The fact that there is only a about a 1% difference between the genetic make-up of chimpanzees and human has been called "the most overly exposed factoid in modern science". First established in a paper in 1975, it was confirmed a couple of years ago by the Chimpanzee Sequencing and Analysis Consortium. However, a feature in the journal Science points out that this figure has enormous limitations. "For many, many years, the 1% difference served us well because it was under-appreciated how similar we were," says Pascal Gagneau, a zoologist at the University of California, San Diego. "Now it's totally clear that it's more a hindrance for understanding than a help."

The consortium pointed out that many stretches of DNA have been inserted or deleted in the genes. They account for an additional 3% difference. Entire genes are often reduplicated or lost, further distinguishing chimps from humans. Recent research shows that human and chimpanzee gene copy numbers differ by 6.4%.

Is it possible to propose a precise figure for the difference between the two species? Probably not, scientists feel. "I don't think there's any way to calculate a number," says Svante P??bo, a member of the consortium working in Germany. "In the end, it's a political and social and cultural thing about how we see our differences."