Ancient Snakes and Lizards Did Not Lay Eggs, Only Live Birth

The lizard or the egg? In a "what came first" scenario for reptiles, researchers are now arguing that "the egg" can't even be an option. The first lizards and snakes likely exclusively gave live birth, according to a recent study.

The study, published in Ecology Letters, reveals new evidence that the first reptiles likely gave live birth, evolving over time to lay eggs. What is more, certain lineages of lizards and snakes probably changed how they gave birth several times over millions of years.

"Before, researchers long assumed that the ancestor of snakes and lizards laid eggs, and that if a species switched to live birth, it never reverted back," study co-author Alex Pryon said in a recent statement.  "We found this wasn't the case."

According to the study, Pryon reached this conclusion after closely analyzing the squamate evolutionary tree - in which lizard and snake species are categorized. The tree was created using DNA sequencing of thousands of samples from various lizards and snakes. This tree inlcudes all families, subfamilies, genus, and species groups, and provided Pryon with a comprehensive "map" of the evolutionary advancement of lizards and snakes from which to work from.

The study reports that among modern lizards and snakes, only about 2,000 species gave live birth. A group about four times the size of the live birth group currently lays eggs. However, Pryon's analysis found that many of the live birth and egg birth groups are remarkably related, branching from the same live birthing ancestors that lived up to 175 million years ago.

This theory is supported by recently discovered fossil evidence, as well as the fossil records of lizards from the Cretaceous period. These records indicate that the majority of snake and lizard ancestors were live-birthing creatures who adapted to lay protective eggs, only to adapt again and again for live birthing or egg laying, depending on ecological conditions.

The study was published in the January issue of Ecology Letters.