http://www.newscientist.com/news/news.jsp?id=ns99994817

Early humans swapped bite for brain
 
18:00 24 March 04
 
NewScientist.com news service
 
Humans owe their big brains and sophisticated culture to a single genetic mutation that weakened our jaw muscles about 2.4 million years ago, a new study suggests.

The slack muscles relaxed their hold on the human skull, giving the brain room to grow. Other primates remained stuck with mighty muscles that squeezed the skull in a vice-like grip.

The finding is "pretty amazing", says Peter Currie, an expert on skeletal muscle development at the Victor Chang Cardiac Research Institute in Darlinghurst, Australia: "Changes in muscle anatomy are well known to alter the bones to which they attach. The exciting part of this is the mutation in the gene dates to exactly when this transition occurs in the fossil record."

Over the past 2.5 million years, human brains have grown enormous compared to those of other primates. Human brains are now roughly three times the size of those of chimps or gorillas.

One possible reason is that changes in the environment forced early humans to invent tools, and those with the biggest brains had greatest manual dexterity, which led to yet more sophisticated tool use. Alternatively, selection may have favoured larger brains because they permitted more complex cultures.

But why did this process occur in humans and not in other primates? According to Hansell Stedman, an expert on muscle disorders at the University of Pennsylvania in Philadelphia, it was a simple mutation in a gene found in our jaw muscles.

Gene tracking

While studying muscular dystrophy, Stedman and his colleagues were tracking down a gene thought to play a role in muscle contraction using specimens from a macaque.

"We quickly found out this gene was expressed only in the powerful bite muscle," 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 the same gene was also active in a sample of human bite muscle, but there was one crucial difference. Compared to the macaque gene, the human gene had two missing base pairs in a key region.

In follow-up work, Stedman's team studied the gene in people from all over the world, including natives of Africa, South America, Western Europe, Iceland, Japan and Russia. They also studied seven species of non-human primates, including gorillas and chimpanzees. Every human had the mutation, whereas none of the animals did.

Small fibres

To nail down the effect of the mutation, Stedman's team looked closely at the structure of the bite muscle tissue where the gene is expressed. They found that the muscle fibres in humans were far smaller than those in other primates, suggesting the mutation reduced muscle mass in people. The weaker muscles would have exerted considerably less force the skull, allowing it to grow and expand.

Detailed genetic analysis suggests the human mutation occurred approximately 2.4 million years ago. Shortly after that, the earliest known members of the genus Homo appeared - with smaller jaws, and larger brains.

Paul Pettitt, an expert on human origins at the University of Sheffield, thinks the mutation could explain the earliest appearance of brains bigger than 500 cubic centimetres in Homo habilis and Homo rudolfensis around two million years ago. Both these species had significantly smaller jaw muscles compared to their ancestors, the australopithecines.

"This is a fascinating discovery which potentially sheds light on the origins of the genus Homo," says Pettitt. "It's certainly a very plausible reason for a relatively late beginning of the rise of brain size above and beyond the ape norm."

Journal reference: Nature (vol 428, p 415)

Anil Ananthaswamy