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 modeled the glucose needs of humans and their ancestors from 4 million years ago until recently, finding natural sugars may have supplied the energy needed 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 Jennie Brand-Miller, a professor of human nutrition at the University of Sydney Charles Perkins Center and School of Life and Environmental Sciences.

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

"While animal foods were important, the brain's need for glucose has received little attention from anthropologists," Brand-Miller said.

Over a period of 4 million years, human brain size increased from around 300 grams in early hominins to about 1,500 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," 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."