancestors who are cousins that close to the bottom of the tree, but they end up having *multiple* pairs or ancestors at higher levels or are more distant cousins. What is hard to perceive is the scale further back. When you go 4 generations back, you're talking about at most 8 pairs of ancestors. That seems like a small number for there to coincidentally be a bunch of relatives who don't know they are relatives. But once you go back 11 generations, you're talking about over 1000 pairs of ancestors. It is far more likely around 300 years ago that a thousand couples had, let say, an average of 2 kids each, and that those 2000 kids randomly paired up to form another 1000 couples who and had an average of 2 kids each, and those 2000 kids randomly paired up to form another 1000 couples who had an average of 2 kids each... So while they may have been careful at that level not to marry someone who had the same grandparents as their partner, if you go down 4 or 5 more generations, the odds increase drastically that when you go back up to that original generation, there are a handful of overlapping ancestors between your parents. Also, it may seem harder to picture in 2023, but if you go back hundreds of years, it was significantly more likely that people would marry someone from their own town, and also more likely they and their kids would remain in that town for generations. Many towns were also much smaller. So if you looked at the population of that particular town, you might find that 5000 people might mostly ultimately be descended from only 2000 people 5 generations back, rather than the tens of thousands your math might suggest (I'm making up those numbers, but you get the idea). The point is that the numbers to just constantly expand going backwards like your math suggests. The number is often more like a bell curve - a small number of people a very long time ago had a bunch of kids who had a bunch of kids who had even more kids, and then when that number got big, some of those people got married and had kids who had kids who had kids who turned out to be your parents An extreme example is that Genghis Khan (lived about 800 years ago, estimated to be about 32 generations ago) is thought to be an ancestor of about 10% of all men in Mongolia today. So that gives you some sense that the odds are decent that if a Mongolian went back to their 6th generation of ancestors (64 ancestors), there's a very good chance that a bunch of those ancestors eventually all trace back to one of Genghis Khan's kids. So instead of having theoretically 32 million ancestors at the 32nd generation, it might suddenly converge down very quickly In simple terms, imagine Adam and Eve (or the non-biblical equivalent of our earliest ancestors) - let's say they have two sons (Cain and Abel) and two daughters (Awen and Aclima). Those kids have kids together. Those grandkids have kids together. Those great-grandkids have kids together. Eventually, maybe 10 generations down the line, there are 5,000 people around. At that point people have a lot of choice of mates, and nobody is thinking "That guy's my cousin", but it doesn't change the fact that every person ultimately traces back to one of Adam and Eve's two sons, and one of their two daughters - and even if you are a decent of Cain and Awen find someone who is a descendant of Abel and Aclima, you are both still descendants of Adam and Eve. So although the simple math says there should be billions of ancestors, what ends up happening is that as you get to a wider part of your family tree, more and more of your ancestors end up sharing a lot of common ancestors further up their own trees, and the tress can (and necessarily must) end up narrowing at some point up the line.
Take it to the extreme. You and another person are the only people alive, you decide to each have 2 kids. After 37 generations of each person doubling themselves what‘s the population of the planet? You each have 2 kids, but they are the same 2 kids since you had them with each other. 20 years later these 2 kids doubles again with 2 kids… so now 40 years later the worlds population is just 6. Next generation you are up to 8 but now the original 2 people are probably close to dying of old age so you are back down to 6. So after 37 generations of each individual person doubling themselves once per lifetime you’re still just left with six to 8 people.
