Gorillas' Hidden History Revealed
AMERICAN ASSOCIATION OF PHYSICAL ANTHROPOLOGISTS, 28-31 MARCH, PHILADELPHIA, PENNSYLVANIA
Ann Gibbons

Although gorillas are our closest living relatives other than chimpanzees, their evolution is something of a mystery. There are no fossils of gorillas and little DNA from wild ones. Now, a new study of nuclear DNA from the two species of wild gorillas offers a glimpse of their mysterious past and of how new species of primates arise.

Unlike their cousins the chimps, these shy herbivores turn out to have diverged slowly into two species, apparently taking the better part of a million years, according to a talk at the meeting by molecular anthropologist Linda Vigilant of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. "It shows us how little we've known about speciation in gorillas," says anthropological geneticist Anne Stone of Arizona State University in Tempe.

Previous studies of the paternally inherited Y chromosome from gorillas suggested that the two species--eastern gorillas and western gorillas--interbred until recently. But maternally inherited mitochondrial DNA suggested that they separated more than 1 million years ago. Nuclear DNA studies sampled too few individuals to clear up the confusion, says evolutionary biologist Michael Jensen-Seaman of Duquesne University in Pittsburgh, Pennsylvania.

Vigilant and her colleagues isolated DNA from the blood, liver, or feces of 18 of the 14,000 wild gorillas left on the planet, including three eastern gorillas from Uganda and the Congo; the western gorillas were chiefly from Cameroon. The team sequenced 14,000 base pairs of noncoding (and therefore presumably not under selection) nuclear DNA from each gorilla. Disparities turned up at 79 different sites, similar to the genetic diversity in chimpanzees, but twice as high as in humans, who are remarkable for their lack of variation. "This shows us again how odd humans are," says Stone.

Vigilant's team used the number of genetic differences to calculate a mutation rate, which allowed them to date the timing of the initial speciation to about 900,000 to 1 million years ago. That's just when the two species of chimpanzees went their separate evolutionary ways, suggesting that changes in climate broke up the dense forests that are home to both chimps and gorillas.

Although chimpanzees sorted into two species rapidly, gorillas took much longer and continued to mate at low levels until 164,000 to 230,000 years ago, with males moving more than females, says Vigilant. That's surprising because today the two species live 1000 kilometers apart and most gorillas never venture far from home. "I find it amazing that gene flow persisted for hundreds of thousands of years," she says.

East met west. Eastern gorillas (right) bred with their western cousins until recently.
CREDITS: J. KALBITZ/MPI-EVA; M. M. ROBBINS/MPI-EVA

 

Her study offers a rare window on speciation in apes, which serves as "a model for understanding human evolution," says Jensen-Seaman. Until now, researchers have focused on the quick split between species of chimpanzees. In fact, says Jensen-Seaman, gorillas' "sloppy back-and-forth gene flow--a long, drawn-out process"--may be the norm.