Natural sugars may have helped fuel early human brain expansion, study finds

A study published August 6 in the journal Science suggests that naturally occurring sugars from fruit and honey were a major energy source for early hominin brains before the advent of cooking. The human brain consumes roughly 20 percent of the body's energy at rest and up to 66 percent during childhood, running primarily on glucose derived from carbohydrates. While scientists have long emphasized meat, marrow and fat as fuels for brain expansion, those foods contain few carbohydrates.

Lead author Jennie Brand-Miller, a human nutrition scientist at the University of Sydney, and colleagues built a model tracking dietary needs across four million years of hominin evolution, from a chimpanzee-like ancestor through Australopithecus afarensis, Homo habilis, Homo erectus and early Homo sapiens foragers. At each stage they estimated brain and body size, the proportion of plant versus animal foods consumed, and the glucose required by the brain and other tissues.

The model calculated that the earliest hominins may have obtained more than 65 percent of their energy from naturally occurring sugars in fruits and other sweet foods. Honey from raided bee nests and ripe fruit would have provided readily available glucose without the need for cooking.

Cooking starchy plants such as wild yams, tubers and later grains eventually offered an alternative glucose source and reduced reliance on sugary foods. The researchers stress that animal products remained important; they describe sugar and animal-based nutrition as parallel contributors rather than competing explanations.

Paleoanthropologist Marina Lozano of the Catalan Institute of Human Paleoecology and Social Evolution, who was not involved in the study, said the model supports the view that both animal protein and plant-derived glucose were key. She noted, however, that debates persist over when widespread cooking emerged, and some researchers argue that large brains were already established before solid evidence for cooking appears.

The findings also offer a possible evolutionary context for modern human sugar cravings. Brand-Miller said the desire for sweetness should be recognized as an important, regular part of human nutrition rooted in deep evolutionary history.

The study acknowledges that reconstructing ancient diets involves speculation, particularly regarding the timing and prevalence of cooking. The authors' earlier work in 2015 similarly proposed cooked starch as crucial for brain growth, a hypothesis that has drawn skepticism about the archaeological evidence for early fire use.