Evolution in Action: Lizard Moving From Eggs to Live Birth

A yellow-bellied three-toed skink carrying embryos, visible as light orbs inside its body.

Evolutionary records shows that nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only. "By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other."

In general, the results suggest the move from egg-laying to live birth in reptiles is fairly common—at least in historic terms—because it's relatively easy to make the switch. It's basically the early stages of the evolution of a placenta in reptiles: The mother forms eggs, but then retains them inside her body until the very last stages of embryonic development. The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranes—all that remains of the shells.

"We tend to think of this as a very complex transition...but it's looking like it might be much simpler in some cases than we thought."