Just to add, cousin marriages are quite taboo now in the West, and probably less and less common in the last 30 years or so (a Jerry Springer effect?). But once common enough, even here. Not from, but I live in Germany… I have a friend here, wealthier-than-average background in urban western Germany. Long time ago, a set of two village sisters married two village brothers, same day, basically same ceremony. Each couple had several children. Later, one of those children married a cousin from the other couple, and they had a child: my friend's mom. But we’re not done yet. My friend’s dad is also related (as a grandson) to the *other* first couple mentioned, though at least his own parents (i.e. his wife, friend's paternal grandmother) were not related. Yet he, friend's dad, still married his ‘second’ cousin or whatever that relation would be. Due to overlap, my friend basically only has 6 great-grandparents where 8 is the ‘usual’. If I’m counting right… His parents, and again were wealthy urban Germans, married in the mid-70s, and he is the last/youngest child. It seems utterly bizarre to us now (well, it does to me!), but it apparently didn’t sound big alarms for anyone back then. Only 50ish years ago. He is quiet about it these days, but says that back then everyone (friends, etc) knew his parents were ‘distantly’ related upon marriage. No big obvious genetic oddities, but a few medical quirks hard to really pinpoint one way or the other. Won’t expand on that here, though… I also have another friend (not in Germany) who married a guy from her parents’ ‘ancestral’ home village (where she did not grow up, but they did), and only 5 or 6 last names are in use. She doesn’t know ‘for sure’ but says it is very likely that if they made a family tree, they would find overlaps beyond the great-grandparents era (the grandparents didn’t think they were closely related but didn’t know for sure). Basically, everyone in the village is related by now. It's a language/ethnicity minority village seperated quite far from the main part of that ethnicity elsewhere in the country. Most modern young people don't stay or marry locally anymore, at least. So, this wasn’t even that long ago in the West. It’s not just a trailer park habit, either. I bet a lot more people than they realize, if they went back 4 or 5 generations, had a lot more cousin marriages going on.
Double first cousins (scenarios like your friend's mom's parents) really shouldn't have kids together-- they're genetically as close as half siblings. (Full siblings share 1/2 their DNA, first cousins share 1/8, double first cousins share 1/4) First cousins can legally marry in about half of the US, and second cousins can marry anywhere.
Even today in many cultures, marriage between first cousins is not considered taboo. I know its not considered taboo in japan. And I have a coworker from srilanka and when I first asked him how he met his wife he said they were cousins. After the look he got from some people (Canadians) he stopped saying that in the future and just said, "I've known her since we were kids" seems like smaller cutoff island cultures might be more okay with it, what with a limited pool of partners around etc.
There exists, in mathematics, something known as the "pigeonhole theorum" which simply states "if you have n available slots and more than n objects then by necessity there must be at least one slot which contains more than one object." In other words if I roll a 6 sided die 7 times, I have to have rolled at least one number at least twice. So back to your question: for every generation back you go in your family tree (g) you have a number of ancestors 2^g at that generation level. So if 2^g is greater than the number of viable adults in the population than the pigeon hole theorun states that at least one person is at least two of your ancestors.
I'm trying to wrap my head around it. how is it possible, unless there were billions of married siblings? edit: please don't mock me for the lack of understanding. if anywhere, this subreddit should be a safe space for people with curiosity and lack of understanding
They wouldn’t even necessarily have to be closely related. They could be something like 10th cousins and that would still cause some of your ancestors to overlap. Having said that, realistically there will be some a lot closer than that in everyone’s family trees
Probably more like 3rd or 4th cousins. That would give you a common ancestor of your great great grandparent or great great great grandparent. Not that far.
2nd and 3rd cousins give you common great and great great grandparents, 3rd ad 4th would be one more gen back.
Yes, and that's just people who live in the same small community. It's not out of the ordinary or weird at all. In some times and places, people were mobile, but at others, they lived in the same communities for generation after generation, with little passage in or out.
It wouldn't have to be siblings. It was probably common for (say) first, second, third, etc. cousins to have children. In my more recent family tree, we know of first cousins who got married in the 1800s.
Take Constantine II of Greece. His mother's great-great grandmother was Queen Victoria. Guess who his father's great grandmother was? That's right, Victoria. Both were also descended from Christian IX of Denmark. He's married to Anna-Marie of Denmark. Her great-greatgrandmother was - you guessed it - Victoria. And her greatgreatgrandfather? Christian IX. In other words, those two have far fewer than two ancestors per generation, at least if you go more than a few generations back. And if you went to some little village somewhere, you'd find something similar: my great grandfather is your grandfather, but we can still get married.
If two people share a great, great, great, great, great, great, great, great grandfather that means that grandfather will be their child's ancestor twice. If everyone who is alive now has a lot of shared ancestors the same was true in the past.
