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Scientists Devise Method To Address Conflict Between Molecular Clock, Fossil Record Of Mammalian Evolution

Humans can trace the origins of many of their mammalian relatives back either 65 million years or 130 million years, depending on which group of scientists they choose to trust. Now a research team led by University of Chicago paleontologist Mike Foote has developed a mathematical model that could resolve this scientific family feud.

The clearest fossil evidence indicates that modern placental mammalsa diverse group that includes humans, bats and molesfirst evolved about 65 million years ago, around the time the dinosaurs went extinct at the end of the Cretaceous Period. But according to genetic data, this same group of mammals should have appeared 130 million years ago, early in the Cretaceous Period.

Foote and his colleagues, John Hunter of the New York College of Osteopathic Medicine, Christine Janis of Brown University and Chicago's Jack Sepkoski, use their mathematical model to determine how incomplete the fossil record would have to be to leave this 65-million-year gap. They also estimate the quality of the mammalian fossil record to see if it was consistent with the genetic data. They report their findings in the Feb. 26 issue of the journal Science.

"We find that the quality of the fossil record is something like 10 to 100 times greater than the quality that would be required by this hypothesis of missing species diversity," said Foote, Associate Professor in Geophysical Sciences at Chicago. "It's such a large discrepancy, we ended up concluding that it's difficult if not impossible to maintain that there are 65 million years of fossils missing from the history of modern placental mammals. This result calls into question the use of a strict molecular clock to date the origins of major biologic groups."

The trigger for the study was a paper published in the April 30, 1998, issue of the journal Nature, written by Sudhir Kumar and S. Blair Hedges of Pennsylvania State University. Kumar, Hedges and other scientists using molecular techniques
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Contact: Steve Koppes
s-koppes@uchicago.edu
773-702-8366
University of Chicago Medical Center
26-Feb-1999


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