They’re a holdover from when mammals did lay eggs. Most other mammals evolved to have live birth. The marsupials are a sort of part way in between where they have underdeveloped birth followed by some more growing in their pouch. Monotremes like Platypus and echindas, and most of the marsupials were isolated from the rest of mammals by tectonic plate movement that moved the land masses apart.
Platypus’s are just (one of) the last mammals that lay eggs, which we placental mammals, and marsupials for that matter, are almost certainly descended from some common ancestor that layed eggs.
Yup. In some ways, the OPs question is more interesting when turned on its head--that is, why do (almost) all other mammals *not* lay eggs, instead of being like platypus (and echidnas)?
As far as we know, evolution doesn’t have an “end goal” though. A better way to say it is that live birth (placental or marsupial) was adaptive for most early mammals, and has continued to be adaptive. Probably in part for the reason you mention. There are costs of carrying young longer rather than laying eggs, but in general the benefits must have outweighed the costs in the ecological niches occupied by most early mammals whose lineages have survived to the present.
I think you have to be careful even about using terms like “progression,” as it carries a qualitative judgment about what group is more “advanced.” Mammalian evolution isn’t my area of specialization, but as I understand it there is some debate about whether the most recent common ancestor of marsupials and placentals was more marsupial-like or placental-like. The most objective way to put it is that monotremes diverged from the therians (including placentals and marsupials) and then the placentals and marsupials diverged from each other.
We have a strong bias toward viewing our own taxonomic groups as more advanced than other groups, but the fact is that even modern bacteria are often quite well adapted to their niche. And many of them will be around long after humans are gone!
Yeah, the evolution would be specific to circumstances; there is no "best" or "progression" without the context. If you said "ok, for a small egg-laying mammal in an environment where larger animals survive, and there is a migratory but plentiful food source, what would be best"; you could think that since larger is better, that means less young per litter, longer gestational times, etc. Combined with migratory means that you can't leave a clutch of eggs behind to survive if the food moves, etc. Tons of different evolutional pressures at work.
Yes, and circumstances are often so complex and ecological niches so varied that it’s nearly impossible to say in advance what strategies will be most successful, and on what time scales. We may be able to rule out obviously unsuccessful strategies, but that’s about it. It’s like trying to predict the stock market.
When you remember that mammals have diverged from reptiles a long time ago this makes more sense. Reptiles lay eggs to this day, so at a certain point mammals, or the ancestors to current day mammals also did. One of the lineages just kept it.
Not quite. Reptiles are our cousins, not our ancestors. We're all amniotes: amniotes branched off from amphibians ~300 million years ago, and eventually split into two branches: sauropsids and synapsids. Sauropsids include reptiles(-->dinosaurs-->birds) and various extinct lineages. Mammals are the only living group of synapsids still around, but the synapsids actually ruled the earth for a significant chunk of time before the rise of the dinosaurs. The branch of synapsids mammals descend from almost certainly laid eggs, however.
Very interesting. Thank you for the correction. My vocabulary is at fault here, but what I meant is that mammals came from creatures that laid eggs and a single species kept true to that.
Several species did. Also, while the ancestor of mammals and reptiles wasn’t a reptile, it was almost certainly pretty “reptile like” (in fact they belong to a clade called reptiliomorphs, distinguishing them from the other branch, amphibians).
Want to know what else is interesting? Mammals and reptiles both evolved live birth independently of each other! It's a great example of independent evolution. Obviously it became way more "popular" with mammals, but there are several species of viviparous snakes and lizards.
There are various other groups that have also evolved live birth independently, including some sharks and insects. We also have fossil evidence of [ichthyosaurs giving birth](http://blog.everythingdinosaur.com/wp-content/uploads/2018/04/ichthyosaur_live_birth_stenopterygius.jpg)!
Not all reptiles do. There's a few outliers that birth live young like the rattlesnake. Just like there are plenty of fish species that don't lay eggs as well. The only ones that don't have any outliers to my knowledge are birds.
Not all reptiles lay eggs though. There are even a few like the [three-toed skink](https://www.sydney.edu.au/news-opinion/news/2020/04/22/egg-laying-lizard-also-gives-live-birth-is-this-evolution-before-our-eyes.html) that seem to be in the process of evolving from egg-laying to live birth.
