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Descending Scales

Greg Ross

Snakes have been around for at least 100 million years, but it's still not clear whose family reunion they should go to. Biologists agree that they descended from lizards, but exactly where this first happened has been a matter of debate since the 1800s, when two contending theories emerged.

The so-called terrestrial hypothesis holds that snakes evolved from land lizards that shed their limbs. The marine hypothesis contends that they appeared first at sea and slithered ashore later.

New genetic researchClick to Enlarge Image

Which is correct? In recent years, the marine hypothesis found some support when a group of extinct marine snakes from the Cretaceous were discovered to have vestigial limbs. But a new study using genetic analysis comes down firmly on the landlubbers' side.

By scrutinizing evolutionary changes in two genes, biologists Nicolas Vidal and S. Blair Hedges of the Pennsylvania State University set out to construct a family tree for modern snakes and lizards. The team collected data from 64 different species, going to extra lengths to get tissue samples for all 19 families of living lizards, including some rare specimens from Africa, China and the Philippines.

"The importance of having all the families of lizards is that we simply don't know the closest family of lizards to snakes," Hedges says. "So if you're missing one or two, that could be the one. You really have to have all of them."

If the terrestrial hypothesis is correct, today's snakes are most closely related to land lizards. If the marine hypothesis is correct, modern snakes can claim a closer genetic cousin in the sea. (When snakes first appeared, the only seagoing lizards were giant mosasaurs. The mosasaurs left no tissue samples, so Hedges and Vidal used their closest living relatives, monitor lizards, as genetic stand-ins.)

After compiling gene sequences from each species, the team compared them using a variety of statistical methods, looking for patterns of mutation. Their findings, published in the U.K. Royal Society journal Biology Letters, suggest that snakes are not closely related to monitor lizards—or, hence, to ancient sea lizards. Because all other lizards at that time lived on land, the Penn State study supports the idea that land lizards evolved into modern snakes.

"The tree unfortunately doesn't show exactly what lizard family is the closest relative of snakes significantly," Hedges says. "But what it does show significantly is that the closest relative is not the monitor lizards, the varanids, which is the only connection that would have supported the marine hypothesis."

The terrestrial hypothesis makes intuitive sense to Hedges, who used to explore caves in high school. "The first thing you realize is that your limbs are a detriment to moving through small openings," he says. "So it's very easy and logical to understand why natural selection would result in a limbless organism."

In fact, "we see this in many groups of lizards that have small limbs or completely missing limbs, reduced to where there are no limbs externally visible. Quite often those lizards burrow in the ground or live in holes. So it's very logical that the ancestor of snakes might have been a burrowing sort of lizard."

One that learned to walk before it could crawl.

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