Now we know just what that mysterious monolith in 2001 was REALLY doing.

From The Australian: A SINGLE mutation in one jaw muscle gene is the first key genetic difference to be found between between humans and apes. What's more, the mutation - estimated to have occurred 2.4 million years ago - may have been the critical change that sent ancestral humans down a different evolutionary path than their primitive primate cousins, claim researchers at the University of Pennsylvania in Philadelphia.

"This is the first genetic distinction between humans and chimpanzees that can be linked to the fossil record," the team's leader, Hansell Stedman, said yesterday.

Details on the story continue from the Sydney Herald: The US team had initially been on a medical rather than evolutionary quest, looking for human genes linked to a family of proteins called myosins that make muscles contract. They discovered a new myosin gene, called MYH16, that had one small mutation which made it inactive. This mutation turned out to be present in all the people they tested from Africa, South America, western Europe, Iceland, Japan and Russia, which made the researchers wonder why it was so common.

They then analysed the DNA of seven species of apes, including our closest relatives, the chimpanzees, and found they all had the active, rather than the mutated, version of the gene. By studying a wide range of muscle types, they discovered the gene was only switched on in monkeys in facial muscles that control chewing and biting. These muscles are much larger and more powerful in apes and monkeys than in humans, and put a lot of stress on the top of the skull.

The final step in the team's sleuthing was to compare the sequence of "letters" of DNA in the MYH16 gene from humans with those from five other animals, including dogs. This allowed them to estimate that the mutation arose by chance about 2.4 million years ago, after humans and chimpanzees had diverged. This fitted well with the fossil evidence that early humans with large, round skulls and small jaws began to appear about 2 million years ago. The findings are reported today in the journal Nature.