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 supplied large amounts of energy to support growing brains before the advent of cooking made starch-rich foods like tubers and grains easier to digest. 

“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,” said 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.

The findings offer a different perspective on the longstanding scientific view that meat consumption was the key driver of human evolution. They may also provide insight into why we often crave sweet food. 

“While animal foods were important, the brain’s need for glucose has received little attention from palaeoanthropologists,” 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 also 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 fetus, 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), 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 flours and 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.