( e of geographic isolation. The or...)Gene study shows three distinct groups of chimpanzees

The original boundaries between groups may have been the emergence and growth of rivers, such as the Congo River, which is thought to be about 1.5 million years old. "Chimps don't swim," Becquet said. "For them, water provides a very effective border." The ongoing loss of habitat has increased the physical separation between the three groups.

The extent of accumulated genetic difference enabled the researchers to speculate about when the different populations separated. They estimate that bonobos, which live south of the Congo River, split off from the ancestors of modern chimpanzees about 800,000 years ago. Western chimps appear to have separated from central and eastern chimpanzees about 500,000 years ago and central and eastern chimps divided about 250,000 years ago.

"Even though the chimp genome has been sequenced, it's amazing how little we know about their evolution and the level of variation within chimpanzees," said Przeworski. "These are our nearest relatives, closer to humans than they are to gorillas, yet we know so little about them, and even less about gorillas and orangutans."

The chimpanzee genome differs from the bonobo genome by about 0.3 percent, which is one-fourth the distance between humans and chimps. Yet chimps and bonobos have radically different social systems, cultures, diets and mating systems.

On the other hand, in this study, looking at three "subspecies" of common chimpanzees, "we found significant genetic variation," said Przeworski, "but there's very little detectable difference between the populations in terms of appearance or behavior."