Monday, November 17, 2008

Snake-Fang Evolution Mystery Solved -- "Major Surprise"

Biologist Freek Vonk goes eye to eye with the longest venomous snake species in the world—a female king cobra—in the Indonesian rainforest.

James Owen
for National Geographic News

July 30, 2008

The diverse and deadly array of venomous snakes living today all arose from a single fanged ancestor, a new study suggests.

Vipers, cobras, and other snakes that have fangs at the fronts of their jaws surprisingly begin life like snakes that have poisonous fangs at the back of their jaws, said a team led by Freek Vonk of Leiden University in the Netherlands.

The discovery suggests venomous fangs—the lethal evolutionary invention that led to snakes becoming so successful—arose only once about 60 million years ago.

The origins of venom-injecting snakes have long been the source of scientific controversy, because the contrasting fang positions of diverse snake groups pointed to independent evolution.

But a study of the embryos of eight front- and rear-fanged species has found that fangs always first appear at the back of the upper jaw before migrating forward in vipers and cobras.

This previously unidentified transformation in the unborn young occurs due to "rapid growth of some parts of the upper jaw relative to the others," Vonk explained.

Single Origin

"It's a major, major surprise," the zoologist said of the findings, which will appear tomorrow in the journal Nature.

"There was no significant evidence before for such a single origin of fangs."

Furthermore, the team may have identified the prehistoric mechanism that allowed snake fangs to develop from teeth.

The findings suggest the rear of part of the reptiles' tooth-forming layer in the upper jaw long ago became uncoupled from the rest of the jaw, enabling the back teeth to evolve independently with the venom gland.

"This uncoupling idea is totally new," Vonk said.

"Snakes evolved the fangs once, probably by an uncoupling between some rear and front teeth, and then after that they just played with the [fang] position within the embryo."

This theory makes absolute sense from an evolutionary standpoint, he added, since snake fangs are unique among vertebrates.

"It would be very difficult to assume that snakes had miraculously invented fangs at different occasions," Vonk argued.

The jaw uncoupling may have occurred as far back as 60 million years ago, "at the base of the advanced snake tree."

Venom Systems

After that, Vonk added, "the different lineages could evolve their highly sophisticated venom systems."

Of the 3,000-odd snake species living today, advanced snakes (which are nearly all venomous) number around 2,700, he noted.

Johannes Müller, a reptile fossil expert and curator at the Museum für Naturkunde in Berlin, Germany, said, "I think this is a very important study which, from a novel perspective, sheds new light on our understanding of advanced snake evolution.

"The study shows that advanced snakes seem to share a common, and highly modified, developmental mechanism in the upper jaw," said Müller, who wasn't involved in the new study.

"It supports the view that the developmental tool kit for becoming venomous evolved only once, and the different advanced snake lineages then took advantage of it in various ways," he added.

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