Natural selection in East Asian populations has favoured genetic mutations leading to bigger brains, according to a new study by Chinese researchers that did not find a similar preference in Europe or Africa. The study has shed new light on a controversial issue that has puzzled scientists for decades: why is the average Asian brain bigger than the average European or African one?
The world’s largest survey of brain sizes, conducted by American scientists three decades ago using more than 20,000 modern human skulls from around the globe, found that the average cranial volume among East Asians was 1,415 cubic centimetres, compared with 1,362 for Europeans and 1,268 for Africans.
Subsequent studies have confirmed those results. Among them was a magnetic resonance imaging survey last year which found that East Asians had a higher cranial vault, which allowed their skulls to house a bigger brain.
Researchers proposed a range of hypotheses to explain the differences, with some suggesting that living in a cold climate could lead to a boost in brain size because in such conditions a bigger brain would be better at maintaining a constant temperature at its core, where most thinking took place.
But the climate theory could not fully explain differences in the brain sizes of people living in the same latitudes, such as Chinese and Europeans.
The Chinese researchers said a gene called CASC5 – one of eight regulating human brain size – might provide more clues. Unlike most of the other genes, which also regulated the brain sizes of monkeys or early human species such as Denisovans and Neanderthals, genetic mutations of CASC5 in Homo sapiens are relatively young, only occurring after our species left Africa between 50,000 and 100,000 years ago.
The researchers, led by Professor Su Bing, from the Chinese Academy of Sciences’ Kunming Institute of Zoology, compared CASC5 mutations in different populations for the first time.
They found a “high frequency” of four mutations closely related to increased brain size among East Asian populations including Chinese, Japanese and Mongolians. But such mutations rarely occurred in Europe or Africa.
“At the population level, our results suggest a selection of CASC5 in East Asian populations, which seems to favour a larger grey matter volume of the brain,” the researchers said in a paper published in the journal Human Genetics late last month. “By contrast, no signal of selection was detected in Europeans and Africans.”
“Precisely why this occurred is not entirely clear,” they added.
Besides climate, other forces that might drive such selection included social structure and cultural preference, Su told the South China Morning Post this week, while adding that such theories were pure speculation at this stage.
“Precise answers require further studies,” he said.
Su said the study in no way suggested that Asians were smarter than other humans.
“Scientific research has found no evidence, none at all, to support the existence of intellectual difference among races,” he said.
However, scientists generally agreed that humans had made significant sacrifices in return for increased brain size, Su said.
The brain consumed lots of energy, and a bigger brain made birth more difficult and drained resources from the rest of the body, resulting in many issues such as decreased physical strength. Europeans were generally bigger and stronger than Asians, Su said, but whether the physical difference was associated with brain size required further investigation.
“The Darwinian selection may still be going on today, but I think the brain size difference among races will eventually disappear due to the widespread genetic exchange occurring around the world today,” he said.
An anthropologist based in Beijing said the study tackled an important but sensitive issue in human evolution.
“The findings may fuel racist debate,” said the anthropologist, who requested anonymity.
Data in the study also showed a high frequency of genetic mutations occurring in South Asian populations, who lived in a warmer climate, and the anthropologist said it would be interesting to investigate whether Darwinian positive selection favouring bigger brains had also occurred there.
If so, it might suggest that brains grew bigger as humans spread further from Africa.