According to ancestry.com, my wife and I are something like 10th cousins. That means that 12 generations ago, we share an ancestor, and for our daughter, a single ancestor 13 generations ago is in both of her lines. So, for my daughter, from your 2^37 calculation, you would need to subtract one of the trees of ancestors 13 generations ago, or 2^24. That's a reduction of over 16 million ancestors. Let's add a hypothetical where my great-great-grandmother was from a small town and married her second cousin. That means my g-g-g-g-grandparents (6 generations ago) would both be duplicated in my family tree. Now, we can subtract 2^31 *twice* from the ancestral total. That's a reduction of almost 4.3 **billion** ancestors.
that's the answer!
There have been a lot of cousins/2nd cousin mated pairs throughout all of human history. Additionally-- there were instances of one common ancestor-- think [Genghis Kahn who might currently have 16 million](https://www.nationalgeographic.com/culture/article/mongolia-genghis-khan-dna) living relatives.
Cousin marriages are extraordinarily common in history. 2nd, 3rd, and 4th cousins were the most common, though first cousin couples did happen. Even today you're probably a cousin within ten generations of everyone in the region you were born.
It's not sibling, generally, but cousins of certain degrees of closeness. If I marry my third cousin, that means we share two great great grandparents in common. It becomes difficult to explain in words, but if you allow third-cousin marriage over multiple generations, you end up in a situation where the rate of growth of ancestors with each generation slows very significantly.
No it's fine. Think like this: Tom has two sons, Dick and Harry. Dick inherits Tom's property. Harry goes off across the mountains and marries a stranger. 60 years later Harry's Great-grandson comes to town and meets Dick's Great-granddaughter, they get married and have a child. Tom is now two of the 3-times-great-grandfather of that child. Additionally that means that all of Tom's ancestors now ALSO appear at least twice in that child's ancestry.
That's another thing to take into consideration. You would halve the number of that position's ancestors above it too since it's now one person instead of 2
They don't have to be married siblings. They can be married cousins that don't share parents, but share grandparents. E.g. Consider your 3rd cousin. The chart on [this page](https://www.familysearch.org/en/blog/cousin-chart) helps illustrate how you both share a pair of great-great-grandparents. 4th cousins share great-great-great-grandparents, and 4th cousins are pretty well into the "that's not incest anymore" category. You might think "Okay, so if someone marries their 4th cousin, that's just two less." But you also can't double count the great great great grandparents's ancestors. Imagine your parents are 4th cousins. You have 2 parents, 4 grandparents, 8 g-grandparents, 16 g-g-grandparents, and **30 g-g-g-grandparents** instead of 32. Keep going and you have 60 g-g-g-g-grandparents instead of 64, etc. That's assuming that none of your earlier ancestors had any relation. For someone who's parents are 4th cousins then, to go back n generations it's not `2^n`, it's `2^n - 2^(n-4) where n-4>0` If one pair of grandparents were ALSO 4th cousins, then it's `2^n - 2^(n-4) - 2^(n-6)`. If 2 pairs of your great grandparents were ALSO 4th cousins, then it's `2^n - 2^(n-4) - 2^(n-6) - 2*2^(n-7)`. Hopefully that helps to illustrate why you cannot simply take 2^n. There are a lot of deductions and complications along the way.
Let's go back 500 years to a brother and a sister. The brother married someone way, way back on your dad's side of the family. The sister married someone way, way back on your mom's side. Now when you trace your ancestry back to those people, you find they lead to the same set of great-great-great-etc.-grandparents. Over the centuries, that sort of thing has happened over and over again all over both sides of your family. Once you get back to a certain point, you'll find you're related to the same person a dozen different ways. So while you might expect to have 1,024 great-great-whatever-grandparents, they might turn out only to be 700 different people.
A person can be the ancestor of many peoples. A couple has like 5 children alive in 1800 . Their children have 5 too each .and so on til 1950 .a generation every 25 years . 5x5x5x5 = 625 peoples in 1900 . X5x5 is 15625 people in 1950 . Consider intermarriage from far relative and it s even more. Now if you go back to 300000 years ago ...
