The findings go against an accepted and long-held school of thought, a scientist at George Washington University said.
"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," biology Professor Alex Pyron said. "We found this wasn't the case."
Several recent dinosaur fossil discoveries and the fossil record of a few lizards from the Cretaceous Period showed embryos in the mother, confirming live birth, the researchers said.
The findings put the evolution of live birth a lot further back in time to 175 million years ago, showing live birth has much more ancient roots as an evolutionary strategy than previously believed, the researchers said.
About 2,000 species of lizards and snakes have live birth, while the other 8,000 species lay eggs, although the new study suggests the ratio hasn't all been that way, and could change again in the future.
More studies could reveal the genetics at work behind the evolutionary switching of reproductive modes, Pyron said.