Scientists have long wondered why humans and chimpanzees are so different in appearance and behaviour despite sharing about 97 per cent of the same genes – but now they've found an answer in how the genes work.

U.S. geneticists compared genes from four primates and found some human ones showed a much higher rate of mutation, according to research published in Thursday's issue of the journal Nature.

The changes were concentrated in a group of key genes known as transcription factors that control the "gene expression," or activities, of other genes.

Genes are the codes for making proteins.

"When we looked at gene expression, we found fairly small changes in 65 million years of the macaque, orangutan, and chimpanzee evolution," Yoav Gilad, a human genetics specialist at the University of Chicago, said in a news release.

"The big question is why are humans so different," said Gilad. "What sort of changes in the environment or lifestyle would drive such a rapid shift in the expression of genes – in this case in the liver– in humans and in no other primate?"

When scientists sequenced genomes, they discovered that humans and chimps share most of their genes.

They had long speculated that the way genes expressed themselves could explain the differences, but little was known about it worked in different species.

The research team led by Gilad compared more than 900 genes from humans, macaques, orangutans and chimpanzees.

They found that the major changes in human genes occurred in the last five million years of human evolution, when humans split from a common primate ancestor.

Gilad speculated that part of the high rate of mutations might be rapid changes in diet as humans learned to use fire to cook food, an adaptation found in no other animal.

The researchers also note that changing the ancient genes could be harmful.

They point out that mutations in five of the 100 most stable genes are linked to liver cancer.