A groundbreaking study by researchers from the University of Sydney and the University of Glasgow reveals that dietary sugars, rather than animal foods, may have been the primary catalyst for the significant expansion of the human brain over the last 4 million years. Hominin brain size increased nearly fivefold, from approximately 300 grams in early relatives like Australopithecus afarensis to around 1,500 grams in modern Homo sapiens. These findings challenge the traditional belief that meat consumption was the key factor in human evolution.
Paranthropus boisei. Image credit: © Roman Yevseyev.
Despite constituting just 2% of an adult’s body weight, the human brain utilizes roughly 20% of the body’s resting energy, and up to two-thirds in young children.
Glucose is the brain’s preferred fuel source. Although the body can generate some glucose from alternative sources, this process is inefficient, emphasizing the necessity for early humans to obtain a steady supply of dietary sugars.
“The human brain is disproportionately large for our body size and demands significant energy,” explained Professor Jennie Brand-Miller, the lead author of the study from the University of Sydney.
“Glucose, crucial for brain function, would have primarily come from carbohydrates present in fruits and honey throughout much of human evolution.”
The research team assessed the glucose requirements of the brain, red blood cells, kidneys, and reproductive tissues at various stages of human evolutionary history.
They then compared this energy demand with the probable carbohydrate content of ancestral diets, ranging from chimpanzee-like diets dominated by plant-based foods to those of contemporary warm-climate hunter-gatherers that include more animal protein.
Results indicate that adult glucose needs vary from 150 to 250 grams daily, depending on sex and reproductive status, with children needing about 125 grams.
Early hominins likely sourced a large portion of their glucose from fruits, obtaining over 65% of their energy from sugars.
After mastering fire, the consumption of cooked starches began to supplement sugars and proteins as significant glucose sources.
“Many people associate carbohydrates with breads and cereals, but for the majority of human evolution, carbohydrates came from fruits, honey, and underground plant foods,” noted Professor Brand-Miller.
The earliest evidence of seed grinding dates back to 65,000 years ago, marking the first use of grinding stones to produce flour.
The researchers highlight supporting evidence from fossilized teeth, isotope ratios in ancient remains, color vision adapted for spotting ripe fruits, and genetic changes linked to sugar metabolism and taste.
They suggest that the seasonal fluctuations in the availability of fruit and honey may have influenced foraging behaviors and, in some instances, contributed to the extinction of hominin groups that struggled to secure sufficient carbohydrates.
While the findings do not discount the significance of meat in the human diet, they argue for a reevaluation of sugars in the narrative of human evolution and its relevance to understanding modern diet-related health issues.
“A comprehensive understanding of carbohydrates in conjunction with meat, fats, and proteins paints a fuller picture of our evolutionary trajectory,” emphasized Professor Brand-Miller.
“Our study indicates that sugars and starches may have significantly contributed to the evolution of the human brain and many of the characteristics that define us as a species.”
“The evidence suggests that carbohydrate-rich foods played a crucial role in meeting the energy demands of our expanding brains.”
“Instead of fearing carbohydrates, individuals should prioritize high-quality sources such as whole fruits, legumes, and nutritious grains as part of a balanced diet.”
The complete findings are published today in the journal Science.
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Jennie Brand-Miller et al. 2026. A central role for dietary sugars in human evolution. Science 393 (6811): 574-581; doi: 10.1126/science.aed8437
Source: www.sci.news
