Chimpanzees have evolved more extensively than humans since the two species split from their common ancestor, according to research that challenges the conventional wisdom that Homo sapiens has been favoured by natural selection.

A comparison of the genomes of the two close cousins has revealed that many more chimp genes than human ones have been the subject of positive evolutionary selection.

The findings have surprised scientists, as it has long been assumed that the many unique features that separate people from other apes, such as our large brains and upright walking posture, must have been the result of evolutionary pressures that weighed most strongly on humans.

The study, from a team at the University of Michigan, suggests instead that chimps may actually have adapted more to their environment as a result of natural selection. It underlines that evolution is not a matter of progress towards a goal, and that it is incorrect to assume that more intelligent species are “more evolved”.

“Our results show that the number of positively selected genes is substantially smaller in humans than in chimps,” said Jianzhi Zhang, who led the study. “These observations . . . refute the anthropocentric view that a grand enhancement in Darwinian selection underlies human origins.”

The work may also indicate that chimps have many more adaptive variations than have been categorised by scientists so far, and that these have simply been less obvious than human ones because of our greater familiarity with our own species.

“In sharp contrast to common belief, there were more adaptive genetic changes during chimp evolution than during human evolution,” Dr Zhang said. Chimpanzees are humanity’s closest living relatives, with the two species sharing almost 99 per cent of their functional genes. Both apes are descended from a common ancestor that lived about seven million years ago.

In the study, which is published in the journal Proceedings of the National Academy of Sciences, the researchers compared 14,000 human and chimp genes to see how they had been affected by evolution since the split. They particularly looked for genes that showed evidence of positive selection — those that had evolved quickly because of an adaptive benefit in the environment — rather than those that had changed more slowly by random genetic drift. They found 154 human genes that showed evidence of the rapid positive selection that marks out adaptive traits, but 233 chimp genes with the same qualities.

Dr Zhang said that the lower than expected proportion of adaptive genes among humans could mean that genetic drift — random mutations that spread even though they are not advantageous — could have played a bigger role in human evolution than is usually thought.

This could reflect the young age of Homo sapiens, which emerged in anatomically modern form about 150,000 years ago, and the very low populations of the earliest groups.

Drift can have a bigger influence in small populations, and humanity is known to have passed through a bottleneck in which only a few thousand individuals survived. It is also possible that there are more adaptive genes in the human genome that have been positively selected, but that these emerged too recently to have been detected by the study.

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