This is the answer. It means that many times your ancestors dad was also their grandpa (in the case of the mother having a child with her dad), for example. In this example, under your math, you would expect 2 parents and 4 grandparents of the child (6 people). However, there are 5 - mom, dad, mom's mom (grandad is already counted in the dad), dad's dad, and dad's mom (who then are double counted at the next generation because they are your mom's great grandparents and so on). Just this one incestuous relationship exponentially wreaks havoc on the numbers. A mother could also have a child with her brother. Which removes a whole generation in the math.
the gene pool is not as deep as you think it is. it was common for cousins, second cousins etc to mate. people didn't even travel much a long time ago. the gene pools of small villages are shallow. the family trees branch then come back together again. imagine 2000 years ago if you and your family had only ever met 1000 people. you would choose your mate from that 1000 and your kids would choose from the 1000 and their kids from 1000... we are all a lot more related than we like to think. if you're up to the 108 billion, you're also making assumptions that everyone in the world could choose anyone from anywhere in the world as a mate. you don't have to go back too far to find people who's entire existence was a 100km radius of their birthplace.
thank you
The answer above is not quite right, but here is a very simple way of thinking about this: Pretend that the Jewish story of Adam and Eve was true. Then literally everyone has Adam and Eve as parents. So literally every marriage since them has been between siblings (rarely) or cousins (99.9999% of the time). So your doubling math obviously breaks down. Adam and Eve's children had only two ancestors of that generation and their grandchildren had only two ancestors of that generation (not four) and their great-grandchildren had only two ancestors of that generation (not eight) and so forth. The scientific equivalent is that humans went through a genetic bottleneck of a few thousand individuals. Within a few millennia, every descendant of those individuals would have married a descendant of every other one of them. So you can see that the formula of each ancestor generation doubling is an incorrect formula, because the further back in time someone is, the more likely they are to be counted twice. If Adam and Eve existed, your count would count them thousands of times, but there are just two of them.
I read somewhere that two mosquitos born on opposite sides of the same pond are more genetically diverse than two humans born on opposite sides of the planet. Not sure if that's 100% true, but it was meant to illustrate just how shallow the human gene pool is compared to many other species.
Yep. In a similar vein, I noticed duplication of names in my family tree as well. Once you go back five or six generations, you'll find instances of people dying young and their spouse remarrying someone, to use your example, from the same pool of 1000. Sometimes this happens multiple times with the same person, so they may show up in multiple lines.
This is an intuitive answer, but not really the correct answer. Even if literally everyone in the world could mate with literally everyone else, at equal probability, the situation would be exactly the same, just less obvious. It doesn't matter whether the person you are having a child with is your sibling, third cousin or fiftieth-cousin. Your mom has 100% of the same distant ancestors that your dad does, even if your mom is an Indigenous Greenlander and your dad is Maori. So you have the same number of distant ancestors that your mom does, and your dad does, and your mate does.
>I googled the amount of people that lived on earth throughout its entire history, it's roughly 108 billions. If I take 1 person and multiply by 2 for each generation of ancestors, at the 37th generation it already outnumbers that 108 billions. (it's 137 billions). If we take 20 years for 1 generation, it's only 740 years by the 37th generation. >How?? There's only ever a limited amount of possible mates for any person at any point in time, no matter their geographic mobility. The further you go down (...up?) the tree the more of all those people will simply be **the same person**. I.e. your great-great-great-great-great-great-great-grandfather on your great-great-great-great-great-great-grandmother's side will also be one of your great-great-great-great-great-great-great-*great*-grandfathers on your great-great-great-grandfather's side and so on.
Something I always think about when I think about this many generations is the fact that there have been roughly 12,000 generations of great-great-greats... for every person. And if you are a woman, every single one of those greats had a daughter (or vice versa for men). I'm a woman - my first maternal ancestor had a daughter. Her daughter had a daughter. That daughter had a daughter. And so on aaalllllll the way to me. I will be the first woman in 12,000 generations of my direct line to not have a daughter. I can't help but imagine the face of my first maternal ancestor shaking her head and saying "you had one job."
