THE CASE OF THE MISSING LINK

MISFILED BONES HAVE LED RESEARCHERS TO DISCOVER HUMANITY'S ANCESTOR

Martin Pickford held two "turtle" bones up to the light, glimpsing a network of
capillaries and veins. These were no turtle bones, Pickford realized; they
resembled pieces of a primate skull. It was 1981 and Pickford, a researcher
from the Institute of Paleontology in Paris, learned that the expedition
responsible for unearthing the "turtle" bones had also yielded an intriguing-and
incomplete-skull from a chimp-sized primate dubbed Proconsul.

His next mission: matching the bones with the skull. Pickford called upon Johns
Hopkins paleontologist and primate expert Alan Walker, who hunted down the skull
and, with great delicacy, nudged the fragments back in place. The fit was
perfect. Proconsul, the scientists discovered, had been much brainier than
originally thought. In fact, according to a decade of new research conducted by
Pickford, Walker, and others, the 18-million-year-old primate is probably the
earliest known missing link between the apes and man. "Proconsul makes a good
model for a common ancestor," Walker says. "Its features are generalized enough
to embody traits common to the apes and us."

In 1984, to establish Proconsul as a legitimate missing link, Pickford and
Walker headed for Kenya, where the esteemed paleontologist Mary Leakey had found
Proconsul on Lake Victoria's Rusinga Island decades before.

The expedition provided the researchers with enough fossils to study Proconsul
for years to come, Piecing together the bones in a process that continues to
this day, Pickford and Walker have been able to establish the life cycle of this
prehistoric primate. "We have males and females, babies through adults," Walker
says, Indeed, the tedious process of preservation and reconstruction has
revealed details as delicate as the ear's bones and semicircular canals. The
seventh and eighth cranial nerves have been reconstructed as well.

Other researchers interested in the life of our ancient ancestor have begun
studies of their own. David Beynon of the University of Newcastle upon Tyne in
the United Kingdom, for instance, has been trying to determine the life span of
Proconsul by examining its teeth. Beynon's unusual technique, which involves
"reading" teeth much as researchers read the rings on a tree, should tell the
life spans of Proconsul males and females.

Another researcher, Chris Ruff of Johns Hopkins, is using the principles of
engineering to calculate Proconsul's body weight. According to Walker, Ruff's
analysis of limb bones should reveal not only the types of stress Proconsul had
to bear, but also the differences-if any-between males and females. In the
future, Ruff may also be able to illuminate the lives of Proconsul individuals.
Did a particular male suffer from a broken arm or a spinal disorder? Did a
female die from a head injury, or perhaps in childbirth? Ruff's new techniques
may be able to answer some of these questions.

But even when these details and more are known, scientists will still debate
whether Proconsul was truly our ancestor. "There are few ways to demonstrate
that something is a real ancestor," Walker says. "Of Proconsul, one can say,
simply, that here is an example of a population from which our ancestors most
likely arose." He thinks a bit and then amends his response. "Genetically, we
all go back to the primordial slime, don't we?" he adds. "Details of that are
difficult to trace."