Human Evolution The Role of Sugars in the Evolution of the Human Brain

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.

A 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.

“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 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 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.”

Modelling the Nutritional Demands of a Growing Brain

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, 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.

“The earliest evidence of seed grinding is 65,000 years ago, when we see the first evidence of grinding stones to make flour.”

The Importance of Glucose During Reproduction

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.

“When we think about the foods that supported human evolution, we need to consider not only brain growth but also the nutritional requirements of pregnancy and raising children,” said Professor David Raubenheimer, co-author and Professor of Nutritional Ecology at the Charles Perkins Centre. “The foetus and placenta place additional demands on glucose, increasing the need for carbohydrate-rich foods. This helps us understand why pregnant women often crave fruit.”

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

“Our desire for sweetness is instinctive,” said Professor Brand-Miller. “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.”

Incorporating Natural Sugars into Our Diet

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.”

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The findings build on previous University of Sydney research showing honey is nature’s original antibiotic.

“Honey is not ‘empty calories’ as we are often led to believe. For most of human evolution, honey was a highly valued food source that provided energy and other beneficial properties,” Professor Brand-Miller said. “Understanding the role of carbohydrates alongside meat, fat and protein provides a more complete picture of the human evolutionary journey. 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.”

What can Modern Humans Make of These Primordial Findings?

“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 prioritise a range of carbohydrate foods as part of a healthy diet,” Professor Brand-Miller said.

Original Article: Sweet — sugars, diet and human evolution; Science; DOI:10.1126/science.aed8437

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