Unlike our human ancestors, whose fossil remains are relatively plentiful, chimps have always been conspicuously absent from the fossil record. Many experts doubted such specimens could exist because most chimps live in the rain forests of West and central Africa where acidic soil and high rainfall levels hamper fossil preservation.

Early humans, on the other hand, lived in areas more arid areas conducive to fossil preservation but relatively hostile to chimp survival, such as the East Africa Rift Valley. 鈥淚t鈥檚 the last place you鈥檇 expect to find chimps,鈥� says anthropologist Jay Kelley of the University of Illinois at Chicago, US.

But in a sediment of the Kapthurin Formation in the Eastern Rift Valley, Sally McBrearty of the University of Connecticut and Nina Jablonski of the California Academy of Sciences did find three fossils with the unambiguous characteristics of chimp teeth.

Branching out

The two incisors and a molar, probably all from the same individual, date from around 500,000 years ago. They were found in sediments that include fossils of two early humans 鈥� Homo erectus or Homo rhodensiensis 鈥� suggesting that chimp and human ancestors were contemporaries. A possible fourth tooth is still awaiting analysis.

On their own, the fossils will not tell us much, says palaeontologist Tim White of the University of California, Berkeley, 鈥淪o little of the anatomy is preserved that these fossils do not teach us much new about chimpanzee anatomical evolution.鈥� But, the finding does suggest that chimp ancestors were more capable of adapting to a broader range environments than previously thought. 鈥淭here is no logical necessity why chimps had to remain in the forest,鈥� says McBrearty.

She believes her discovery 鈥渉as opened the door鈥� for others to look for chimp fossils in unexpected places. It might even be possible to find much older chimp fossils from a time near the split in human and chimp lineages. White believes this is quite probable. 鈥淚t is only a matter of time,鈥� he predicts.

The chimp genome sequence, which consists of 2.8 billion pairs of DNA letters, will not only tell us much about chimps but a comparison with the human genome will also teach us a great deal about ourselves.

鈥淭he major accomplishment is that we now have a catalogue of the genetic differences between humans and chimps,鈥� says lead author, Tarjei Mikkelsen of the Broad Institute in Cambridge, Massachusetts, US.

In keeping with previous studies comparing much smaller portions of the chimp and human genomes, the new comparison shows incredible similarity between the genomes. The average number of protein-changing mutations per gene is just two, and 29% of human genes are absolutely identical. What is more, only a handful of genes present in humans are absent or partially deleted in chimps.

鈥淣o silver bullet鈥�

But the degree of genome similarity alone is far from the whole story. For example, the mouse species Mus musculus and Mus spretus have genomes that differ from each other to a similar degree and yet they appear far more similar than chimps and humans.

Domestic dogs, however, vary wildly in appearance as a result of selective breeding and yet their genome sequences are 99.85% similar. So most of the differences between chimp and human genomes will turn out to be neither beneficial nor detrimental, in evolutionary terms.

The real challenge then will be finding the changes that played a major role in the evolution of chimps and humans since the two lineages split, 5 to 8 million years ago. Nothing obvious has leapt out of the initial analysis. 鈥淔rom this study, there鈥檚 no silver bullet of what makes chimps chimps and humans humans,鈥� says Evan Eichler of the University of Washington at Seattle, US.

Rapid evolution

Comparing the two genomes has thrown up numerous candidates for what makes humans different though. One such set came by comparing 13,454 specific genes in the chimp and human genomes, looking for signs of rapid evolution. For each gene, the researchers compared the number of single letter mutations that alter the encoded protein versus silent mutations that have no effect.

Silent mutations are possible because most amino acids are coded by more than one 3 letter DNA 聭word鈥� 鈥� for example, proline is coded by CCU, CCC, CCA, and CCG, so a change at the third position makes no difference to the protein.

Comparing the two types of mutations allowed the team to spot genes that have had changes favoured by natural selection while taking into account the background mutation rate. And 585 of the genes studied in this set 鈥� many involved in immunity to infections and reproduction 鈥� had more protein-altering mutations than silent ones. Mikkelsen believes these will be a good place to look for genes that make humans different from chimps.

But comparing genome sequences can only tell scientists so much. Now begins the methodical job of homing in on the promising parts of the sequence and identifying the differences that count.

鈥淭his is best viewed as an exciting starting point,鈥� says Simon Fisher at the Wellcome Trust Centre for Human Genetics at Oxford University, US. 鈥淚n the same way that knowledge of our own genome sequence has not automatically led to a full understanding of human biology, so the decoding of other primate genomes will not, by itself, reveal exactly what sets us apart.鈥�

Journal reference: Nature (vol 437, p69)