Scientists have come a step closer to understanding one of the key moments in man's evolution when the human brain underwent a dramatic increase in size.
Our brain is three times larger than that of our closest living relative - the chimpanzee - and yet how the transformation happened is one of the biggest mysteries of human evolution.
A large brain is the physical essence of humanity. It enabled language, consciousness and culture and yet scientists can only speculate as to why it evolved to be so much bigger than the brain of our ape-like ancestors.
A comparison of the human genome with that of the chimp - along with several other animals - has now revealed the existence of a key region of man's DNA that appears to play an important role in the growth of the human brain.
Many different animals possess the same region of DNA but it is only in humans that it has undergone a rapid and dramatic evolutionary change.
The difference between chickens and chimps - which are separated by 310 million years of evolution - is just two mutations out of a total DNA sequence of 118 "letters" of the genetic code. Yet the difference between chimps and humans - separated by 6 million years - is 18 mutations in the same DNA region.
Professor Katherine Pollard of the University of California, Davis, said that this region of the human genome has changed more than any other since humans diverged from the rest of the apes.
"It's evolving incredibly rapidly. It's really an extreme case," Professor Pollard said. "We found 18 differences between chimps and humans, which is an incredible amount of change to have happened in a few million years."
The gene, called human accelerated region 1 (HAR1), is one of 49 that the scientists have identified as changing the most since humans diverged from our chimp-like ancestors.
"Some DNA regions have hardly changed at all over many millions of years in most species. My twist was to look for the subset of these regions that have changed just in humans," Professor Pollard said.
Further research showed that the region is most active during the crucial gestation period in the womb when the human brain begins to form the outer cortex, the brain region responsible for "higher" activities such as language and consciousness.
Professor David Haussler, of the University of California, Santa Cruz, said the precise function of the gene is not yet known but it may have been responsible for the expansion of the brain during human evolution.
Professor Haussler said: "At this point we can only speculate about this gene's role in the evolution of the human brain, but it's exciting to find a new gene involved in brain development."
"The evidence is very suggestive that this gene is important in the development of the cerebral cortex, and that's exciting because the human cortex is three times as large as it was in our predecessors," he said.
"Something caused our brains to evolve to be much larger and have more functions than the brains of other mammals."
The region works differently to ordinary genes in that it does not appear to "code" for a protein but instead it produces a short molecule of a DNA-like substance called RNA.
The study, published in the journal Nature, shows that the RNA molecule formed from the human DNA was substantially different in structure to the RNA made from the chimp's DNA.