Scientists have identified a gene that appears to have played a key role in the development of the human brain.
The human brain is highly evolved
They believe it is responsible for the expansion of an area of the brain called the cerebral cortex which controls abstract reasoning.
This is much bigger in humans than their closest relatives - and gives us our extraordinary brain power.
The research, by Howard Hughes Medical Institute, US, is published in the journal Human Molecular Genetics.
Lead researcher Dr Bruce Lahn said: "People have studied the evolution of the brain for a long time, but they have traditionally focused on the comparative anatomy and physiology of brain evolution.
"I would venture, however, that there really hasn't been any convincing evidence until now of any gene whose changes might have contributed to the evolution of the brain."
PRIMATE SPECIES STUDIED
The Howard Hughes team focused on the particular gene, called ASPM, because mutations in its make-up are known to be linked to severe reductions in the size of the cerebral cortex in people who carry them.
They compared the make-up of the human form of the gene with that in six other primate species, each of which corresponds to a key stage in the evolutionary path of modern man.
They ranged from the chimpanzee, which is man's closest living relative, to the owl monkey, a relatively primitive creature analogous to an early stage of human evolution.
They found evidence that the make-up of the gene changed significantly between the species - and the higher up the evolutionary scale they went, the more changes they found.
The biggest difference was found between the human and chimpanzee forms of the gene - confirming that the recent phase of human evolution has been the most speedy and profound.
By contrast, when researchers looked at the make-up of the gene in more primitive animals such as cows, sheep, cats and dogs they found little evidence of significant changes between species.
Man's closest living relative
This implies that the speed of change was much slower further down the evolutionary ladder.
Dr Lahn said: "The fact that we see this accelerated evolution of ASPM specifically in the primate lineage leading to humans, and not in these other mammals, makes a good case that the human lineage is special."
The next stage in the research will be pin down exactly how ASPM functions in the brain.
Research has suggested that the gene regulates the rate at which brain cells are produced in the cerebral cortex.