A team of researchers from the University of Sydney and the University of Glasgow argues that dietary sugars, not animal foods, may have played the decisive role in the expansion of the human brain over the past 4 million years, as hominin brain size grew nearly fivefold, from about 300 grams in early relatives like Australopithecus afarensis to roughly 1,500 grams in modern Homo sapiens. Their findings offer a different perspective on the longstanding scientific view that meat consumption was a key driver of human evolution.
Paranthropus boisei. Image credit: © Roman Yevseyev.
The human brain, though just 2% of an adult’s body weight, consumes about a fifth of the body’s resting energy, and as much as two-thirds in a young child.
Its preferred fuel is glucose, and while the body can manufacture some glucose from other sources, the process is limited and inefficient. This means early humans needed a steady dietary source of the sugar.
“The human brain is unusually large for our body size and requires substantial amounts of energy,” said University of Sydney’s Professor Jennie Brand-Miller, lead author of the study.
“Glucose is the brain’s primary energy source, and for much of human evolution, would have come from carbohydrates found in foods such as fruit and honey.”
In the study, the authors calculated how much glucose the brain, red blood cells, kidneys and reproductive tissues would have required at different stages of human evolution.
They then compared this demand against the likely carbohydrate content of ancestral diets from chimpanzee-like diets dominated by plant foods to those of modern warm-climate hunter-gatherers with a greater share of animal foods.
The results suggest adult glucose needs ranging from about 150 to 250 grams a day, depending on sex and reproductive status, with young children requiring roughly 125 grams.
Early hominins likely got much of this from fruit, deriving more than 65% of their energy from sugars.
Later, after the mastery of fire, cooked starches began to take over as a major glucose source alongside sugars and protein.
“People often think of carbohydrates as breads and cereals, but for most of human evolution carbohydrates came from fruits, honey and underground plant foods,” Professor Brand-Miller said.
“The earliest evidence of seed grinding is 65,000 years ago, when we see the first evidence of grinding stones to make flour.”
The researchers point to supporting clues from fossil teeth, isotope ratios in ancient remains, color vision suited to spotting ripe fruit, and genetic adaptations tied to sugar metabolism and taste.
They argue that the seasonal rise and fall of fruit and honey availability may have shaped foraging behavior, and, in some cases, even contributed to extinctions among hominin groups less able to secure enough carbohydrates.
The findings don’t dismiss the importance of meat in the human diet, but they suggest sugar deserves a far more prominent place in the story of how humans became human and in understanding modern struggles with diet-related disease.
“Understanding the role of carbohydrates alongside meat, fat and protein provides a more complete picture of the human evolutionary journey,” Professor Brand-Miller said.
“Our study suggests sugars and starches may have helped shape the development of the human brain and many of the traits that make us human.”
“The evidence suggests that carbohydrate-rich foods helped fuel the energy demands of our growing brains.”
“Rather than fearing carbohydrates, people should focus on high-quality sources, including whole fruits, legumes and nutritious grains, and prioritize a range of carbohydrate foods as part of a healthy diet.”
The findings appear 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
