A researcher at George Washington University found evidence that suggests a common ancestor of snakes and lizards likely gave birth to live young, rather than laid eggs.

Alex Pyron believes that over time the ancestral species related to the two reptiles have switched back and forth in their preferred reproductive mode, according to a press release.

"This is a very unusual and controversial finding, and a major overturn of an accepted school of thought," Alex Pyron said in a statement. "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. We found this wasn't the case."

In the study, Pyron analyzed an evolutionary tree that contained all groups of squamates -the group that comprises lizards and snakes.  

The tree uses DNA sequencing technology to group thousands of lizards and snakes. It includes all families and subfamilies and most genus and species groups, according to a press release.

In the evolutionary tree, he found that in total, about 115 groups of lizards and snakes, or about 2,000 species, have live birth. The other 8,000 species lay eggs.

Their findings are backed by several recent plesiosaur and mosasaur fossil discoveries and the fossil record of a few lizards from the Cretaceous Period, which had embryos in the mother and had live birth.

He said the findings push their understanding of the evolution of live birth a lot further back in time to 175 million years ago.

Pyron realized "live birth has a much more ancient past as a strategy than previously believed," according to a press release.

In the future, he wants to test the genetics at work behind the evolutionary switching of reproductive mode.

Pyron is working next to analyze all tetrapods-a group composed of animals with four legs, such as amphibians, reptiles, birds, mammals and turtles - to see if there are any new surprises about the evolution of their reproductive modes.

His research was published in the journal Ecology Letters.