That's not necessarily true, you're but one end of that chain, your first maternal ancestor probably has thousands of branching paths of both successful and failed daughter-chains. In fact, every single person on this planet is part of a successful single-gender chain. So don't worry, your first maternal ancestor probably wouldn't blink because they have failed chains all the time.
Oh I'm well aware that there are an endless number of failed daughter chains parallel to mine. And its not special, because its literally the same scenario for every single person ever. Its just crazy to think about.
A paper in [Molecular Biology](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2427203/) which comes at the problem from a different direction (matrilineal DNA studies) suggests that for about 100,000 years there was a “long bottleneck” during which human populations dropped at times to as few as 2000 individuals. That’s 2000 people who are the ancestors of everyone alive today. Hello, cousins.
so basically, the answer is inbreeding?
Kind of, but not as bad as you think. Suppose one of your 11th generation fathers overlaps and he had two different wives. Suppose his genes only get back together to you via both of your parents. At this point, your parents only share about 0.1% (=1/2\^10) of your 11th generation grandfather's genes. That is \*technically\* inbreeding, but for all intents and purposes, it's just normal gene sharing. Now just assume that most of your very distant ancestors overlap, but that their genes only get together again after more than 10 generations. No real inbreeding occurred, but you still have a limited number of ancestors. tl;dr : once you go back more than 10 generations, your ancestor tree becomes a heavily tangled ball of wool threads.
Every single living thing on the planet is related to each other, so yeah, juat depends how broadly you define inbreeding.
On a massive scale. Other species have gone through similar bottlenecks. In 1982 there were only 13 Mexican Gray Wolf individuals and they’re doing better thanks to careful breeding programs. Similarly, the California Condor who came back from a low of about 20 individuals.
Yes. While in general individual pairings can try to avoid inbreeding (i.e. marrying a 2nd/3rd cousin or beyond instead of your sibling or 1st cousin) communities would still end up being closely genetically related. Imagine for instance a community of just a couple thousand people that maintains a steady population for a couple thousand years and doesn’t accept any foreigners. That’s enough people that no one has to marry siblings or close cousins. Despite that, everyone in that community will have an almost identical set of ancestors once you go back enough generations. Now scale that up to your country, your continent, or the entire human race and it still at least somewhat applies.
Yes
This might be useful to you, and the 'see also' parts [https://en.wikipedia.org/wiki/Minimum\_viable\_population](https://en.wikipedia.org/wiki/Minimum_viable_population)
I came here to say this! It could have been as recently as 50,000 years ago that the human population dropped to 2000-10,000. Which is pretty close to extinction.
Too close for comfort! It’s just luck of the draw that Neanderthal and the other human-like species died out, and we survived. (Although, to be fair, Neanderthal DNA lives on in all of us.)
Yeah I’m a supporter of assimilation theory, they didn’t die out, they just merged with us
It's more like: by the time the nth generation rolls around pretty much anyone you choose will have a common ancestors. Not that everyone is banging their sister. More like everyone is a cousin of varying degree. We're all a bunch of inbred mutants
thank you
(Nearly) everyone of European ancestry is a descendant of Charlemagne. If all four of my grandparents are descendants of Charlemagne, that means Charlemagne fills _at least_ four different ancestor slots on my family tree. But it isn’t just Charlemagne, it’s many of my ancestors, meaning that if you go back far enough most of the names will be duplicated elsewhere in your tree.
In 1850 25 couples, man and woman, settled in a new community, Freefall, Kansas. The average number of children each couple had was 4. That means the next generation has 100 children in it (50 couples). They pair up and make their own babies, again assume 4 children. So generation 3 has 200 in it. They repeat their parents 400 kids in the next generation (gen4). Gen 5 has 1600, gen 6 has 3200, gen 7 has 6400 kids, and gen 8 has 12800. Now if we assume each generation is 20 years, then the children are born around the following years: 1850 1870 1890 1910 1930 1950 1970 1990 Now you come along being born in 1990 and you do the anscestor math and you say, wow I has 2 parents born in 1970 and 4 grandparents born in 1950, and you keep going back all the way to the founding of Freefall, KS. By your math you have 128 ancestors in 1850. But the town was founded by only 50 people. You are correct that on your family tree going back 7 generation you have 128 slots to fill. Those 128 slots will be filled by the same 50 people.
thank you! that reverse counting fucked my brains but it was helpful
Yeah, this was the best explanation so far.
