Genetic Study Shows Snake Evolution

["Richard W. Travsky" <rtravsky@uwyo.edu>]

cool!

http://www.science.psu.edu/alert/Hedges1-2004.htm

30 January 2004 --The mystery of where Earth's first snakes lived as they
were evolving into limbless creatures from their lizard ancestors has
intrigued scientists for centuries. Now, the first study ever to analyze
genes from all the living families of lizards has revealed that snakes
made their debut on the land, not in the ocean. The discovery resolves a
long-smoldering debate among biologists about whether snakes had a
terrestrial or a marine origin roughly 150 million years ago--a debate
rekindled recently by controversial research in favor of the marine
hypothesis.

In a paper to be published in the 7 May 2004 issue of the Royal Society
journal Biology Letters, Nicolas Vidal, a postdoctoral fellow, and S.
Blair Hedges, a professor of biology at Penn State, describe how they put
the two theories to the test. They collected the largest genetic data set
for snakes and lizards ever used to address this question. Their
collection includes two genes from 64 species representing all 19 families
of living lizards and 17 of the 25 families of living snakes.
...
"For the marine hypothesis to be correct, snakes must be the closest
relative of the only lizards known to have lived in the ocean when snakes
evolved--the giant, extinct mosasaur lizards," Vidal says. "While we can't
analyze the genes of the extinct mosasaurs, we can use the genes of their
closest living cousins, monitor lizards like the giant Komodo Dragon," he
explains.
...
The team analyzed gene sequences from each of the species, using several
statistical methods to determine how the species are related. "Although
these genes have the same function in each species--and so, by definition,
are the same gene--their structure in each species is slightly different
because of mutations that have developed over time," Vidal explains. When
the genetic comparisons were complete, Vidal and Hedges had a family tree
showing the relationships of the species.

"Our results show clearly that snakes are not closely related to monitor
lizards like the giant Komodo Dragon, which are the closest living
relatives of the mosasaurs--the only known marine lizard living at the
time that snakes evolved," Vidal says. "Because all the other lizards at
that time lived on the land, our study provides strong evidence that
snakes evolved on the land, not in the ocean."

The research suggests an answer to another long-debated question: why
snakes lost their limbs. Their land-based lifestyle, including burrowing
underground at least some of the time, may be the reason. "Having limbs is
a real problem if you need to fit through small openings underground, as
anybody who has tried exploring in caves knows," Hedges says. "Your body
could fit through much smaller openings if you did not have the wide
shoulders and pelvis that support your limbs." The researchers note that
the burrowing lifestyle of many other species, including legless lizards,
is correlated with the complete loss of limbs or the evolution of very
small limbs.

This research was supported by the National Aeronautics and Space
Administration Astrobiology Institute and the National Science Foundation. 

(astrobiology?)

Abstract and text at

http://www.journals.royalsoc.ac.uk/app/home/contribution.asp?wasp=hpb9qmqvyq3jlcuq9evl&referrer=parent&backto=issue,2,33;journal,1,4;linkingpublicationresults,id:110824,1