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Resetting the evolutionary clock with fossils and DNA

>> Michael Foote uncovers fossil data with mathematical models rather than picks and shovels.

PHOTO:  In his Hinds office, paleobiologist Foote excavates piles and files of quantitative data in search of an evolutionary timeline able to stand up against genetic and fossil evidence.A typical expedition for paleontologist Michael J. Foote, SM'88, PhD'89, means a short walk across the science quad from his second-floor office in Hinds Laboratory to the John Crerar Library. The professor in geophysical sciences has nothing against fieldwork-last summer he spent a week helping two Chicago graduate students collect fossils in the Marshall Wilderness of northwest Montana. But he prefers to wield mathematical models to uncover the inconsistencies in the fossil record that would otherwise prevent paleontologists from understanding evolutionary trends.

"You will never see any data that I collected out in the field in one of my papers-at least not yet," says Foote. "What I do is not typically what most people would think of as paleontology."

His colleagues know better. Last November Foote received the Paleontological Society's 2000 Charles Schuchert Award, which is presented to a person under 40 whose work reflects excellence and promise in paleontology. He is the eighth University of Chicago alumnus or professor to hold the award since it was established in 1973. "He may be the most gifted analytical paleobiologist of our generation," says Arnold Miller, PhD'86, a geology professor at the University of Cincinnati. "He routinely unlocks information from the fossil record that would otherwise have remained inaccessible."

Since the days of Charles Darwin, scientists have debated the quality of the fossil record. The biases that affect that record can operate selectively on species, environments, and time periods alike. The hard shell of a clam, for example, is more likely to become buried and preserved on the sea floor than the remains of a bird, whose delicate bones would disintegrate rapidly when exposed to the elements. But even a rich layer of fossilized clams would disappear if laid bare to erosion by Earth's geologic forces.

During his expeditions to Crerar, which he supplements with examinations of specimens at the Smithsonian and other museums, Foote prospects for data to feed into his mathematical models. One of his goals is to determine whether biological diversity is caused by changes in the rate of either the evolution of new species or the extinction of existing species. For example, he has found that, generally speaking, changes in species diversity during the Paleozoic Era-before the age of the dinosaurs-were more strongly linked to changes in the rate of extinction. But after the Paleozoic Era, the opposite occurs. "It's a weird thing that I don't understand yet," he admits.

Streaks of gray have infiltrated Foote's full head of dark hair, lending a mature touch to his otherwise boyish looks. Though he speaks warmly and enthusiastically about his work, Foote is shy when it comes to recognition-Schuchert award or not. He was nominated for the award by the late J. John Sepkoski Jr., a Chicago colleague who received the same honor in 1983. Before Sepkoski died in 1999, he and Foote joined forces to crack the case of the missing mammal fossils. In a 1999 Science paper, Foote, Sepkoski, and two co-authors offered a quantitative solution to conflicting evidence on the origins of modern placental mammals.

The clearest fossil evidence indicates that placentals first 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 mammalian group should have appeared 130 million years ago, early in the Cretaceous.

These genetic claims are based on the assumption that the molecular clock-the rate at which DNA evolves-is relatively constant over time. Many evolutionary biologists argue that the best way to figure out how long ago the ancestors of any two related modern species began evolving in separate directions is simply to look at how much DNA they share, and set the clock backward.

Ironically, the molecular clock was calibrated with fossils. The technique's first great success was in dating the evolutionary divergence of humans, chimpanzees, and gorillas to approximately 5.5 million years ago, a time fairly consistent with the fossil data. But in many other situations, including placentals, the molecular data conflict with the fossil data. In an effort to resolve these conflicts, Foote, Sepkoski, and their Science co-authors set about numerically testing the validity of the fossil data. They developed a mathematical model that would determine how incomplete the mammalian fossil record would have to be to erase the 65 million years' worth of remains that geneticists insist should be there.

A survey of the fossil data worldwide revealed between 225 and 450 species of known mammals from the Cretaceous Period (depending on how one counts certain difficult-to-identify forms). The sample excluded modern placental mammals, which are missing from the Cretaceous fossil record.

"We can empirically estimate the density of fossil occurrence of these species, expressed in terms of occurrences per species per million years," Foote explains. "If modern placentals have similar preservation potential as the other mammal groups, then the density of fossil occurrence of the other groups in the Cretaceous would have to be 10 to 100 times lower than the actual empirical estimates."

In the year since the findings appeared in Science, some geneticists have countered with studies that attempt to reconcile their molecular clocks with Foote's assessment of the mammalian fossil record. The dialogue was to continue February 8 at London's Natural History Museum, with Foote and a rival geneticist giving talks in honor of Darwin's birthday.

In the meantime, who, exactly, should care about Foote's math models besides evolutionary scientists? Anyone who worries about modern biodiversity, says Cincinatti's Miller. As sketchy as it is, the fossil record is the only reference point that scientists have for assessing how threats to species diversity have affected the planet in the past, and how modern threats may affect it in the future.-Steven Koppes

 

 


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