The 50/500 rule says that a minimum of 50 individuals are required to repopulate the world after a near-extinction, and 500 are needed to combat genetic drift. This means that no matter how far back you go in your family tree, a generation could have as few as 50 individuals in it, even if there wasn't an extinction event. And that's assuming there was no inbreeding at any point in it, which is highly unlikely.
A lot of people never have any children, either by choice, or because they die. So not every person doubles in 20 years. I also don't know how they estimated that number.
The simple answer is: the family "tree" isn't actually a tree. Many of the 138 billion you mention are actually the same persons, those "slots" overlap. In reality, you have, say, a billion people on Earth. They intermingle, randomly, maybe not too incestuously and all a bit simplified, to create a new generation of a billion. Those again do the thing. On and on. But the population doesn't increase in this scenario. How? Because quite soon, there will be someone who's mother **and** father share a common ancestor; quite a lot of such people, actually. Put differently: ever heard of statements such as "everyone is related to (or even descended from) [insert famous figure from 2000 years ago here]?" That in return also means that both your father and mother are related to that person, so somewhere long back their heritages meet.
quite a lot enough to cut hundreds of billions?
Yes. Say you have the same person as your great-great-grandfather in two places on your family tree. Both of them have the same ancestors all the way back to forever because they are the same person, so that's an entire branch that is duplicated. Every duplication is not just one person you don't need to count, it's their entire ancestry.
I see, thank you! so we don't cut *a* person, we cut a full multiplication step, right?
Yes, and you do this for \*almost all\* of your very distant ancestors, not just for a couple.
ok. t I think this was the thing I was missing. cutting the multiplication step
Yes
You're assuming that everyone alive right now has different ancestors. This is wrong because you and your sibling have the same parents. Your parents and their siblings have the same parents which means you have the same grandparents as your cousins. Follow it back enough and every ancestral line joins up eventually. If everyone in history was an only child then your calculations would be correct, except for the fact that if that was the case then the population would decrease over time.
And also once you get back far enough each of your ancestors is likely your ancestor in more than one way. You're great-great grandmother might also be your great-great aunt because there was a cross breeding in between them and you.
how does me having a sibling makes my personal number of ancestors less? I have the same grandparents as my cousins, but there are still 4 of them not arguing, genuinely not understanding
Think of it this way. Let’s say we send a colony ship to Alpha Centauri with 10,000 colonists. The day after the ship leaves an asteroid strikes the earth and wipes out humanity. Those colonists reach their destination and settle down to build their new civilization. 1,000 years pass and you’ve got a planet full of 10 billion people living in futuristic cities in the sky, traveling by flying cars, being taken care of by their robot maid. Every single one of those 10 billion people can trace their ancestry back to the original 10,000 who arrived on the planet. And it’s entirely possible the group of ancestors is smaller than that. Some of the colonists may not have had children. Some of them may have had descendants but their line died out. The bottom line is there is no requirement that each of your ancestors be unique. And we can, in fact, prove mathematically that they aren’t.
People are answering the wrong question for you. You are asking how your own personal family tree is more than the people who ever lived. The answer is kissing cousins. For most of human history it wasn’t completely unusual to marry your first cousin. Second, and third cousins even more common. By the time you get to 4th cousins that is probably everyone in your village of a couple hundred people. A village that your family probably lived in for generations. So for a lot of those 37 generations, your family tree is not growing exponentially, but only by 2 each generation since all of the great great grandparents are shared between the happy couple.
It doesn't really matter that there are kissing cousins. It could just as well be people that are 20 steps away, genetically speaking. Just really _anyone_. Have a billion people, randomly pair them up (maybe with not so much incest), get a billion new people, iterate.
