Skeleton study sheds new light on how snakes evolved
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BLOOMINGTON, Ind. -- Research by paleontologists at Indiana University and the University of Nebraska sheds new light on how snakes evolved their elongated, legless bodies.
Hox genes, which establish the boundaries of the neck, trunk, lumbar, sacral and tail regions in birds, lizards, crocodiles and mammals, were previously thought to have been disrupted in snakes, resulting in a loss of regions in their seemingly simplified body form as they evolved from four-legged lizard ancestors.
P. David Polly of Indiana University and Jason Head of the University of Nebraska-Lincoln examined regional differences in the shapes of individual vertebral bones in snakes, lizards, alligators and mice. Snakes are different from these other amniote groups in that they lack forelimbs, shoulder girdles and sternal skeletons. Snake vertebrae were assumed to have become less regionalized when the limbs were lost.
“If the evolution of the snake body was driven by simplification or loss of Hox genes, we would expect to see fewer regional differences in the shapes of vertebrae,” Head said. “Instead, what we found was the exact opposite. Snakes have the same number of regions and in the same places in the vertebral column as limbed lizards.”
Not only did Head and Polly find that snakes were as differentiated as lizards, but when they compared regions in snakes with Hox gene expression, they found the two matched.
“This suggests that Hox genes are functioning in the evolution and development of the vertebral column in snakes; but instead of patterning distinct, rib-less regions like the neck and lumbar spine of mice, they control more subtle, graded changes in shape,” Head said.
Those findings combined with information from fossils indicate that the direction of snake evolution is the opposite of what had been concluded from developmental genetics alone.
“Our findings turn the sequence of evolutionary events on its head,” Polly said. “It isn’t that snakes have lost regions and Hox expression; it is that mammals and birds have independently gained distinct regions by augmenting the ordinary Hox expression shared by early amniotes.
“Snakes have a lot more vertebrae compared to lizards and they have lost the shoulder girdle, but they are just as regionalized,” Polly said.
Head and Polly reached their conclusions using a method called geometric morphometrics and a regression-based analysis of the size and shape of vertebral structures. To determine where one segment ends and the next begins, they use a statistical method called maximum likelihood estimation.
“Analysis of gene functions are necessary, but not sufficient in studying evolutionary transitions,” Head concludes. “In order to fully understand the mechanisms by which new body forms evolve, it is crucial to study the anatomy of modern and fossil organisms.”
The study was published online Jan. 5 by the journal Nature.
The two scientists previously collaborated, along with other researchers, on a 2009 paper that reported the discovery of 60-million-year-old fossils of a snake that was 43 feet long and weighed 2,500 pounds.
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