Why Did Our Mammal Ancestors Stop Laying Eggs?

None of us hatched. All of us were born live, and we might seem to think of this as the default for mammals. But that’s not so. Some mammals still lay eggs, just as our ancestors did for millions upon millions of years. The question is when our forebears made the switch. 

A handful of living mammals start their lives by breaking out of eggs. The duck-billed platypus and spiny echidna belong to an archaic group of mammals called monotremes that split off from other early mammals more than 100 million years ago. And, to this day, these mammals reproduce by laying small, spherical eggs that protect the gestating puggles (that is, a baby platypus or echidna) inside, until they’re ready to push their way out.

The Evolution of Birth

The way monotremes keep the generations going is a look into the deep past. Over 300 million years ago, the early ancestors of mammals split away from their reptile relatives. These protomammals are known as synapsids, and for millions upon millions of years, synapsids laid eggs. That makes live birth relatively new. The way that placental mammals (including humans) birth more developed young only came about relatively late in the evolutionary story. Same goes for marsupials giving birth to tiny, underdeveloped joeys.

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(Credit: structuresxx/Shutterstock)

Part of the task facing paleontologists is determining when live birth evolved, and how many times. “I’d say that all non-mammalian synapsids laid eggs and that live birth evolved once, at the common ancestor of marsupials and placentals,” says University of Washington paleontologist Christian Sidor. This is the simplest interpretation of the evolutionary history, he notes. While most reptiles lay eggs, live birth independently evolved over 100 times in species such as blue-tongued skinks and boa snakes. As far as protomammals and their relatives go, though, the available evidence hints that live birth only evolved once. To identify when mammals switched to live birth, paleontologists have to sniff out very rare and hard-to-discern evidence of how our ancient forebears and their relatives reproduced.

Protomammal Eggs

Two years ago, paleontologists Eva Hoffman and Tim Rowe announced that they had found an entire clutch of a protomammal called Kayentatherium. This weasel-like protomammal belonged to a group called cynodonts, related to the earliest mammals, and lived during the Jurassic about 185 million years ago. The sheer number of offspring found at a single site hinted that these synapsids still laid eggs. The litter size was more than twice the largest count for any living mammal. In fact, these prehistoric pups could have been quite a handful. A series of fossilized footprints made by an early mammal about 170 million years ago shows signs that the creature was carrying a heavy load on their back, perhaps a litter of offspring that hung on like baby possums do today.

Finds like the Kayentatherium babies and tracks help refine the timeline, but the critical evidence of the first mammals to have live young still awaits discovery. “Mammal paleontologists haven’t actually spent a lot of time on this question, mainly because there’s so little solid skeletal or fossil evidence to provide any answers,” says University of Oxford paleontologist Elsa Panciroli. Finding fossil eggs — as paleontologists have for dinosaurs and other fossil reptiles — would be a huge help. But it’s possible that protomammals laid soft-shelled eggs that were more likely to rot away than fossilize when buried. Finding any fossil is like searching for a needle in a haystack, but uncovering something so fragile is a truly rare event.

Both marsupial and placental mammals have live young. Since those two groups of mammals split from each other about 160 million years ago, that means that prehistoric mammals that lived just prior to the heyday of Stegosaurus and Allosaurus might hold pivotal clues. And, in fact, the sought-after evidence may be all in the hips.

“One of the main lines of investigation has been the pelvis,” Panciroli says. A group of vaguely squirrel-like mammals called multituberculates serves as an example. “In multituberculates,” Pancrioli points out, “the pelvis is so narrow that it’s been suggested that no realistic-sized egg could have fit through it.” It’s likely that these mammals birthed live, tiny young — similar to the way marsupials do today. The same is true of the oldest known member of our own placental lineage, named Eomaia. The beast’s name means dawn mother, the capacity of its hips suggesting that the earliest members of our fuzzy family reproduced more like us and less like a platypus.

Dino Influence

The million-dollar question, Panciroli says, is why some synapsids made the switch. After all, protomammals did just fine — even thrived — for over 100 million years by laying eggs. And live birth isn’t inherently superior. The answer might have something to do with the evolutionary downsizing mammals underwent during the Age of Dinosaurs.

During the Mesozoic, when reptiles ruled, mammals and their close relatives were very small. The largest was about the size of a badger. That’s great for evading the attention of dinosaurs, but it meant that whatever eggs early mammals laid must have been very small. “If early mammals laid tiny eggs, the young must have been much less developed when they hatched,” Panciroli says. But if the young developed inside, they could become more mature before being ushered into the outside world and would be less vulnerable after being born. From there, Panciroli points out, milk might have allowed early mammals to nourish pups that weren’t ready to forage on their own yet.

The answers are still in the rocks, and the fossil record continues to surprise. Paleontologists have a sense of what evidence to look for and when, and an unexpected fossil can always change the story. Still, the happenstances of life at small size may have given mammals the critical characteristics that make them what they are today. A warm-blooded metabolism, insulating fur, small size and perhaps even live birth all evolved when dinosaurs ruled, evolutionary happenstances that have let beasts stand the test of time.

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