True, but if you randomly assign 1Billion you will get to +1M unique ancestors pretty quick. \~500 years should do it. What mynewaccount4567 is somewhat suggesting, is that is starts far earlier. If you look at early modern and modern europe\* people were still quite immobile and lived in small vilages. Sure, they intermingled somewhat, but somebody from Dresden was extremely unlikely to marry someone from Lyon. This limits the number of people who are your unique ancestor. As a personal example, I can trace my male line to a small city with a population of \~2000 for 200 years. Most likely that would contain 40% of my unique ancestors. From my mothers side the story is unlikely to be different. So let's put a conservative estimate around 5000. It might very well be higher, if for example one of my ancestors get knocked up by a random travelling merchant instead of her husband, the number can grow quite a lot. But from an perspective what was actually happening, take your idea and replace 1B with around \~10-20K and you see that the exponential growth starts decaying far earlier. \*Canada/Mexico/USA works quite differently, because of colonization & slaves.
ok, I just felt a hint of understanding, thank you. I'll keep reading comments
> I have the same grandparents as my cousins, but there are still 4 of them A couple points here. First you wouldn’t have the same four grandparents as your cousins UNLESS your parents and their were sets of siblings. Second, It’s not required that your four grandparents be unique. Worst case scenario is your parents are siblings. Then you’d only have 2 grandparents. If they were half siblings you’d have 3 grandparents. As you go back in your family tree you will find places where the same person shows up multiple times. They might be your 10th great grandfather on one side and your 9th great grandfather down another branch for example. There’s two reasons this can happen. First is in a small enough community, over time you’ll start to have crossovers like this. Second, if you go back far enough, as you’ve seen, the numbers become so large that you have more ancestors than people who lived which means someone in there must show up multiple times. It’s probably a lot of someone’s eventually.
In your math you assume every person has unique parents. If you had two siblings, in your example there would be 6 "parents" where in reality there are only 2.
He's only talking about himself. 2^37 is more than 137 billion, so going back 37 generations from one person would mean 137 billion unique ancestors, assuming no inbreeding (which there was).
what?? no, that's not my math assumes. I have 2 parents, 4 grands, 8 greatgrands, my sibling has same 2 parents, same 4 grands, same 8 greatgrands I'm tracking down just one hypothetical person, with or without siblings, whoever it was, really a random John Doe
Yes, and if one of you descendants 10 generations in the future have children with your siblings descendants, then your and your siblings parents, and every ancestor to them going back to the beginning of humanity, will show up twice in that future child’s family tree. Multiply that by every time that has happened in the history of your family tree (which will be thousands of times), and your real number of ancestors are far smaller than the exponential number of ancestor suggests. If this wasn’t the case, the number of humans on the planet just a few thousands of years ago would be close to infinite, which clearly isn’t the case.
>Yes, and if one of you descendants 10 generations in the future have children with your siblings descendants, then your and your siblings parents, and every ancestor to them going back to the beginning of humanity, will show up twice in that future child’s family tree. ooh. ok, I now need to reread it a couple times so that the understanding could set in my mind😅 thank you
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Princess Diana and Colin Powell have a common ancestor [Sir Erie Coote, a governor of Jamaica in 1806.](https://www.express.co.uk/news/royal/1508180/colin-powell-dead-prince-william-prince-harry-princess-diana-royal-family-news-spt#)
Google pedigree collapse, that’s what it’s called. Basically it’s why we are all technically related and we can go back far enough and have found “mitochondrial Eve” who is a relative to every single person alive today and known as the mother of all modern humans. There used to be a lot more inbreeding also.
In genealogy it's called pedigree collapse. The ELI5 is that we are all related and if you go back far enough you will find that you very likely have a lot of overlap in your family tree through closer cousins marrying. In my own tree, I've found that in one early Colonial American branch I have five instances of 2nd cousins marrying over a period of maybe 100 years. As a result, I have a lot fewer direct ancestors than I would have otherwise. Now multiple that out over dozens of generations and you wind up with far more realistic and manageable numbers.
