Human brains grew fivefold in 4 million years, thanks to natural plant sugars

Recent research from the University of Sydney and the University of Glasgow has found that fruit, honey, and other naturally sweet foods may have played a key role in human brain evolution. In the study, published in the journal Science, researchers modeled the glucose needs of humans and their ancestors from 4 million years ago to recent times.

According to the study, over a period of 4 million years, human brain size increased from about 300 grams in early hominins to about 1,500 grams in modern humans. As brain size increased, so did the demand for glucose. Natural sugars may have helped provide the growing brain with the energy it needed. This was at a time when cooking began and starch-rich foods like tubers and grains were no longer easily digested.

The study, led by nutritionist Jenny Brand-Miller of the University of Sydney, was co-authored by archaeologist Karen Hardy of the University of Glasgow. “Raw starch does not release glucose, which means that early humans may have relied on naturally sweet foods long before cereals became a regular part of the diet,” said Brand-Miller, Professor of Human Nutrition at the Charles Perkins Centre and School of Life and Environmental Sciences at the University of Sydney and lead author of the study.

“During human evolution, much attention has been paid to the need for meat and protein in the diet. But we also need energy, and this is best provided by glucose from plants,” said Hardy, Professor of Prehistoric Archaeology at the School of Humanities at the University of Glasgow and co-author of the study.

These findings offer a new perspective on the long-standing scientific focus on meat consumption as a factor driving human evolution and also help explain why we crave sweet foods.

According to Brand-Miller, “Although animal-based foods were important, the brain’s need for glucose has received largely little attention from anthropologists.”

She added, “The human brain is unusually large relative to our body size and requires a significant amount of energy.” According to Brand-Miller, for most of human evolution, glucose may have been the primary source of energy for the brain, coming from carbohydrates found in foods such as fruit and honey.

The researchers developed a model of approximately 4 million years of human evolution to determine whether dietary carbohydrates could meet the growing glucose needs of the developing brain. They calculated what they called the “essential glucose requirement.” This included glucose needed for the brain, red blood cells, kidneys, and reproductive tissues, including the foetus, placenta, and mammary glands. They then compared this to potential food sources available to the ancestors of humans at different stages of development.

The model covered six evolutionary stages and assessed how changes in the proportion of animal and plant foods affected carbohydrate availability.

The experts combined evidence from diet composition, metabolism, physiology, fossil studies, tooth structure, archaeology, primate diets, and genetics to estimate how humans met their glucose needs as brain size increased.

According to Brand-Miller, archaeological evidence suggests that grains entered the human diet much later. She said, “People often think of carbohydrates as bread and cereals, but for most of human evolution, carbohydrates were obtained from fruits, honey, and plant foods found underground.”

The study also sheds light on nutritional needs during pregnancy and lactation. Researchers estimate that women during their reproductive years require significantly more glucose than other adults, due to the increased demands of the foetus, placenta, and milk production.

According to Hardy, “Pregnancy and lactation require additional energy that cannot be adequately supplied by animal-based foods.” She added, “Our modelling shows that glucose requirements in pregnant women have steadily increased throughout evolution, as their brain and body size have increased.”