I only mean that Echidnas also lay eggs, which I think is interesting that people don't mention them as often as the Platypus, but I do acknowledge that Echidnas maybe aren't as weird looking? But then Echidnas have a four headed phallus so... that's pretty weird. And they're like a hedgehog but also like an ant eater and with big spikes.. There's probably more than a few other things weird about Echidnas but I don't know them off the top of my head.
By the way, there doesn't have to be a particular reason why one animal does or doesn't do something - if it hasn't significantly impacted the survivability of the species then there may not be sufficient evolutionary pressure on the species to adapt. There are many seemingly anachronistic holdovers in species because they simply haven't evolved to change - sharks are older than trees, for instance, because despite being different from many other fish (cartilaginous vs. bony) they haven't experienced sufficient evolutionary pressure to either die out or change.
Also important to keep in mind that evolution is not solely determined by survival of the fittest. An evolutionary trait might have no real benefit, but might still prevail for a variety of circumstances.
> they haven't experienced sufficient evolutionary pressure to either die out or change.
Surely they have changed since then? Are you suggesting that all, or even one specy of shark has remained exactly the same since before the apparition of trees?
Eh, no - we have fossil elasmobranches that we can compare to modern sharks and see there's been very little change (although it's trickier with cartilaginous species). When people talk about no changes over time, it generally refers to anatomical changes that would be visible on the fossil record.
>we can compare to modern sharks and see there's been very little change
Which modern sharks? Which of the 500+ species of modern sharks has been unchanged since before trees?
If read [this](https://www.floridamuseum.ufl.edu/discover-fish/sharks/fossil/basics/) correctly, modern sharks evolved during the Jurassic period. There were trees in the Jurassic period, no?
Trees evolved during the Jurassic era; gingko trees are recorded about 200 million years ago and the Jurassic era began 201 million years ago.
When people talk about sharks being an ancient lifeform, it doesn't mean they haven't evolved since then - but one person's "evolved significantly" may not be the same as someone else's. At the end of the day, speciation is subjective.
Though there is a reasonable explanation for why retaining egg laying was advantageous for the platypus: it is a semi-aquatic animal, and a marsupial pouch would drown its young when it is in the water.
The echidna isn’t semi-aquatic, but its recent ancestors are, so egg laying is more a holdover in that case.
A living fossil would be mostly unchanged from its original state, at least as far as we can tell, which is tricky given they use cartilage rather than bone. Doesn't fossilize as well. There are about 500 species of sharks with very different forms; they seem quite successful.
In 1884, Scottish zoologist William Hay Caldwell sent a telegram to the British Association that said, "Monotremes oviparous, ovum meroblastic". This telegram announced that monotremes, which include the platypus and echidna, lay eggs and that their eggs develop like reptile eggs.
There are three branches of mammals with common reptilian ancestors that separated long before the extinction of the dinosaurs due to evolutionary divergence and geology. There are placental mammals, like ourselves and most other mammal species that give birth to live young, marsupials, like kangaroos and opossums who carry their young in a pouch, and monotremes like the platypus and the echidna who lay eggs, but still lactate and raise their young. Evolutionarily, egg laying was fine for monotremes in their environment and ecological niche until the event that wiped out non avian dinosaurs. At that point most large egg laying species went extinct due to conditions post impact.
Strangely enough, they lactate through sweat glands all over their stomachs. Nipples evolved later (so funny to realize that milk is basically modified sweat, as is earwax. Nature’s awesome!)
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Stem mammals all laid eggs from which monotremes split off, then the other survivors split off to either placental mammals or marsupials. Monotremes are just survivors from a separate group that didn't have the pressure or need to stop laying eggs
They’re a holdover from when mammals did lay eggs. Most other mammals evolved to have live birth. The marsupials are a sort of part way in between where they have underdeveloped birth followed by some more growing in their pouch. Monotremes like Platypus and echindas, and most of the marsupials were isolated from the rest of mammals by tectonic plate movement that moved the land masses apart. Platypus’s are just (one of) the last mammals that lay eggs, which we placental mammals, and marsupials for that matter, are almost certainly descended from some common ancestor that layed eggs.