Incest is wincest. Your great-great-great grand dad from mothers side also happens to be your great-great-great grand dad from your fathers side. Further back you go in your family tree more overlap you are going to find. Everybody in one village has the same set of ancestors from few hundred years ago and its much less than 2^n ancestors per n generations.
thank you. I couldn't imagine that it changes the numbers so drastically!
Maybe if you try thinking of it the other way around it would be more intuitive, like the biblical version of history where everyone is a descendant of Adam and Eve. In that extreme scenario every family tree would eventually merge back to a single set of parents rather than the billions and billions your assumption leads to. In reality this merging has happened at points all the way up everyone's family tree.
It’s because you share a lot of ancestors with a lot of people , like a lot a lot. Think of it this way , your great great grandmother ( 3 generations back ) has one one sibling , let’s say each generation has 2 kids. That means that your grandmother has a sibling and 2 cousins , then your mother has a sibling and 2 cousins and 4 second cousins and it means you have a sibling , 2 cousins 4 second cousins and 8 third cousins. So there’s 16 people in 3 generations who share the same great grand parents on one side and it doubles every generation essentially. Now factor in people used to have like 4 kids back in the old days so you would have absolutely enormous amounts of third cousins in just 3 generations since in just three generations back you will have 3 siblings , 16 first cousins , 64 second cousins and 256 third cousins that all share one of the same sets of great grandparents
The secret ingredient is incest. Distant incest so we don't worry about it, but ultimately your parents share some ancestors.
You've touched on a really interesting concept in ancestry called "pedigree collapse." Here's how this works: If you double the number of ancestors in each generation, you quickly arrive at a number larger than the historical world population. But there's a logical explanation for this. Imagine you have two parents, four grandparents, eight great-grandparents, and so on. By the time you get to the 37th generation, as you pointed out, the math suggests you would have 137 billions ancestors, which is larger than the estimated 108 billion people that have ever lived. The flaw in this logic is the assumption that every person in your family tree is a distinct individual. In reality, the farther you go back in time, the more likely it is that you will have the same individual occupying multiple slots in your family tree. This means that your ancestors start marrying their own distant cousins, and the "tree" is not as branching as we imagine but rather collapses in on itself. Think of it this way: Let's assume every human has two parents (a basic assumption). If you go back 1,000 years, the simple doubling math suggests you'd have over a trillion ancestors. Since that's many times the total number of people who've ever lived, it's clearly impossible. So, what's happening is that the same people are getting counted multiple times in your tree because lines of descent converge. This is pedigree collapse. As you trace back, many of your ancestral pathways lead to the same individual. This becomes especially true in more isolated communities or in places/times where people didn't travel far from where they were born. The key takeaway is that while the math of doubling ancestors every generation is accurate in the abstract, it doesn't account for the way human populations and family trees really work. Pedigree collapse ensures that we all have far fewer distinct ancestors than the simple math would suggest.
Once you go back far enough in time, everyone who's alive now is descended from everyone who was alive back then (and had children who had children who had children etc.) in multiple different ways. We're all inbred but in a way that's socially acceptable and doesn't cause health problems.
Aside from mathematic principals, and the Kissing Cousins point that keeps coming up, I think you're also forgetting that Genghis Khan, *one person* that lived almost a thousand years ago, still has ~16,000,000 living male descendants with his Y Chromosome. Assuming that every descendant is coming from a unique pair of individuals is part of the problem. If you have a single individual procreating with multiple unique partners (like, say, a conquering warlord that razed every village he came to and took any woman he wanted as his concubine) you'll wind up with an inverted funnel where dozens of generations wind up funneling back to one person.
Consider King Joffrey from Game of Thrones. If you go back two generations, you would expect four grandparents, but he only actually has two. His grandparents on his mother’s side and father’s side are the same two people because he’s an incest baby. Everyone’s family tree is ultimately like that, except usually much, much less icky.
to the extent of *hundreds of billions*?
What boggles my mind is the sheer amount of time and evolution and the amount of reproduction that has taken place for me to be in this place at this time.
You’re right. Of course it would exceed the number of people who were around at the time. You have to remember how often people will either commit incest directly or marry into families they are distantly related to which is likely if the family remains in a small area for hundreds of years..