Could Sugar, Not Protein, Have Driven the Evolution of the Human Brain?

A controversial evolutionary theory puts fruit, honey and cooked starch ahead of meat as the main dietary forces behind the growth of the human brain.

The human brain accounts for only about 2% of body mass but uses roughly 20% of the body’s energy at rest. In children, the share can reach two-thirds. Researchers from the University of Sydney and the University of Glasgow argue that glucose supplied by carbohydrates may have mattered more than protein in meeting that demand.

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It was as if a light bulb switched on.

The prevailing explanation has emphasized meat and cooking. Hunting would have added protein and fat, while fire would have made meat easier to digest. But the brain runs primarily on glucose. Protein can be converted into glucose, the researchers say, but the process is inefficient and may not meet the especially high demands of children and women during pregnancy and breastfeeding.

The team modeled diets across six stages of human evolution over four million years, from Australopithecus afarensis, including the individual known as Lucy, to modern hunter-gatherers. It estimated glucose needs for organs and tissues, including additional requirements during pregnancy and nursing, then reconstructed possible sources of carbohydrates, fats and protein.

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This is a very valuable analysis, but it is built on uncertain foundations.

In the model, early humans obtained more than two-thirds of their energy from natural sugars in tropical-forest fruit and honey. After humans began controlling fire about one million years ago, cooking made starches in roots and grains easier to convert into glucose. The researchers argue that this helped support a second major expansion of the brain. They also say that movement into colder, drier environments favored genes that improved sugar and starch absorption. Humans can carry up to 20 copies of an amylase-related gene involved in starch digestion, compared with one in chimpanzees.

The theory also offers an evolutionary explanation for modern cravings for sweet foods and the prevalence of type 2 diabetes and cardiovascular disease. But Matt Sponheimer of the University of Colorado Boulder called the analysis valuable while warning that it rests on uncertain foundations. The researchers acknowledge that diet’s role in human evolution is necessarily inferential, while pointing to color vision, tooth form, cavities and isotope ratios as evidence consistent with high sugar intake among early humans.

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The role of diet in human evolution is inevitably a matter of inference, but objective evidence from color vision, tooth form, cavities and isotope ratios suggests that early humans consumed large amounts of sugar through fruit-heavy diets.