In 1799, the Keeper of the Department of Natural
History at the British Museum didn’t know what to make of a bizarre specimen that had
just arrived from Australia. It appeared to be the skin of a beaver or
a mole with a beak sewn onto it? Being a good curator, he carefully examined
every inch of the specimen, and published a detailed description of its many weird features. But … he wasn’t convinced it was even
a real animal. He wrote, “I ought perhaps to acknowledge
that I almost doubt the testimony of my own eyes with respect to the structure of this
animal's beak.” The beak met the body so perfectly, and the
animal was so strange that for a hot second, there he suspected the specimen was a fake. But the arrival of more specimens confirmed
that he was dealing with a 100% genuine creature with the bill of a duck, the tail
of a beaver, and the feet of an otter, with venomous spurs. The animal, of course, turned out to be a
duck-billed platypus. But perhaps the weirdest thing about it was
that it was a mammal that laid eggs. The playtpus belongs to an unusual group called
monotremes that diverged from the branch of the mammal family tree that leads to marsupials
-- and later placentals -- around 186 million years ago. And although they might not look like it,
eggs were a huge evolutionary innovation. They saved those animals known as amniotes
from extinction and may have set the stage for their invasion of dry land. While the identity of the first animal to
lay eggs is still a mystery, paleontologists can estimate when this creature came about
and what it might’ve looked like. And they’re teaming up with geneticists
and biologists who study living animals to understand why and how most mammals lost this
incredible egg-laying innovation. Turns out, that might have all started with
a chance encounter with a virus. The story of the egg spans millions of years,
from the first vertebrates that dared to venture onto dry land to today’s mammals, including
the platypus, and of course birds. yknow like, chickens? Yes, I’m here to tell you fam: The egg came
first. OK, now, there are eggs, and then there are
eggs. The earliest eggs of any kind were laid in
water accompanied by a thick layer of jelly, like frogs do today. And yeah, it’s literally called just … jelly. And this jelly is a good home for amphibian
embryos, as long as everything stays wet. But about 385 million years ago, in the Devonian
Period, tetrapods started venturing away from the water’s edge. Competition was high in aquatic ecosystems,
and the only animals on land were invertebrates, like insects and spiders, which probably enticed
tetrapod ancestors out of the water. But in order to become truly terrestrial,
tetrapods had to be able to keep their embryos wet on dry land. One of the first vertebrates to live, and
maybe reproduce, on dry land was Casineria It lived during the Carboniferous Period,
around 334 million years ago, in what's now Scotland. And it mixed some of the amphibian features
of its ancestors, like unfused shoulder bones, with features associated with reptiles, like
strong leg and back bones. And some experts think it resembled a new
group of creatures that would appear in the fossil record about 20 million years later:
the first true amniotes. These animals laid a new kind of egg, one
surrounded by a tough, leathery shell. The shell helped protect the embryo from diseases
and attacks, but it was also porous enough to let gases reach the embryo so it could
breathe and grow. And the waterproof membranes and the amniotic
fluid inside the egg created a self-contained pond so the embryo stayed hydrated. Suddenly, animals didn’t have to spend their
lives in the swamp anymore. It was a huge boost for adventurous tetrapods
that were colonizing dry land. Then, parts of the planet that had been tropical
and humid started to cool and dry up, an event called the Carboniferous Rainforest Collapse,
which I've talked about before. The swamps started to disappear, and
many amphibian groups went extinct. But the amniotes, being better adapted to
drier conditions, expanded into a variety of new niches, and dominated the land. But what were the steps between embryos in jelly and embryos in shells? Well, we don’t know because of what’s
known as the egg gap. Some of our earliest fossil records of amniote
eggs are from dinosaurs like Mussaurus patagonicus, which lived around 215 million years in Argentina. And that’s at least 100 million years after
when we think the egg first appeared. So the initial 30% of the history of the amniote
egg unfolded without leaving a trace. Because, eggs, especially soft-shelled, leathery
eggs like we’d expect to see in early amniotes, don’t fossilize well. So without that proof, paleontologists have
to rely on inferences, based on closely related animals for which they do have hard evidence,
a technique known as phylogenetic bracketing. Since reptiles, mammals, and birds are all
amniotes, amniote eggs must have existed in their last common ancestor, the first true
amniote. Fossils of the earliest unquestionable reptile,
and therefore the earliest known amniote, come from Nova Scotia, and are dated to around
310 million years ago. They’re from a cute little creature called
Hylonomus. So, the egg must be older than that. What we do know is that the tale of the egg
diverged into two separate storylines, around 315 million years ago, when amniotes split
