Wanna See a Snake with Two Legs?

Before there were snakes, there most likely were snakes with legs. And now, a new study has revealed a rather creepy middle step: a prehistoric snake that had no front legs, but two tiny back legs. You know, just kinda sitting there.

A team of Yale University paleontologists conducted a comprehensive analysis of DNA sequence data and anatomical information for different groups of living and extinct snakes to create an evolutionary family tree. From there, they used computer algorithms to statistically determine which features the most recent ancestor of modern day snakes would have had.

"Our analysis suggested that the most recent common ancestor of snakes likely would have been a nocturnal, stealth-hunting animal found in forested ecosystems, probably in the southern hemisphere," Daniel Field, a Yale Ph.D. candidate and co-author of the study, told me over the phone.

The analysis, published this week in BMC Evolutionary Biology, also suggested that this early snake, which lived about 110 million years ago, would have had two small hind legs on its body, complete with ankles and toes.

Field explained that snakes are descendants of early four-legged lizards, eventually evolving to have no legs at all, although scientists aren't sure exactly why. Field said it could be that early snakes were evolving in water and could glide more easily without legs, or it could be that they lived more exclusively in burrows, where legs would again be a bit of an impediment (hey, worms don't need 'em). It could also be some combination of the two or another catalyst entirely, but for one reason or another, snakes ditched their limbs.

But Field said he and his colleagues were pretty surprised to find this most-recent snake ancestor still had little legs trailing behind it.

"Some fossil snakes are known that actually retain hind limbs. However it was always thought that these fossil snakes were very early, primitive snakes that simply had not yet lost their legs and were not closely related to any living snakes," Field said. His team's analysis suggests these fossils are probably closer cousins to today's snakes than paleontologists had previous thought.

Looking at the artist's rendering above, which was created based off of Field's and his colleague's findings, those little legs don't seem all that powerful or useful, and Field told me it's hard to say what, if anything, they were used for.

"Honestly, we don't have a great idea of what function those little hind legs would have served," he told me. "My personal opinion is that those hind limbs might not have been useful for much at all. At that point in evolutionary history, it's possible that the small, little hind limbs were nothing more than vestigial limbs remnants, not serving much of an important anatomical function whatsoever. They were on their way out but had not yet disappeared."

Field said the analysis is the most comprehensive to date, but as with all things in science, the understanding of snake evolution will continue to change. He told me he's eager to get out into the field and search for fossils that might provide deeper insight into this grandfather of modern snakes and its creepy little legs.

"It will be interesting to see if our conclusions are upheld in the future when snakes from the right time and place turn up in the fossil records," he said.