Sally McBrearty (top) and Nina Jablonski. Credit: A. HILL (TOP); D. LIN/CALIF. ACAD. SCI.

Examining a fossilized molar in Nairobi last October, Nina Jablonski was thinking just one thing: “This is no monkey.” It should have been. After all, Jablonski, an anthropologist at the California Academy of Sciences, was in the Kenyan capital to study a collection of monkey fossils unearthed at the Kapthurin Formation in the East African Rift Valley.

But the size and shape of the tooth were wrong. “My first thought was, ‘Oh my gosh, this is a gibbon’,” Jablonski says. “Then I realized, no, it was almost certainly a chimp.” Further searching among the monkey fossils turned up a second anomaly; this time, an incisor.

The collection had been amassed by Sally McBrearty, an anthropologist at the University of Connecticut, and her team who were in Kenya looking for stone tools and fossils. Jablonski's musings echoed McBrearty's suspicions. “I believed it was an ape,” McBrearty recalls. The result was the paper co-authored by the pair on page 105 of this issue.

Jablonski compared the fossils with teeth of modern chimps at the National Museums of Kenya. The molar, with its distinctive crown features, was a perfect match. “I sent off a very excited e-mail to Sally,” Jablonski says. “She was thrilled because she had suspected that the teeth did not belong to a monkey.”

McBrearty immediately sent her team back into the field. By March, the researchers had found more chimp teeth, bringing the total to four, three of which are described in this issue.

“This is the first unequivocal example of a modern chimpanzee in the fossil record — no question,” Jablonski says.

McBrearty's team has yet to uncover any other chimp remains. Teeth are often the only fossils left, as they are harder than other parts of the body and less likely to be crushed or decay. “I'm hoping to go back and spend more time looking for the rest of this creature,” says McBrearty.

The discovery of the chimp teeth was a major surprise, Jablonski says. No fossils of modern apes had ever been found in Africa — and to cap it all, the remains came from the Rift Valley. Today, chimps are found only in western and central Africa; the Rift Valley was seen as a geographical barrier to the species, preventing its spread farther east. But the teeth, which are about 500,000 years old, put chimps in a part of the continent where they weren't known to exist. And, more importantly, they put the chimps side-by-side with a hominid.

Although she is aware of implications for evolution and anthropology, Jablonski sees some irony in the discovery of the teeth. She has been working on monkey fossils for 30 years, and has made many finds that characterize that group's evolution, but few people outside her speciality “gave a hoot or a holler” about such work. The chimp teeth attracted much more attention because of the close evolutionary link between the apes and humans.

Jablonski has now returned to her primary research interest. “I'm happily going back to monkeys,” she says. “But I certainly won't ignore any chimps that cross my trail.”