into two major groups. One was the sauropsids, which would include
reptiles, dinosaurs, and birds. And the other was the synapsids, the ancestors
of mammals. And these two groups put their eggs in very
different baskets. Sauropsids mostly kept laying eggs throughout
their evolutionary history, although they’d end up laying different kinds of eggs. Some reptiles continued laying eggs like those of early amniotes, which were soft and leathery, like turtles still do today. But birds, and their predecessors the dinosaurs,
eventually started forming a different type of egg - one that had calcium in the shell. This made them much stronger and also more
likely to fossilize, which is why we have those fossil eggs from Mussaurus. Now, the other group, the synapsids, took a much more complicated path. Like sauropsids, early synapsids probably
laid primitive amniote eggs with flexible shells, but there’s no fossil evidence of
their eggs either. But we know that in just over 100 million
years, the ancestors of mammals made huge strides. They went from scaly, lizard-like creatures
like Ophiacodon to fully warm-blooded, furry creatures with mammalian jaws and middle ear
bones. And at some point, synapsids even evolved
mammary glands, when they were still laying eggs. At first these glands possibly served as a
source of water to help keep eggs hydrated. But, later they became a source of milk for
feeding young after they hatched. Milk is something all mammals do - even the
ones that lay eggs. Enter the monotremes. This new group of synapsids branched off around
186 million years ago, during the early Jurassic Period. This was the last group of synapsids to branch
off that still laid eggs. And these ancient egg-layers probably reproduced
a lot like today's monotremes do. The mother platypus, for example, keeps her
fertilized, shelled eggs inside for a long time before she lays them. While they’re inside, the embryos take in
a lot of nutrients from the mother and are developed enough that they hatch in less than
2 weeks. And we think monotremes have been doing it
more or less that way this whole time. But finally, around 160 million years ago in the Late Jurassic Period, the ancestors
of placental mammals and marsupials appeared. These were the therians, like Juramaia, a
toothy, shrew-like mammal known from China. Now, the steps that happened between having
eggs and having actual pregnancies in our mammal ancestors are mysterious. Because, you guessed it, there’s not much
fossil evidence. But, once again, researchers can turn to living
mammals to try to figure out what happened. One piece of the puzzle could be changes in
gene expression. Because, the main membranes of an amniotic
egg -- known as the sac, the chorion, and the allantois -- are all present in animals
that give birth to live young. They’re just used in different ways. Mammals still form an amniotic sac around
their young. But the placenta -- the organ that provides
material to the embryo from the mother -- is actually formed from the chorion and allantois
membranes of the egg. So mutations may have changed how the pieces
of the amniotic egg were used, leaving mammals with an entirely new reproductive strategy
as a result. Another idea is that therians followed a path
similar to the monotremes, where eggs were retained in the mother’s body for longer
and longer periods of time. This also might’ve happened through changes
in the expression of genes, in this case in the genes involved with egg retention or embryo
development. Yet another piece of the puzzle could be unusual
proteins called syncytins These proteins help cell membranes fuse together. And their most important role in mammals is
developing the placenta. What’s interesting about syncytins is that
they didn’t evolve in mammals; they’ve been part of our genomes since before therians
were even a thing. Syncytins are actually what’s left of an
ancient form of glycoproteins, which make the outer shells, or envelopes, of some viruses. But they’re also found in modern mammals. So researchers think that, more than 190 million
years ago, some retrovirus attacked one of our mammal ancestors, and inserted bits of
its genetic material into its host’s DNA that made that viral protein. And over time, that viral DNA was repurposed
into new genes in mammals. So today’s mammals, including you, still
carry pieces of this ancient virus with them. But instead of connecting parts of a virus
together, they’re used to build a placenta. So it may be that a chance encounter with
a virus kick-started the process that led to mammals no longer laying eggs. The evolution of the shelled egg was a milestone in animal evolution, helping those first amniotes
take over the land. And the sauropsids and the synapsids put their
own spins on the amniotic egg, ones that we can see today in living birds, like the chicken!,
and in monotremes, like the platypus. So by digging into the genomes of these living
amniotes, we can start to tell the story of how and when the egg came first. Big thanks to this month’s eggs-ellent Eontologists: Kallie is making me say this Patrick Seifert, Jake Hart, Jon Davison Ng,
Sean Dennis, and Steve! Remember, backers have access to our Eonites
Only Podcast, so go patreon.com/eons and pledge your support. And as always I wanna thank you for joining
me in the Konstantin Haase Studio. Be sure to subscribe at youtube.com/eons for
more Shell-ebrations of ancient life. Kallie.... Don't make me say this!
