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Gene Study Shows Mammals Lived Before Extinction Of The Dinosaurs

Overwhelming evidence from the largest evolutionary study of gene sequences ever performed shows that the major groups of mammals emerged well before the extinction of the dinosaurs, according to Penn State researchers Sudhir Kumar, postdoctoral fellow, and S. Blair Hedges, associate professor of biology, whose research will be published in the April 30, 1998, issue of the journal Nature.

"The evolution of mammals appears to have occurred gradually by the isolation of breeding groups when the continents broke apart, not suddenly by the rapid filling of ecological niches left vacant when the dinosaurs became extinct," Hedges says. The massive gene study suggests that modern orders of mammals first evolved when the continents were separating during the Cretaceous era about 100 million years ago--much earlier than some previous estimates based on fossil studies, which link the evolutionary event to mass extinctions 65 million years ago.

"This is the first time we ever have been able to estimate when all these lifeforms appeared on Earth," Hedges says. "Fossils can't give us this information, partly because there are huge periods of Earth's history from which not enough fossils have been found to make reliable estimates."

To gauge the pace of evolution, Kumar and Hedges mined a burgeoning collection of gene sequences being accumulated at Genbank, the public genetic databases maintained by the National Institutes of Health. "During the past few years there has been a tremendous explosion in the number of known gene sequences, so we had ten times as much data to work with as we did just two years ago," says Hedges, who published a similar but much smaller study with Kumar and others in 1996.

The scientists sifted through many thousands of vertebrate gene sequences from hundreds of species to find those that develop mutations at a constant rate over time, which Kumar and Hedges
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Contact: Barbara K. Kennedy
science@psu.edu
814-863-4682
Penn State
30-Apr-1998


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