"We were going through these [fossils] at the end of the season and I realized this was something unusual," she said.
Jablonski then came to Kenya to identify the fossils. She separated a few fossils that weren't easily identified and started looking at a single molar.
When she realized it belonged to an apea type of fossil that had not yet been foundshe raced over to another section of the museum that stores bones of modern animals. She pulled open the drawer for chimpanzees and compared the two.
"It wasn't just a possible match," she said. "It was a good match." She also identified another tooth, an upper incisor, from the same collection.
Two more chimpanzee teeth were later found at the site, an upper incisor and a tooth tentatively identified as a molar. The possible molar will be a subject for future study.
The fossil teeth are very similar to the teeth of modern chimpanzees, Jablonski said. "There's nothing that would cause us to believe that they were substantially different from chimpanzees today."
Even more exciting would be the discovery of early chimpanzee fossils from four to five million years ago, the time when hominins started to appear, said Sanders in Michigan.
Chimpanzees are key to learning about human evolution, because the chimpanzee is the closest living relative of humans. Uncovering chimpanzee fossils from an earlier time period might help people learn more about how and why chimpanzees and humans went their separate ways.
Few Get Fossilized
Modern chimpanzees live primarily in forests in western Africa, far from the arid Rift Valley site where these first fossils emerged.
But the valley's geology, along with the presence of water-loving species like the hippopotamus and the crocodile, suggest the fossil chimp and hominins experienced a lush, more wooded environment than what's present today.
While stories about new discoveries might make fossils seem routine for those who know how to look for them, fossil finds are actually quite rare.
Most animal bones aren't preserved when the creature dies, McBrearty says. The animal has to be buried and its bones replaced by minerals in order to form fossils.
Forest creatures of any species, including chimpanzees, are especially tough to track down in the fossil record. When a forest dweller dies, scavengers and invertebrates quickly step in. Within weeks the entire animal often disappears.
Forest soils are often acidic, making quick work of remains that haven't been destroyed by other creatures.
In contrast, the Rift Valley in eastern Africa provides suitable conditions for preserving fossils, McBrearty says. Here, lakes and streams deposit sediment that erodes off the highlands, burying potential fossils in the valley below.
The valley is also prone to earthquakes that expose the fossils for researchers on the hunt.
McBrearty wants to keep up the search for chimpanzee fossils in the same fossil-friendly area.
While the chance of unearthing more ape fossils may be low, Jablonski said, "this find indicates that it's certainly worth looking."
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES