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Fruit, honey and other naturally sweet foods may have helped fuel the evolution of the human brain, according to new research from the University of Sydney and the University of Glasgow.

Published in Science, the study modelled the glucose needs of humans and their ancestors from four million years ago until recently, finding natural sugars may have supplied energy needed to support growing brains before the advent of cooking made starch-rich foods like tubers and grains easier to digest.

Led by nutritionist Professor Jennie Brand-Miller at the University of Sydney, the study is co-authored by archaeologist Professor Karen Hardy of the University of Glasgow.

Lead author Professor Jennie Brand-Miller, Professor of Human Nutrition at the University of Sydney Charles Perkins Centre and School of Life and Environmental Sciences said: “Raw starch does not release glucose, meaning early humans likely relied on naturally sweet foods long before grains became a regular part of the diet.”

Co-author Professor Karen Hardy, Professor of Prehistoric Archaeology at the School of Humanities, University of Glasgow, added: “The human evolutionary diet has a strong focus on meat eating and the need for protein.  But we also require energy and this is best provided by glucose obtained from plants.”

The findings offer a different perspective on a longstanding scientific focus on meat consumption as a driver of human evolution. They also offer insight into why we crave sweet food.

“While animal foods were important, the brain’s need for glucose has largely received little attention from anthropologists,” Professor Brand-Miller said.

Over a period of four million years, human brain size increased from around 300 grams in early hominins to about 1500 grams in modern humans. As brains grew, so did demand for glucose, which is required by other tissues including red blood cells, kidneys and reproductive organs.

“The human brain is unusually large for our body size and requires substantial amounts of energy,” Professor Brand-Miller said. 

“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.”

The researchers developed a model spanning approximately four million years of human evolution to examine whether dietary carbohydrates could meet the increasing glucose demands of a growing brain. They calculated what they describe as “obligatory glucose demand” – the glucose required by the brain, red blood cells, kidneys and reproductive tissues including the foetus, placenta and mammary glands – and compared this with likely food sources available to our ancestors at different stages of evolution.

The model included six evolutionary stages and assessed how changing proportions of animal and plant foods affected carbohydrate availability.

The researchers combined evidence from food composition, metabolism, physiology, fossil isotope studies, dental morphology, archaeology, primate diets and genetics to estimate how humans met their glucose needs as brain size increased.

This modelling suggests earliest hominins (such as ‘Lucy’, whose brain was the same size as a chimpanzee, as well as Homo habilis and Homo erectus) were likely highly reliant on carbohydrate-rich foods, particularly fruit. Later, as the use of fire and cooking became more widespread, starches from underground food sources such as wild yams, water lily rhizomes and African potato contributed to glucose intake.

According to Professor Brand-Miller, archaeological evidence suggests grains entered the human diet much later.

“People often think of carbohydrates as breads and cereals, but for most of human evolution carbohydrates came from fruits, honey and underground plant foods,” she said.

The study highlights nutritional demands during pregnancy and lactation. The researchers estimate reproductive females required substantially more glucose than other adults because of the needs of the foetus, placenta and milk production.

Professor Hardy, who is the principal investigator of the UKRI-funded Powerful Plants project, said: “Reproduction, including pregnancy and lactation require additional energy in the form of glucose that cannot be adequately supplied by animal produce.

"Our modelling shows that pregnant females had a growing need for glucose during our evolution as their brains and body size increased.”

The findings may also help explain why sweet foods appeal to children.

Professor Brand-Miller said: “Our desire for sweetness is instinctive. Children need more energy in the form of carbohydrate than adults because their brains are claiming as much as two thirds of their basal metabolic rate during development.”

Professor Brand-Miller said the research offers insight into our longstanding preference for sweetness.

“For most of evolution, fruit and honey were valuable sources of energy that helped meet the body’s glucose needs.

“Sugars in nature are typically packaged within foods that also contain fibre, vitamins, minerals and other beneficial compounds.”

Professor Karen Hardy explains: "Refined sugars such as are found in sweets and snacks are very different to the natural sugars found in fruit.  These and an overabundance of fast release carbohydrates that are common today, such as in white rice and many types of bread can cause health problems."

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Research

Brand-Miller, Jennie, Hardy, Karen, Copeland, Les, et al., SWEET – sugars, diet and human evolution, Science (2026).

DOI: 10.1126/science.aed8437

First published: 6 August 2026