WE MAY owe our big brains and sophisticated culture to a single genetic mutation that weakened our jaw muscles about 2.4 million years ago. The slack muscles relaxed their hold on the human skull, giving the brain room to grow, a new study suggests. Meanwhile, other primates were stuck with stupendous jaw muscles that squeezed the skull tight.
Over the past 2.5 million years, the human brain has grown much bigger than those of other primates, and it is now roughly three times the size of chimp or gorilla brains. A possible reason is that environmental changes forced early humans to invent tools, and those with the biggest brains had the greatest manual dexterity, which led to yet more sophisticated use of tools. Or perhaps selection favoured larger brains because they allow more complex culture.
But why did this process occur in humans and not in other primates? According to Hansell Stedman of the University of Pennsylvania in Philadelphia, it was because of a simple mutation in a gene that is active in our jaw muscle tissue.
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As part of a study of the disease muscular dystrophy, Stedman and his colleagues looked at muscle from a macaque monkey to track down a gene thought to play a role in muscle contraction. 鈥淲e quickly found out this gene was expressed only in the powerful bite muscle,鈥� says Stedman. Bite muscle is the most powerful of the jaw-closing muscles and completely encloses the skull in all non-human primates.
The team found that humans have the same gene, which is also active in human bite muscle 鈥� but with a crucial difference. Compared to the macaque gene, the human gene has two missing base pairs in a key region. Stedman鈥檚 team studied the gene in people from all over the world 鈥� including natives of Africa, South America, western Europe, Iceland, Japan and Russia 鈥� and in seven species of non-human primate. Every human had the mutation, whereas none of the animals did.
When the team looked closely at the structure of the bite muscle, they found that in humans the muscle fibres were far smaller than those in other primates, suggesting that the mutation has reduced the mass of this muscle in humans. The weaker muscles would have exerted considerably less force on the skull, allowing it to grow and expand (Nature, vol 428, p 415).
Detailed genetic analysis suggests that the mutation responsible for the difference occurred approximately 2.4 million years ago. Shortly afterwards, the earliest known members of the genus Homo appeared 鈥� with smaller jaws and larger brains.
Peter Currie, an expert on skeletal muscle development at the Victor Chang Cardiac Research Institute in Darlinghurst, Australia, calls the finding 鈥減retty amazing鈥�. 鈥淐hanges in muscle anatomy are well known to alter the bones to which they attach,鈥� he says. 鈥淭he exciting part of this is that the mutation in the gene dates to exactly when this transition occurs in the fossil record.鈥�
Paul Pettitt, an expert on human origins at the University of Sheffield, UK, thinks the mutation could explain the earliest appearance of brains bigger than 500 cubic centimetres in Homo habilis and Homo rudolfensis around 2 million years ago. Both these species had significantly smaller jaw muscles than their ancestors, the australopithecines. 鈥淭his is a fascinating discovery which potentially sheds light on the origins of the genus Homo,鈥� says Pettitt. 鈥淚t鈥檚 certainly a very plausible reason for a relatively late beginning of the rise of brain size above and beyond the ape norm.鈥