Researchers from China may have just found the key to a mystery that has stumped scientists for decades: Why is the average Asian brain significantly larger compared to the average European or African?

According to a new study by the Chinese Academy of Sciences’ Kunming Institute of Zoology, the answer may, in fact, lie with Darwinian selection in East Asian populations, reported South China Morning Post.

The study, published in the journal Human Genetics late last month, posits that genetic mutations have led to bigger brains in the group due to natural selection. Such preference was notably missing in Europe or Africa.

The discovery was first put forth by a team of American researchers three decades ago after studying more than 20,000 present-day human skulls from different parts of the world in what was considered the largest survey of brain sizes.

Findings revealed that the East Asians’ average cranial capacity (the volume of the interior of the cranium) was 1,415 cubic centimeters, which was bigger than the Europeans’ which was averaged at 1,362 and 1,268 for Africans.

Other similar studies that followed also supported such findings but none could explain why.

According to the Chinese research team led by Professor Su Bing, a gene called CASC5, one of the eight genes that regulate the human brain size, may hold the answer to the puzzle.

The gene is unique since its genetic mutations in humans are relatively new, and was only triggered after our ancestors left Africa around 50,000 to 100,000 years ago.

After isolating and comparing CASC5 mutations in different groups, the researchers found a “high frequency” of four mutations that increased the brain size among East Asians. Compared with Europe or Africa, mutations (growth) are much rarer.

“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 wrote in the paper. “By contrast, no signal of selection was detected in Europeans and Africans.”

“Precisely why this occurred is not entirely clear,” they added.

The scientists, however, points out that the drivers behind such change are still unclear for the time being. While theories exist, Su admits that such would just be speculation at this stage.

“Precise answers require further studies,” he stated.

Su, however, clarified that the size difference does not account for any intellectual advantage of the Asian brain over others.

“Scientific research has found no evidence, none at all, to support the existence of intellectual difference among races,” he said.

He further pointed out that as humans evolve and the brain increases in size, it requires more energy and resources it from the rest of the body. Some scientists believe that this has resulted in, among others, a decrease in physical strength.

Further investigation to substantiate such observation, however, is required, according to Su.

“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.