Scientists have theorized for decades about how the human brain evolved and grew dramatically in size. Now, new research points to one common ingredient that may have played a crucial role in the brain’s evolution over time: sugar.
A research team at the University of Sydney and the University of Glasgow created a model comparing modern nutrition with that of our ancestors over a span of four million years. The findings suggest that our ancestors relied on naturally occurring carbohydrate-rich foods, which could have played an important role in not only their overall dietary nutrition but also the evolution of the human brain.
These new findings challenge an older scientific model that emphasizes meat as a primary driving force behind human evolution.
“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 Center and School of Life and Environmental Sciences, in a statement.
While meat played a role in our ancestors’ diets, the new research suggests that the amount of energy required by a growing brain also needs to be considered. Throughout human evolution, brain size increased from roughly 300 grams in early hominins to approximately 1,500 grams in modern humans. Glucose is the brain’s primary energy source, creating increasing nutritional demands as brain size grew over time.
“While animal foods were important, the brain’s need for glucose has largely received little attention from anthropologists,” Professor Brand-Miller said.
The Research
To better understand this hypothesis, the team created a model encompassing six evolutionary stages. Researchers estimated the “obligatory glucose demand” of the brain and other glucose-dependent tissues, including red blood cells, kidneys, and reproductive tissues. They then compared these glucose requirements with the carbohydrate sources that would have been readily available to humans during different stages of evolution.
“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 model incorporated evidence from numerous fields, including metabolism, food composition, fossil isotope studies, dental morphology, archaeology, primate diets, and genetics.
Overall, the research suggests that early hominins, including Australopithecus, Homo habilis, and Homo erectus, were highly dependent on carbohydrate-rich foods, with fruit representing an important source. Later, as humans began using fire to cook food, starch-rich plants could also provide significant amounts of glucose. Grains, meanwhile, appear to have entered the human diet much later. The researchers point to archaeological evidence of seed grinding dating to around 65,000 years ago.
“People often think of carbohydrates as breads and cereals, but for most of human evolution carbohydrates came from fruits, honey and underground plant foods,” said Professor Brand-Miller. “The earliest evidence of seed grinding is 65,000 years ago, when we see the first evidence of grinding stones to make flour.”
The study also examined the role of glucose in reproduction and pregnancy. Pregnancy and lactation significantly increase glucose demands, since energy is required not only by the mother but also by the developing fetus and placenta, as well as for milk production.
The new findings may also help explain humans’ longstanding attraction to sweet foods. For much of human evolution, naturally sweet foods such as fruit and honey provided concentrated sources of energy while also supplying fiber, vitamins, minerals, and other nutrients.
That doesn’t mean people should start making a run for the candy aisle. The findings do not suggest that modern humans should consume more refined sugar. Instead, the researchers distinguish between naturally occurring sugars found in whole foods and highly processed foods containing large quantities of added sugar.
Sugar was also not the human brain’s only source of fuel, nor were carbohydrates the sole dietary factor influencing human evolution. Rather, the researchers argue that glucose requirements have often received less attention than other dietary factors, including meat, fat, and protein. Our ancient appetite for naturally sweet, carbohydrate-rich foods, however, may have helped fuel the extraordinary expansion of the human brain.
Chrissy Newton is a PR professional and the founder of VOCAB Communications. She currently appears on The Discovery Channel and Max and hosts the Rebelliously Curious podcast, which can be found on YouTube and on all audio podcast streaming platforms. Follow her on X: @ChrissyNewton, Instagram: @BeingChrissyNewton, and chrissynewton.com. To contact Chrissy with a story, please email chrissy @ thedebrief.org.