Why Humans Have Big Brains: Genes Shed Light on Evolution

Our big brains are what have given us an evolutionary advantage over other species. They allow us to perform feats that we would otherwise be incapable of. Now, scientists have moved a step closer to understand the genetic changes that allowed us to develop such big brains, which may tell us a bit more about the process of human evolution.

During evolution, different mammal species have experienced variable degrees of expansion in brain size. This process of larger brain evolution is called encephalization. Although it occurred, though, the process is not well understood by scientists--especially since evolving large brains come at a very high cost.

In order to better understand this brain evolution, the researchers examined the genomes of 39 species of mammals. More specifically, they focused on the size of gene families across these species. Gene families are groups of related genes which share similar characteristics and are often linked with common or related biological functions. Large changes in the size of gene families can help explain why related species evolved along different paths.

"We found that brain size variations are associated with changes in gene number in a large proportion of families of closely related genes," said Humberto Gutierrez, one of the researchers, in a news release. "These gene families are preferentially involved in cell communication and cell movement as well as immune functions and are prominently expressed in the human brain. Our results suggest that changes in gene family size may have contributed to the evolution of larger brains in mammals."

Mammalian species in general tend to have large brains in comparison to their body size. While this represents an evolutionary advantage in some respects, it's also costly. The brains require large amounts of energy to function.

"The brain is an extremely expensive organ consuming a large amount of energy in proportion to its volume, so large brains place severe metabolic demands on animals," said Gutierrez. "Larger brains also demand higher parental investment. For example, humans require many years of nurturing and care before their brains are fully matured."

That said, this new research reveals that variations in the size of gene families associated with encephalization provides evolutionary support for the specific physiological demands associated with increased brain size. This, in turn, shows a little bit more about the process of evolution.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences.