How fossil embryos prove mammal ancestors laid eggs

HostWe usually think of mammals as creatures that give birth to live babies. We have fur, we make milk, and we definitely don't sit on nests. But there was a long stretch of time where our very old ancestors were still laying eggs in the dirt, just like a lizard or a bird.

HostHow did we find a way to prove something that happened so many millions of years ago?

GuestIt all started with a lucky find in the red rocks of a desert in Arizona. People found the bones of an animal that looked like a small, scruffy dog. This creature was a very old cousin of ours from a time before the first true mammals even existed. When they got the bones back to the lab and used high-power beams to look inside the rock, they saw something they didn't expect. Tucked right next to the mother were the tiny skeletons of thirty-eight babies.

HostWait, thirty-eight? That's a huge number. I don't see how that proves they came from eggs, though. Some snakes give birth to live young, and they have big groups of babies too. Why could this mother not have just been pregnant with a lot of pups?

GuestThat's a fair point. But when we look at mammals today, even the ones that have a lot of babies don't get anywhere near thirty-eight. A mother dog or a pig might have ten or twelve. To have nearly forty babies at once, the mother's body would've to be giant, or the babies would've to be very small. These babies were so tiny they could fit on your thumbnail. Their bones weren't even fully formed yet. In a live-birth animal, those babies would never survive being born. They were at a stage of growth where they still needed to be tucked away in a shell to stay safe and finish growing.

HostBut we didn't actually see any shells in that rock, right? If there are no eggs to look at, it feels like we're just filling in the blanks because we want them to be egg-layers.

GuestIt's more than a guess. We can look at how the babies are huddled together in the rock. They're all in one tight clump, exactly the way a nest looks. There was no way to fit that many babies inside the mother's body at that size. Also, their skulls were much bigger than their bodies, which is a trait we see in egg-laying animals. But the real proof is in the math of the brain. If you look at the mother's skull, her brain was still very small. It was a lizard-sized brain in a mammal-like body.

HostI'm still stuck on the mammal part. If it has a tiny brain and lays forty eggs, why do we not just call it a reptile and move on?

GuestWe know it belongs to our side of the family because of its ears and its teeth. This creature had the special jaw bones that eventually turned into the tiny bones in our own ears. It had teeth that could chew and grind, unlike a reptile that just gulps its food. It likely had fur too. So it's this weird middle point. It has the body of a mammal, but it's still using the old reptile way of having dozens of kids at once. It was a bridge between the two worlds.

HostSo why did we stop? If having forty kids worked for them, why did we switch to having just one or two? Carrying a baby inside you seems a lot harder than leaving an egg in a hole in the ground.

GuestThe change was all about the brain. To get a big, smart brain, you need a lot of time and a lot of steady food. An egg is like a small lunch box. It only has so much food inside in the form of a yolk. Once the baby eats all the yolk, it has to hatch. It simply can't grow any more while it's in there. By keeping the baby inside the body, the mother can keep feeding it through her own blood. That lets the baby stay in a safe, warm spot for many months. That extra time is what allowed the brain to keep growing and folding and getting more tricky. We traded the safety of a nest for the chance to have a brain that can think.

HostSo we traded a whole army of babies for one smart one.

GuestThat's the trade we made. Those thirty-eight babies in Arizona didn't have room in their tiny eggs to grow big brains. Their mother could've forty kids, but none of them were going to be very bright. As we moved toward being the mammals we're now, we chose to put all our energy into just a few kids. We gave up the safety of having forty chances at survival so that we could've one chance at being smart.

HostThat was a huge gamble. You're saying we basically bet our entire survival as a species on being smart instead of being many.

GuestThis find shows that the gamble paid off. You can see the shift right there in the rocks. You go from these huge groups of tiny, simple babies to the bones found later on where the litters get smaller and the heads get bigger. The egg was like a ceiling. To break through it and become the mammals we're now, we had to leave the shell behind.

GuestThose little bones are the last trace of a time when we still lived like lizards.

HostThe red rocks of Arizona hold the proof that our own story began with a choice to stop laying eggs and start growing bigger minds.