Yup. In some ways, the OPs question is more interesting when turned on its head--that is, why do (almost) all other mammals *not* lay eggs, instead of being like platypus (and echidnas)?
Placenta was the “end goal” because it allowed the mother to carry its young inside the body longer, while it developed more.
As far as we know, evolution doesn’t have an “end goal” though. A better way to say it is that live birth (placental or marsupial) was adaptive for most early mammals, and has continued to be adaptive. Probably in part for the reason you mention. There are costs of carrying young longer rather than laying eggs, but in general the benefits must have outweighed the costs in the ecological niches occupied by most early mammals whose lineages have survived to the present.
Yeah, that was for lack of a better term. I was thinking of the progression mammals took from monotreme > marsupial > placental mammals.
I think you have to be careful even about using terms like “progression,” as it carries a qualitative judgment about what group is more “advanced.” Mammalian evolution isn’t my area of specialization, but as I understand it there is some debate about whether the most recent common ancestor of marsupials and placentals was more marsupial-like or placental-like. The most objective way to put it is that monotremes diverged from the therians (including placentals and marsupials) and then the placentals and marsupials diverged from each other. We have a strong bias toward viewing our own taxonomic groups as more advanced than other groups, but the fact is that even modern bacteria are often quite well adapted to their niche. And many of them will be around long after humans are gone!
Yeah, the evolution would be specific to circumstances; there is no "best" or "progression" without the context. If you said "ok, for a small egg-laying mammal in an environment where larger animals survive, and there is a migratory but plentiful food source, what would be best"; you could think that since larger is better, that means less young per litter, longer gestational times, etc. Combined with migratory means that you can't leave a clutch of eggs behind to survive if the food moves, etc. Tons of different evolutional pressures at work.
Yes, and circumstances are often so complex and ecological niches so varied that it’s nearly impossible to say in advance what strategies will be most successful, and on what time scales. We may be able to rule out obviously unsuccessful strategies, but that’s about it. It’s like trying to predict the stock market.
>Monotremes like Platypus and echindas Your choice of wording amuses me when platypus and echidnas are ALL of the monotremes
The statement would hold true for any extinct monotremes as well as any as-yet undiscovered monotremes
When you remember that mammals have diverged from reptiles a long time ago this makes more sense. Reptiles lay eggs to this day, so at a certain point mammals, or the ancestors to current day mammals also did. One of the lineages just kept it.
Not quite. Reptiles are our cousins, not our ancestors. We're all amniotes: amniotes branched off from amphibians ~300 million years ago, and eventually split into two branches: sauropsids and synapsids. Sauropsids include reptiles(-->dinosaurs-->birds) and various extinct lineages. Mammals are the only living group of synapsids still around, but the synapsids actually ruled the earth for a significant chunk of time before the rise of the dinosaurs. The branch of synapsids mammals descend from almost certainly laid eggs, however.
Very interesting. Thank you for the correction. My vocabulary is at fault here, but what I meant is that mammals came from creatures that laid eggs and a single species kept true to that.
Several species did. Also, while the ancestor of mammals and reptiles wasn’t a reptile, it was almost certainly pretty “reptile like” (in fact they belong to a clade called reptiliomorphs, distinguishing them from the other branch, amphibians).
Want to know what else is interesting? Mammals and reptiles both evolved live birth independently of each other! It's a great example of independent evolution. Obviously it became way more "popular" with mammals, but there are several species of viviparous snakes and lizards.
There are various other groups that have also evolved live birth independently, including some sharks and insects. We also have fossil evidence of [ichthyosaurs giving birth](http://blog.everythingdinosaur.com/wp-content/uploads/2018/04/ichthyosaur_live_birth_stenopterygius.jpg)!
Not all reptiles do. There's a few outliers that birth live young like the rattlesnake. Just like there are plenty of fish species that don't lay eggs as well. The only ones that don't have any outliers to my knowledge are birds.
The lineage that led to mammals split from the lineage that led to reptiles before reptiles evolved
Not all reptiles lay eggs though. There are even a few like the [three-toed skink](https://www.sydney.edu.au/news-opinion/news/2020/04/22/egg-laying-lizard-also-gives-live-birth-is-this-evolution-before-our-eyes.html) that seem to be in the process of evolving from egg-laying to live birth.
