Live birth began in the ancestors of snakes and lizards at least 175 million years ago according to new research published by Alex Pyron, Robert F. Griggs Assistant Professor of Biology in the Columbian College of Arts and Sciences at the George Washington University, on Dec. 17, 2013, in the journal Ecology Letters.
This discovery radically changes some preconceptions about evolution. Most biologists thought that an animal that evolved from producing eggs to produce live birth never returned to producing eggs but the new research invalidates that concept.
Pyron based his conclusion on an analysis of modern snakes and lizards and an examination of the DNA of the ancestors of modern snakes and lizards including the animal’s most ancient ancestors. Twenty percent of modern snakes and lizards give birth to live offspring while the other 80 percent lay eggs.
The fossil record includes a few lizards from the Cretaceous Period that have embryos. The embryo indicates a live birth.
The conclusion is that snakes and lizards switched from laying eggs to live birth several times over the millions of years that reptiles have been on Earth.
Pyron plans a similar examination of mammal ancestors that may reveal an egg laying mammal from the distant past. Five modern mammals lay eggs including the platypus.