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

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 have claimed that various groups of plants and animalsnot just placental mammalsmust have had a long history that is simply not found in the fossil record.

These claims are based on the assumption that the molecular clock, the rate of genetic change any given species undergoes, is relatively constant over time. Thus, a measurement of the genetic difference between any pair of species would reveal how long ago their ancestral lineages went their separate ways.

The fossil record shows species continually give rise to new species and continually die off, with just a few happening to survive for any given slice of time. Evolutionary biologists have known this for more than a century, since the days of Charles Darwin.

If that is the case, Foote said, "there are a lot more species that have to escape preservation in order to make this hypothesis of missing time plausible." And it is not just the number of species that matters, but also their duration on Earth. "The longer the duration is, the more opportunities there would be for fossilization," Foote said.

In the study's next step, Hunter and Janis compiled all the available data worldwide on the fossil occurrences of Cretaceous mammals, except modern placental mammals, which are missing from the Cretaceous fossil record. These data allowed the researchers to measure the quality of the mammalian fossil record and the duration of mammalian species.

Hunter and Janis found between 225 and 460 known species of non-placental mammals from the Cretaceous Period, depending on how they counted forms that are hard to identify. But the numbers are significant no matter how you count them, said Sepkoski, Professor in Geophysical Sciences at Chicago. And the numbers say that the fossil record is much better than some molecular biologists and paleontologists have assumed.

"We're just trying to make the whole question more explicit and more testable," Foote said. Until now, some molecular biologists and paleontologists have tended to go back and forth, blaming each other for bad data.

"We thought the right approach was to ask, what would the fossil data have to say in order to favor one hypothesis or the other, and what do the data actually say?" Foote said. "If this had come out the other way, if our conclusion had been, yes, that it's very easy to believe there's 65 million years of missing mammal history, I would have been perfectly happy.

"But, Sepkoski added, "I would have been skeptical."