Monday, January 30, 2017
Natural selection in East Asian populations has favored 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 an 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 centimeters, 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 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 that 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 favor a larger grey matter volume of the brain," the researchers said. "By contrast, no signal of selection was detected in Europeans and Africans."