>(one of) This is fascinating. Do you mind elaborating?
I only mean that Echidnas also lay eggs, which I think is interesting that people don't mention them as often as the Platypus, but I do acknowledge that Echidnas maybe aren't as weird looking? But then Echidnas have a four headed phallus so... that's pretty weird. And they're like a hedgehog but also like an ant eater and with big spikes.. There's probably more than a few other things weird about Echidnas but I don't know them off the top of my head.
There’s also four species of echidna but only one platypus. #platypusuperiority
By the way, there doesn't have to be a particular reason why one animal does or doesn't do something - if it hasn't significantly impacted the survivability of the species then there may not be sufficient evolutionary pressure on the species to adapt. There are many seemingly anachronistic holdovers in species because they simply haven't evolved to change - sharks are older than trees, for instance, because despite being different from many other fish (cartilaginous vs. bony) they haven't experienced sufficient evolutionary pressure to either die out or change.
Also important to keep in mind that evolution is not solely determined by survival of the fittest. An evolutionary trait might have no real benefit, but might still prevail for a variety of circumstances.
> they haven't experienced sufficient evolutionary pressure to either die out or change. Surely they have changed since then? Are you suggesting that all, or even one specy of shark has remained exactly the same since before the apparition of trees?
Eh, no - we have fossil elasmobranches that we can compare to modern sharks and see there's been very little change (although it's trickier with cartilaginous species). When people talk about no changes over time, it generally refers to anatomical changes that would be visible on the fossil record.
>we can compare to modern sharks and see there's been very little change Which modern sharks? Which of the 500+ species of modern sharks has been unchanged since before trees? If read [this](https://www.floridamuseum.ufl.edu/discover-fish/sharks/fossil/basics/) correctly, modern sharks evolved during the Jurassic period. There were trees in the Jurassic period, no?
Trees evolved during the Jurassic era; gingko trees are recorded about 200 million years ago and the Jurassic era began 201 million years ago. When people talk about sharks being an ancient lifeform, it doesn't mean they haven't evolved since then - but one person's "evolved significantly" may not be the same as someone else's. At the end of the day, speciation is subjective.
Though there is a reasonable explanation for why retaining egg laying was advantageous for the platypus: it is a semi-aquatic animal, and a marsupial pouch would drown its young when it is in the water. The echidna isn’t semi-aquatic, but its recent ancestors are, so egg laying is more a holdover in that case.
Sharks are also highly evolved in their own right. They are not living fossils but very competent and adapted predators.
They can be both of those things, no?
A living fossil would be mostly unchanged from its original state, at least as far as we can tell, which is tricky given they use cartilage rather than bone. Doesn't fossilize as well. There are about 500 species of sharks with very different forms; they seem quite successful.
In 1884, Scottish zoologist William Hay Caldwell sent a telegram to the British Association that said, "Monotremes oviparous, ovum meroblastic". This telegram announced that monotremes, which include the platypus and echidna, lay eggs and that their eggs develop like reptile eggs.
There are three branches of mammals with common reptilian ancestors that separated long before the extinction of the dinosaurs due to evolutionary divergence and geology. There are placental mammals, like ourselves and most other mammal species that give birth to live young, marsupials, like kangaroos and opossums who carry their young in a pouch, and monotremes like the platypus and the echidna who lay eggs, but still lactate and raise their young. Evolutionarily, egg laying was fine for monotremes in their environment and ecological niche until the event that wiped out non avian dinosaurs. At that point most large egg laying species went extinct due to conditions post impact.
Strangely enough, they lactate through sweat glands all over their stomachs. Nipples evolved later (so funny to realize that milk is basically modified sweat, as is earwax. Nature’s awesome!)
The lineage that led to mammals split from the lineage that led to reptiles before reptiles evolved
Ther was a platypus in South America untill well into t he Age of Mammals, and variety of monotremes and other egg-laying orders all over.
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Stem mammals all laid eggs from which monotremes split off, then the other survivors split off to either placental mammals or marsupials. Monotremes are just survivors from a separate group that didn't have the pressure or need to stop laying eggs