Snakes Lose Sea Legs

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Science  06 Feb 2004:
Vol. 303, Issue 5659, pp. 759
DOI: 10.1126/science.303.5659.759c

Snakes slithered into existence on land—not out of the surf, claims a new study. The finding is the latest strike against the marine-origins theory, which first surfaced in Victorian times.

Extinct marine snakes with legs, such as this one, were probably a side branch of the snake evolutionary tree that was otherwise rooted on land.CREDIT: ILLUSTRATION BY KAREN CARR

Until recently, most experts believed these reptiles evolved underground, from burrowing lizards. But the idea that snakes first lost their limbs at sea gained new life 7 years ago, when scientists found a fossilized marine snake with tiny hind limbs. They theorized that it was a missing link between snakes and the giant Cretaceous-era marine reptiles called mosasaurs.

Now, scientists at Pennsylvania State University, University Park, have put snakes' ancestors back on terra firma. Evolutionary biologists S. Blair Hedges and Nicolas Vidal compared two genes, RAG1 and C-mos, in more than 60 living snake and lizard families. The analysis reveals that another group of lizards is related to the carnivorous monitor lizards—the undisputed close relatives of ancient mosasaurs. But there's no close genetic link between those monitors and snakes, the pair report online in the 29 January issue of Biology Letters.

Hussam Zaher, a snake-origins expert at the University of São Paulo in Brazil, says he'd like to see more lizard species in the analysis. Nonetheless, he agrees that “the evidence supporting the marine theory has been seriously questioned.”

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