Humans may have their puny jaws to thank for growing big brains, according to new American scientific research.
Researchers have discovered a genetic defect that led the ape-like ancestors of humans to develop weak jaw muscles 2.4 million years ago.
The mutation preceded the first appearance of modern human-like features in the fossil record, including a large braincase.
This led the American scientists to reach a remarkable conclusion. It was the weakened jaw -- effectively an inherited "disease" -- that allowed human skulls and brains to grow.
Nancy Minugh-Purvis, from the University of Pennsylvania in Philadelphia, said: "The coincidence in time of the gene-inactivating mutation and the advent of a larger braincase ... may mean that the decrease in jaw muscle size and force eliminated stress on the skull which released an evolutionary constraint on brain growth.
The researchers found the small, previously overlooked, mutation while comparing an "anonymous" piece of human DNA with genes known to power muscle contraction.
Tests of DNA samples from a number of different human populations showed it was no anomaly. The gene-inactivating mutation existed in people from Africa, South America, Western Europe, Iceland, Japan and Russia.
But the mutation was not present in the DNA of seven species of non-human primates, including chimpanzees, the scientists reported in the journal Nature.
Studies of monkeys revealed that the protein obtained from MYH16 was only found in a group of related head muscles chiefly involved with chewing and biting.
In humans, the mutation meant that although the manufacturing machinery was still present, none of the muscle protein could be made.
The functional gene appeared to be the reason why the jaw muscles and their bony attachments were much larger and more powerful in monkeys and apes than in humans. In macaque monkeys, the chewing and biting muscles were nearly 10 times as large.
By looking at the coding sequence of the gene, the researchers estimated that the mutation appeared about 2.4 million years ago.
This was after the lineages leading to humans and chimpanzees diverged. About 400,000 years later, the weaker-jawed, larger brained skulls of the earliest known members of the species family Homo started to appear.
These creatures were the forerunners of modern humans, Homo sapiens.
However the researchers cannot explain why the early human ancestors held on to the mutation in the first place.
If the trait had been a handicap, it would have been selected out.
In an accompanying article, Pete Currie from the Victor Chang Cardiac Research Institute in Sydney, Australia, wrote: "Several explanations could be advanced to counter this ideological roadblock, such as a contemporaneous shift in diet [say, to an increased reliance on meat eating], or a growing dependence on hands rather than the jaw in food preparation."
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