When size doesn't matter
2002-02-18 12:34
Paris - 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, a German anatomist, Korbinian Brodmann, published an
influential treatise which 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 conceptualise a goal and plan a way to reach it.
But research published 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,
orangutangs and chimpanzees.
As expected, humans had the largest, with a volume ranging from
238.8 cubic centimetres to 329.8cc (8.17-11.15 fluid ounces)
according to the individual, while in the four species of great
apes, the tally ranged from 50.4cc (1.7 fluid ounces) in a chimp to 116.3cc (3.77 fluid ounces) in an orangutang.
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 36.4-39.3 percent, compared to 36.6-38.7
percent among orangutangs, 32.4-37.5 percent among chimpanzees and 35-36.9 percent 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 29.4-32.3 percent
compared to the rest of their brain hemisphere.
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 only one or two species - and usually under-represented the great apes.
In addition, they sometimes used post-mortem samples from
animals, failing to take into account that the brain shrinks after death, she suggests.
Even though the relative size of the frontal cortex may not
matter, it has clearly played a critical part in the ascent of Man, says Semendeferi.
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 orangutangs, plus four lesser apes (gibbons) and five monkeys. These were compared with scans from 10 humans in normal health. - Sapa-AFP
- SAPA