Ancient snake and lizard ancestors may have given birth to live young instead of laying eggs; over the course of the species' development they may have "flip-flopped" between birthing methods.

"This is a very unusual and controversial finding, and a major overturn of an accepted school of thought," Alex Pyron, Robert F. Griggs Assistant Professor of Biology in the Columbian College of Arts and Sciences at the George Washington University, said in a news release. "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."

The researchers believe these species may have been giving birth to live offspring about 175 million years ago. The idea is further backed up by the fact that fossils of plesiosaurs, mosasaurs, and Cretaceous Period lizards were found with embryos of what were probably live young in their stomachs.

The research team looked at the evolutionary tree branch containing lizards and snakes (squamates) and performed DNA sequencing to reach their conclusion.

The researchers determined that about 2,000 species of lizards and snakes throughout history gave birth, while about 8,000 laid eggs.

"Doctor Pyron is working next to analyze all tetrapods-a group comprised 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. He also wants to test the genetics at work behind the evolutionary switching of reproductive mode," the news release reported.

The research was published in the journal Ecology Letters.