A Serpentine ‘Explosion’ 125 Million Years Ago Primed Snakes for Rapid, Diverse Evolution

Researchers say an evolutionary “singularity” led to several small, quick changes in snake species, from legless bodies and flexible skulls to chemical-sensing abilities

The cat-eyed snake, light brown with a dark brown stripe squiggling along its body, slithers on a green leaf in the Peruvian Amazon
The cat-eyed snake slithers in the Peruvian Amazon. Dan Rabosky, University of Michigan

Not long after the origin of snakes—when certain lizards began to lose their legs more than 150 million years ago—a burst of evolutionary innovation paved the way for the variety of serpentine shapes, sizes and behaviors we see today.

From 30-foot green anacondas in South America to four-inch Barbados threadsnakes you could mistake for a noodle, the roughly 4,000 snake species in oceans, freshwater, forests and deserts exemplify the many unique forms and functions the reptiles have achieved over millions of years of evolution.

Researchers have now helped shed light on this diversity, identifying an “evolutionary explosion” early in snakes’ history that helped them evolve at a rate about three times faster than contemporary lizards, according to a paper published last week in the journal Science.

“The rate at which snakes evolve new features and evolve new kinds of diets has basically been kicked into overdrive,” Daniel Rabosky, the senior author of the study and an evolutionary biologist at the University of Michigan, tells Scientific American’s Jack Tamisiea. “Lizards are puttering around on a moped, while snakes are on a bullet train.”

A close-up of a green anaconda head, staring directly at the camera, as it emerges from the water. A green leaf obscures the view slightly, on the left side of the frame.
A green anaconda, which can grow up to 30 feet long and weigh up to 550 pounds, emerges from Venezuelan waters. Daniel10ortegaven via Wikimedia Commons under CC BY-SA 3.0

That accelerating moment, which occurred roughly 125 million years ago, is the type of evolutionary jump that Charles Darwin once called an “abominable mystery,” and what the research team refers to as a “singularity.” Essentially, instead of the typical slow crawl of natural selection, snakes experienced many small changes in quick succession. Over the expansive timespan of prehistory, these added up to a sudden shift in the direction of the animals’ evolution.

To illuminate the details of this time for snakes, the research team analyzed the genomes of more than 1,000 squamates (the order that includes snakes and lizards) and examined partial DNA from about 80 percent of all known snake and lizard species. They combined these findings with statistical models to create the most detailed evolutionary tree of lizards and snakes to date.

From this analysis, the team found that the “singularity” appeared to have coincided with key changes to snakes’ anatomy. Their skulls became flexible, better for attacking and swallowing prey; they developed the ability to detect airborne chemicals with their tongues; and they lost their legs, becoming thinner and longer, better for traversing new terrains.

“We thought maybe they’d show something exceptional in one area but maybe not in another,” Alexander Pyron, a biologist at George Washington University (GWU) and an author of the study, said in a GWU press release. “But, no, it’s every single thing—increased rates of body form evolution, increased rates of diet evolution, increased rates of niche evolution. Snakes stand out as a huge cut above every other group of lizards.”

A hairy bush viper, yellow with brown spikes, curls on a green leaf
A hairy bush viper, endemic to Central Africa, is venomous. Bree Mc via Flickr under CC BY 2.0

By studying the stomach contents of more than 68,000 dead specimens, mostly from museum collections, the researchers also identified snakes as becoming early dietary specialists, evolving the ability to eat prey that other lizards didn’t touch—including vertebrates and some toxic, hard to digest creatures. Along the way, it surely didn’t hurt their hunting prowess that some snakes evolved to see infrared light, and some became venomous.

“The paper demonstrates that snakes are an evolutionary ‘singularity’ that has changed the face of the Earth,” Michael Lee, an evolutionary biologist at Flinders University in Australia who wasn’t involved in the research, tells Scientific American.

Still, scientists have much to learn. The findings didn’t pinpoint which of snakes’ several unique traits gave them such an advantage, why the sudden singularity occurred, nor exactly how their specialized diets may have contributed to such a rapid evolutionary pace.

“Snakes are special and weird,” Nick Longrich, an evolutionary biologist and paleontologist at the University of Bath in England who was not involved in the new study, tells Popular Science’s Lauren Leffer. “I think that, here, they’ve successfully quantified it.”

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