Snakes seem like relatively simple creatures—basically a glorified sock with holes on either end. Yet, these creatures have managed to adapt to nearly every continent on the planet over the past 100 million years.

But how exactly did snakes evolve, and what made these slithery creatures so successful across the planet today? It was likely a combination of three factors that all arose at roughly the same time.

“It all happens in one singular evolutionary burst around 100 to 110 million years ago, and then after that they pretty much have free reign to the planet every time they go into a new habitat,” says Alex Pyron, a biologist at The George Washington University in Washington, D.C.

Did Snakes Evolve From Lizards?

In some sense, snakes are really just a subset of lizards. Lizards began to evolve roughly 220 million years ago during the Triassic. In fact, snakes are just one of more than 30 groups of lizards that each evolved away their legs separately over the eons. Many of those lineages still survive today, including the European glass lizard, the giant legless skink, and the striped legless lizard.

As far as researchers can tell, snakes evolved from lizards roughly 100 million to 120 million years ago. The oldest snake ancestors yet discovered still had four legs—a 2015 study described four such species. The oldest of those four was Eophis underwoodi, discovered in England and dating back to the Middle Jurassic about 167 million years ago. Other early snakes only retained one pair of hind legs.

In terms of other lizard families, snakes are most closely related to monitor lizards—the group that includes Komodo dragons—and the alligator lizards found in western Mexico, Canada, and the U.S. All three of these lizard groups carry some venom, though the snake groups that diverged early from the others like boas or blind snakes have a lot less of it. It’s only in the more recent groups of snakes like vipers that you see greater chemical efficiency in venom development.

“In snakes, [venom] reaches its purest and most effective form,” Pyron says.

Read More: What Are the Most Venomous Sea Snakes in the World?

The Difference Between Legless Lizards and Snakes

While snakes and legless lizards may appear similar at first glance, a few things separate the groups. Most lizards have eyelids, for example, while snakes lack them.

One of the main characteristics that may have helped snakes become so plentiful is their articulated skull. Some people refer to snake skulls as unhinged, Pyron says, but that isn’t exactly accurate. It’s more that they have a high degree of flexibility, allowing them to open their jaws incredibly wide. This adaptation is the reason why pythons can ingest whole deer, for example.

“It frees them from the constraints that lizards have, of not being able to eat prey bigger than their own head,” Pyron says. “The skull articulation allows them to eat a huge range of prey, more than any other lizard group.”

Read More: These 3 Prehistoric Snakes Are the Stuff of Nightmares

Where Did Snakes First Appear?

Research by Pyron and his colleagues suggests that the earliest ancestor of all living snakes likely evolved in Gondwana—a large supercontinent that included most of the Earth’s current continents. Specifically, the first snakes likely came from the part of Gondwana that eventually split off to become Africa and South America.

One of the reasons snakes have been so successful may be due to the fact that they adapted three of their defining characteristics all around the same time. New research published by Pyron and his colleagues in Science found that the articulated jaw, the loss of limbs and the elongated bodies all happened in a regularly short order between 100 million years ago and 110 million years ago.

Read More: Evolutionary Insight: Inside the Brains of Reptiles and Amphibians

Snake Evolution

It’s unclear whether the first snakes were aquatic, lived in trees, on the ground, or underground, because all of these different types of snakes began to appear right away. This also makes it difficult to tell why snakes lost their legs and grew so long compared to other lizards in the first place. No other group of legless lizards has species adapted to exploit all of those types of ecosystems—most of the legless lizard groups only specialize in one of them.

Pyron speculates that it could be due to grass swimming. Many grass lizards are quite long, with small legs, since these adaptations make it easier to move quickly through dense vegetation. Conversely, snakes’ ancestors may have lost their legs due to living mostly underground, where they wouldn’t have been very useful. The fact that the first major divergence from ancestral snakes is the blind snake group—a small, burrowing type of serpent—adds some weight to this theory.

Among snakes, both underground and grass swimming adaptations have happened repeatedly, so it’s hard to tell which was the ancestral version. Or whether it was neither—snakes may have adapted longer bodies to swim, similar to eels.

In any case, there are still a lot of unanswered questions with regard to snake ancestors, Pyron says. Part of the problem is the nature of the group’s skeletons—their only bones are skulls and long vertebrae, making it difficult to tell what species many fossils belong to except in the rare case where softer tissues are preserved.

Read More: Meet Titanoboa: The Biggest Snake In the World

Article Sources

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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.