WASHINGTON: Human brains have shrunk over the past 30,000 years, puzzling scientists who say it is not a sign we are growing dumber but that evolution is making brains leaner and more efficient.
The average size brain of modern humans, homo sapiens, has decreased about 10 per cent, the size of a tennis ball, during that period, from 1500 to 1359 cubic centimetres. Women's brains, which are smaller on average than those of men, have experienced an equivalent drop in size.
The measurements were taken using skulls found in Europe, the Middle East and Asia.
''I'd call that a major downsizing in an evolutionary eye blink,'' Dr John Hawks, of the University of Michigan, told Discover magazine.
But other anthropologists note that brain shrinkage is not surprising since the stronger and larger we are, the more grey matter we need to control this larger mass. The Neanderthal, a cousin of the modern human who disappeared about 30 millennia ago, was far more massive and had a larger brain.
The Cro-Magnons, who left cave paintings of large animals in the monumental Lascaux cave more than 17,000 years ago, were the Homo sapiens with the biggest brain.
Psychology professor David Geary, of the University of Missouri, said these traits were necessary to survive in a hostile environment. He has studied the evolution of skull sizes 1.9 million to 10,000 years old. His team found that brain size decreased as population density increased.
But the downsizing does not mean modern humans are dumber than their ancestors - rather, they developed different, more sophisticated forms of intelligence, said Brian Hare, an assistant professor of anthropology at Duke University.
He said the same phenomenon can be observed in domestic animals compared with their wild counterparts. So while huskies may have smaller brains than wolves, they are smarter because they can understand human communicative gestures.
''Even though the chimps have a larger brain [than the bonobo, the closest extant relative to humans], and even though a wolf has a much larger brain than dogs, dogs are far more sophisticated, intelligent and flexible, so intelligence is not very well linked to brain size,'' Professor Hare said.
He said humans had characteristics from both the bonobo and chimpanzee, which is more aggressive and domineering. ''Humans are both chimps and bonobos in their nature and the question is how can we release more bonobo and less chimp.'' Agence France-Presse