Diet & brain evolution: another item on the menu

At some point in the distant past, there was a dramatic increase in brain size in our hominid ancestors. From approximately 2 million years ago, to the present day, brain volume in the hominid lineage has increased by a factor of 3.5: the brain of Homo erectus had a volume of about 400 milliliters, while that of modern humans is roughly 1,400 ml.

The size of the human brain cannot be accounted for merely by an increase in body size, because Homo erectus was similar in size to modern humans, so the driving for this increase in brain size (or "encephalization") is still a topic of debate among anthropologists, evolutionary biologists and neuroscientists.

Traditionally, it was believed that our ancestors evolved a large brain to accomodate language and tool use. But in recent years, a number of theories have focused on the role of diet in human brain evolution. During the course of human evolution, changes in diet were brought about by the control of fire, the domestication of plants and animals, and the development and mastery of stone tool technology.

According to one theory, increased consumption of meat by our ancestors provided the additional energy needed for brain expansion. (Cooking would have further increased the amount of calories obtained from meat.) Another holds that a switch to a seafood-rich diet would have provided polyunsaturated fatty acids which, when incorporated into nerve cell membranes, would have made the brain function more efficiently.

And now, a study published in Nature Genetics adds starchy tubers to the smorgasbord of foodstuffs that may have contributed to the expansion of the human brain. 

The brain of modern humans is an energy-hungry organ. At rest, it consumes about one quarter of the body's energy, despite comprising only 2% of the total body mass. (In comparison, the brains of apes about 8% of the body's energy.) However, the calorific intake of humans is similar to that of other similar-sized mammals with smaller brains.

Hence, large amounts of additional energy were required for the brain expansion that occurred in the hominid lineage. One explanation for how this energy was supplied is that we traded brains for guts during our evolution: a change in diet, presumed to consist of more meat, meant that more calories could be absorbed with less effort, so that the gastrointestinal tract could shrink, thus freeing up energy that could be consumed by neural tissue.      

In the new study, which was led by George Perry of Arizona State University and Nathaniel Dominy of the University of California, Santa Cruz, a human gene called AMY1 was investigated. This gene encodes an enzyme called salivary amylase, which breaks down starch into glucose, which is the only energy source for nerve cells.

The AMY1 gene is unusual, in that the number of copies varies quite widely between populations. The researchers therefore determined the number of copies of AMY1 in populations with a high-starch diet, and compared it to the copy number in populations with a low-starch diet.

First, the genomes of 50 American students of European descent were analyzed. It was found that the number of copies of AMY1 varied from between 2 and 15, and that individuals with more copies of the gene had higher levels of the salivary amylase protein in their saliva. On the other hand, chimpanzees, which have a low-starch diet, were found to have just 2 copies of the gene, and low levels of salivary amylase.

The gene copy number in populations with high-starch diets (European Americans, Japanese, and Hadza hunter-gatherers of Tanzania) was then compared to that of populations with a low-starch diet (the Datog peoples of Tanzania, the Yakut of Russia, and the Biaka and Mbuti, both of which are rainforest hunter-gatherers from, respectively, the southern region of the Central African Republic and the Ituri forest in Zaire).

It was found that individuals from populations with a high-starch diet had, on average, more copies of the AMY1 gene than individuals from populations with a low-starch diet - twice as many of the former than the latter had 6 or more copies.

These results show that populations that have a starch-rich diet carry more copies of the AMY1 gene. The authors believe that they have provided one of the very first examples in the human genome of selective pressure resulting in changes in the number of copies of a gene.

The link between diet and brain evolution is, however, indirect. The implication of the findings is that an increase in the AMY1 copy number in our ancestors enabled them to digest starch more efficiently, providing the energy  needed for expansion of the brain. It is, however, impossible to conclude that adopting a starch-rich diet was such an important event in human evolution.

Reference:

Perry, G. H., et al. (2007). Diet and the evolution of human salivary amylase gene copy number. Nat. Genet. doi: 10.1038/ng2123