By land or by sea: a snake debate

Evolution: The origin of one of our less popular creatures is back in the spotlight with a study by Penn State researchers.

Medicine & Science

February 09, 2004|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

Most people probably don't worry too much about how snakes evolved. They just want to keep them out of their basements and gardens.

But scientists have spent more than a century debating the snake's origin, asking whether it developed on land or in the oceans.

This month, a team of Penn State University researchers weighed in with their findings, announcing that their new family tree shows snakes made their debut on land.

Researchers S. Blair Hedges and Nicolas Vidal collected genetic tissue samples from 19 families of lizards and 17 families of snakes - most of it shed skin - to assemble their tree.

The tree, they say, shows that contrary to widely held theories, snakes are not related to an extinct marine lizard known as the mosasaur. The mosasaur is a distant relative of the Komodo dragon whose ancestors swam in the oceans 100 million years ago.

All of the other lizards alive at the time were terrestrial, so the researchers say that snakes likely evolved on land. The study is due to be published May 7 in the Royal Society journal Biology Letters.

"We're breaking that connection with the mosasaurs, which means there couldn't be a marine origin to snakes," said Hedges, a biology professor.

Some experts criticized the study.

Michael Caldwell, a paleontologist and snake expert at the University of Alberta, said the findings are ambiguous and are nothing more than an "educated guess." He noted the researchers are unable to identify the snake's closest lizard relative.

"If you can't tell me based on your data set who is the closest relative, you can't exclude anything," Caldwell said. "The data are inconclusive."

Hedges said he was unable to pin down the snake's nearest relatives because too much is unknown about snake genetics.

He said the study was the first attempt to resolve the mystery of the snake's origin by assembling a family tree.

"The first thing you notice about a snake is that it doesn't have limbs, so the first interesting question about them is how did they lose their limbs," Hedges said.

He said detailing the origin of snakes may help scientists understand the Earth's prehistoric environment and provide clues about the evolution of other wildlife.

"Snakes are just about everywhere, in all kinds of habitats all over the world," Hedges said. "We have fossils of lizards that don't have limbs that go back to the Cretaceous [which stretched from 140 million to 65 million years ago], but we still don't know how they developed."

The findings conflict with theories developed 130 years ago by a Haverford College fossil hunter credited with theorizing that snakes evolved from the sea. Researchers have been arguing ever since.

Australian scientists announced three years ago that fossils found in South Australia and Queensland confirmed that snakes slithered out of the sea. They said that snakes were likely descendants of a 15-foot- long prehistoric beast whose fossilized remains showed it swam like an eel and had no inclination for burrowing. They named their find Wonambi, an Aborigine term for the "rainbow serpent" recorded in Aboriginal myths.

But other researchers have speculated that snakes evolved on land, losing their limbs as a part of their subterranean lifestyle. Appendages became a problem for creatures that need to fit through small openings underground, they say.

"There is definitely a lot of debate about where snakes came from," Caldwell said.

Caldwell said that snakes may have migrated back and forth from land to water in their evolutionary development, making it hard to pinpoint where they originated.

Some researchers argue that the most primitive living snakes are tiny, blind, wormlike burrowing creatures, thread snakes and worm snakes common in most areas. But even that is unclear. Fossil records found in the Middle East show much larger snakes existed 98 million years ago, long before any fossilized records of the tiny burrowing snakes.

"Are the burrowing snakes primitive, or are they actually derived from an earlier species?" Caldwell said. "It's an open question."

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