Scientists believe they have found a key gene that helped the human brain evolve from our chimp-like ancestors.
In just a few million years, one area of the human genome seems to have evolved about 70 times faster than the rest of our genetic code.
Study co-author David Haussler, director of the Center for Biomolecular Science and Engineering at the University of California, Santa Cruz, said his team found strong but still circumstantial evidence that a certain gene, called HAR1F, may provide an important answer to the question: "What makes humans brainier than other primates?"
Human brains are triple the size of chimpanzee brains, although humans are physically only twice as large.
Looking at 49 areas that have changed the most between the human and chimpanzee genomes, Haussler zeroed in on an area with "a very dramatic change in a relatively short period of time."
That one gene didn't exist until 300 million years ago and is present only in mammals and birds, not fish or invertebrates. But then it didn't change much at all.
There are only two differences in that one gene between a chimp and a chicken, Haussler said. But there are 18 differences in that one gene between human and chimp, and they all seemed to occur in the development of man, he added.
Andrew Clark, a Cornell University professor molecular biology who was not part of Haussler's team, said that if true, the change in genes would be fastest and most dramatic in humans and would be "terrifically exciting."
However, the gene changed so fast that Clark said that he has a hard time believing it unless something unusual happened in a mutation. It's not part of normal evolution, he said.
Haussler attributed the dramatic change to the stress of man getting out of trees and walking on two feet.
And it's not just that this gene changed a lot. There is also its involvement with the cerebral cortex, which is responsible for some of the more complex brain functions, including language and information processing.
"It looks like, in fact, it is important in the development of the brain," said co-author Sofie Salama, a research biologist at Santa Cruz who led the efforts to identify where the gene is active in the body.
The scientists still don't know specifically what the gene does. But they know that this same gene turns on in human fetuses at seven weeks after conception and then shuts down at 19 weeks, Haussler said.