Resetting the evolutionary clock with
fossils and DNA
Michael Foote uncovers fossil data with
mathematical models rather than picks and shovels.
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
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