People tend to think of species that lay eggs as less highly evolved than animals that give birth to live young. Live birth is one of the things that defines mammals, including humans. However, new evidence suggests that some egg laying species may once have given birth to live offspring and then evolved to lay eggs instead. Research published in Ecology Letters on December 17 shows that live birth existed in many species as much as 175 million years ago.

Alex Pyron, Robert F. Griggs Assistant Professor of Biology in the Columbian College of Arts and Sciences at the George Washington University, and his team studied a evolutionary tree containing all groups of squamates. Squamates is a group that comprises lizards and snakes. An evolutionary tree is something like a “family tree” however this tree is constructed using DNA and can cover hundreds of millions of years.

The researchers found 115 groups of lizards and snakes, or about 2,000 species, have live birth and another 8,000 that lay eggs so far. According to Pyron,

“This is a very unusual and controversial finding, and a major overturn of an accepted school of thought. 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.”

Next Dr. Pyron will look at evolutionary trees containing all tetrapods or four legged animals, including amphibians, reptiles, birds, mammals and turtles, to see if there are additional surprises about their reproductive methods. Pyron also hopes to anywise the specific genetics behind reproductive methods and the switch from egg laying to live birth.

Source: George Washington University