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Nature
Published online: 16 April 2007; | doi:10.1038/446841a
Chimps lead evolutionary race
More chimpanzee genes have been positively selected for than human ones.Michael Hopkin 
| Chimpanzees have at least 233 genes thought to be shaped by selection for
beneficial mutations. Alamy |
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Humans
are generally believed to be more highly 'evolved' than our chimpanzee
cousins. But in at least one sense that isn't true, say geneticists who
have hunted for the hallmarks of natural selection in our respective
genomes - and found more of them in chimps.
The discovery suggests that, since our evolutionary paths diverged 6
million years ago, greater numbers of chimpanzee genes have been shaped
by 'positive selection', in which natural selection favours beneficial
mutations.
Researchers at the University of Michigan, Ann Arbor, combed through
14,000 matching genes from the human and chimpanzee genomes. As they
report in Proceedings of the National Academy of Sciences this week, 233 chimp genes showed signs of having been shaped by positive selection (M. A. Bakewell, P. Shi and J. Zhang Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0701705104; 2007). The corresponding figure for our own genes was just 154.
The result overturns the view that, to promote humans to our current
position as the dominant animal on the planet, we must have encountered
considerable positive selection, says lead author Jianzhi Zhang. "We
think we're very different from animals, with our large brain size and
speech," he says.
The gene discrepancy might be due to the fact that, for much of our
histories, chimpanzees had the larger population size. Humans, with a
smaller and more fragmented population, may have been shaped by random,
erratic changes.
 It is possible that the genetic changes underlying brain size are very
few.
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It is difficult to put together a coherent picture, says Zhang, because
it is hard to know which genes would have been crucial in shaping
traits such as our large brain size. "It is possible that the genetic
changes underlying brain size are very few," he says.
A sample of 14,000 genes does not tell the whole story. The team could
not compare the entire genome as the chimp sequence has not been
completed to the same level of detail as the human one. But for genes
with good sequences, they were taken to show signs of positive
selection if they had a high proportion of 'non-synonymous mutations' —
DNA changes that alter the protein sequence produced by the gene —
which could be a 'lever' for natural selection.
Zhang admits it is difficult to spot genes that have been the subject
of more recent positive selection. Such genes could have been
responding to selection pressures — such as changes in climate and food
source — encountered by humans as they began to move out of Africa and
across the planet over the past 100,000 years.
There also seems to be little pattern to the functions of the selected
genes, says Zhang. Among those favoured in chimps are genes for protein
metabolism and stress responses, whereas the human genes are involved
in processes such as fatty-acid metabolism.
Victoria Horner, who works with chimpanzees at Yerkes National Primate
Research Center in Atlanta, Georgia, says: "We assume that chimpanzees
have changed less than us, when that's actually not the case."
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