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A 90-year-old argument that humans conquered the planet thanks in part to the size of a key area of their brain has been mauled in a study that says Homo sapiens is no different from the great apes in this regard.

In 1912, German anatomist Korbinian Brodmann published a treatise that declared man was unique because he had an exceptionally large frontal cortex for a primate. He was, literally, a highbrow.

The frontal cortex, the forepart of the brain running from the forehead to the temples, has been linked with a whole range of higher cognitive functions, including the ability to conceptualize a goal and plan a way to reach it.

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But research published on-line Sunday in a specialist journal, Nature Neuroscience, says that, proportionately, there is no major difference in the relative size of the frontal cortex among humans and their closest relatives.

Anthropologist Katerina Semendeferi of the University of California at San Diego and colleagues used magnetic resonance imaging -- a non-invasive, 3-D scan of the body -- to assess the volume of the cortex among humans as well as gorillas, bonobos, orangutans and chimpanzees.

As expected, humans had the largest, with a volume ranging from 238.8 cubic centimetres to 329.8 cc (8.17-11.15 fluid ounces) according to the individual, while in the four species of great apes, the tally ranged from 50.4 cc (1.7 fluid ounces) in a chimp to 116.3 cc (3.77 fluid ounces) in an orangutan.

But the size of the frontal cortex as compared with the rest of the cerebral hemisphere barely differed among the top primates.

Among humans, it was 36.4 to 39.3 per cent, compared to 36.6 to 38.7 per cent among orangutans, 32.4 to 37.5 per cent among chimpanzees and 35 to 36.9 per cent among gorillas.

Similar tests were carried out on a range of monkeys. Unlike the great apes, which are considered man's closest genetic relative, the monkeys performed somewhat poorly, notching up a comparative frontal cortex rating of 29.4 to 32.3 per cent.

Prof. Semendeferi believes that Brodmann and those that followed him leapt to the wrong conclusion about size because their studies used only a small number of primates -- usually one or two species -- and usually under-represented the great apes. In addition, they sometimes used postmortem samples from animals, failing to take into account that the brain shrinks after death, she suggests.

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Even though the relative size of the frontal cortex may not matter, it has clearly played a crucial part in the ascent of man, Prof. Semendeferi says.

She suggests that during evolution some changes occurred in the human frontal cortex, in sub-sectors of this lobe or in the wiring that links neuronal circuits, and this helped to make Homo sapiens so intellectually advanced.

Her study is based on brain scans carried out on six chimpanzees, three bonobos, two gorillas and four orangutans, plus four lesser apes (gibbons) and five monkeys. These were compared with scans from 10 humans in normal health.

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