Data Suggests Legs and Toes in Ancestor of Living Snakes

By NICHOLAS ST. FLEURMAY 19, 2015

Photo

An artist’s rendering of the most recent common ancestor of all living snakes. Its small hind legs probably served no purpose in locomotion. Credit Julius Csotonyi

From the robust boa constrictor to the venomous rattlesnake, all of the more than 3,400 snake species that slither today may have descended from the same prehistoric forest prowler, whose sinuous body had two small hind legs with toes and ankles, researchers reported on Tuesday.

After analyzing data gathered through fossils, genetic sequencing and anatomical comparisons of 73 snake and lizard species, a team of paleontologists from Yale University has constructed what it calls the most comprehensive snake “family tree” to date. The findings provide an answer to longstanding questions about when, where and how modern snakes originated.

“Having that tree as a backbone let us draw a ton of conclusions for what the ancestral snake would have been like,” said Daniel J. Field, a doctoral candidate in evolutionary biology and an author of the study. The team concluded that the most recent common ancestor of all living snakes was nocturnal, thrived 128.5 million years ago in the Southern Hemisphere and devoured relatively large prey whole using its sharp, hooked teeth as a hunting tool.

To reach this conclusion, the team’s first step was to reconstruct the snake’s family tree from tips to its trunk. To better understand when certain characteristics — like the ability to constrict prey or hunt at night — first appeared, the researchers used the genetic and morphological data they collected to piece together how different groups of living snakes are related to one another.

After mapping the relationships among distinct snake groups, the herpetologists used a series of algorithms to fill in when each trait first arose, in a process called ancestral state reconstruction. Mr. Field and his team identified 11 characteristics that they wanted to place onto the snake’s tree of life. Each trait would answer a question about snake evolution that scientists often debate: Did ancient snakes dwell in the water or on land? Originate from the Northern or the Southern Hemisphere? Ambush their prey or forage for food?

Using the tree, team members could rule out the traits that could not have belonged to the most recent ancestor of living snakes and generate a model of what traits it did exhibit. They found that it did not constrict prey like boas and pythons, but did have remnants of hind legs, which Mr. Field said were most likely vestigial structures that served no purpose in locomotion.

“I was most amazed by how strongly we inferred that the common ancestor retained hind limbs,” he said. The team published its tree of life for snakes in the journal BMC Evolutionary Biology.

The team’s ancestral reconstruction is not without its limitations, Mr. Field said. Because there are no fossil records of the most recent common ancestor of living snakes, Mr. Field and his team have no way of confirming that the creature they recreated with their models is correct.

“Sometimes evolution plays out in unexpected and strange ways,” he said. “We think we’ve got a strongly supported idea, and based on the mathematical reconstruction it is what is most likely to be true.”

Christopher J. Raxworthy, a herpetologist at the American Museum of Natural History in New York, who was not involved in the study, called the work fantastic and said he was impressed by how strongly the genetic and fossil data supported the authors’ findings. But he said he was not surprised that the common ancestor of all living snakes had hind legs, because several living snakes have vestigial hind limbs and pelvic girdles.

Dr. Raxworthy added a caveat to the work. He said that herpetology, like most of evolutionary biology, is on the cusp of a genomic revolution, and that many snake groups most likely will have their entire genomes sequenced in coming years. Because of that influx of genetic information, “it’s almost inevitable that there will be changes to this evolutionary history of snakes,” he said.