Sally McBrearty unearthed some chimp teeth where no one expected to find them.© Andrew Hill

Palaeontologists digging in the dusty wastelands of East Africa have discovered the first known chimpanzee fossil. The modest haul of just three teeth is the first hard evidence of the evolutionary path that led to today's chimpanzees.

As well as shedding light on chimps, the find throws up new questions about human evolution; it seems that chimpanzees may not have been physically separated from humans as was once thought.

That no one had previously found a chimpanzee fossil had long been a frustrating puzzle, comments Sally McBrearty, an anthropologist at the University of Connecticut, who made the find near Lake Baringo, Kenya, with her colleague Nina Jablonski. Set against the many human fossils found in East Africa, the lack of specimens documenting the chimp's evolutionary story was exasperating.

Part of the problem, McBrearty explains, is that chimps tend to live in hot, wet jungle conditions that are not good for the preservation of remains. Humans, on the other hand, are thought to have lived for millennia on the savannah, where bones are less likely to rot.

The great divide

Previous theories suggested that chimps never crossed east of the Rift Valley, but instead stayed in the jungles of western and central Africa. Some even suspected that this physical separation was what set the earliest chimp and human ancestors on contrasting evolutionary voyages. But now McBrearty has stumbled on chimp remains east of this divide.

This means we need a better explanation of why and how chimps and humans went their separate evolutionary ways, McBrearty says. The discovery that chimps were living in semi-arid conditions as well as in the jungle seems to blow apart the simplistic idea that it was the shift to savannah that led to humans walking upright.

The teeth are around 500,000 years old, McBrearty and Jablonski report in Nature¹. So far it is impossible to say whether they belonged to the same species as modern chimps, Pan troglodytes, or to some unnamed, now extinct ancestor. "It wouldn't surprise me if there are lots of extinct chimp species," McBrearty says.

If the teeth do belong to the same species as modern chimps, this would mean the species is quite long-lived. In contrast, modern Homo sapiens has been around for only some 200,000 years. But the earlier human species H. erectus is thought to have lasted around a million years.

Finding the ancestor

The fossils are not old enough to tell us about the common ancestor of chimps and humans, which lived between five and seven million years ago, points out anthropologist Daniel Lieberman of Harvard University in Cambridge, Massachusetts. "But this raises hope that we can find older stuff," he adds.

McBrearty suspects that although there may have been more chimps living in the jungles of western Africa, there are probably more fossils in the dry eastern savannah. It's just that "no one was looking for them" she says.

McBrearty hopes to return to Kenya in December to resume the search. In spite of the baking equatorial heat, December's dryness makes it the best time to probe for delicate remains. 

University of Connecticut

• References

  1. McBrearty S. & Jablonsk N. G. Nature, 437. 105 - 105 (2005). | Article | PubMed | ISI | ChemPort |