Gene expression is the process by which genes are turned on or off. Not all of the estimated 30,000 genes in humans are activated at the same time in every cell.

"We think gene expression is a major part of what separates chimps and humans," said Kevin White, an associate professor of genetics, ecology and evolution at Yale University in the United States.

White and researchers from the University of Chicago in Illinois and the Hall Institute in Parkville, Victoria in Australia looked at gene expression in humans, chimpanzees, orangutans and rhesus monkeys.

They used new gene-array technology to compare the level of expression of 1,056 genes in the four species.

"When we looked at gene expression, we found fairly small changes in 65 million years of macaque, orangutan and chimpanzee evolution," said Dr Yoav Gilad of the University of Chicago, lead author of the study.

But he said it was followed by quick changes in specific groups of genes known as transcription factors, which control the expression of other genes, since humans diverged from their ape ancestors during the last 5 million years.

"This rapid evolution in transcription factors occurred only in humans," Gilad added in a statement.

The research, which is published in the journal Nature, supports a 30-year-old hypothesis by scientists Mary-Claire King and Allan Wilson who suggested that key differences between humans and chimpanzees might be found in the way they express their genes.

Until the mapping of the human genome and the development of gene array technology that allows for large-scale analysis of gene expression, it has not been possible to test the hypothesis.

Gilad, White and their colleagues used samples of liver tissue from five adult males from each of the four species in their study.

They found about 60 percent of the genes had consistent levels of expression in humans and the primates.

But genes for transcription factors were more likely to have changed their expression patterns than the genes they regulate.

"Specifically in the human lineage the transcription factors are changing or evolving in their expression at a faster pace than in the other lineages, particularly as compared with chimps," White said.

The researchers do not know what caused the shift in gene expression in humans but they suspect it could be due to changes in the environment, the acquisition of fire and a preference for cooked food.