Sugar, Not Just Meat, Fueled Growth of Human Brain, New Study Argues

For decades, the dominant story of human evolution has centered on meat. Hunt more, eat more protein and fat, grow a bigger brain. It’s a tidy narrative, and a new analysis argues it’s only half the story. The study makes the case that sugar, specifically the natural sugars found in fruit and honey, played just as central a role in making humans the big-brained species they are today. Long before our ancestors figured out how to cook, they were foraging for the sweetest foods they could find, and that sugar-seeking behavior may have literally fueled the growth of the human brain.

Their argument starts with a straightforward biological fact: the brain runs on glucose, a type of sugar. Despite making up only about 2% of an adult’s body weight, the brain burns through roughly 20% of the body’s resting energy. In 5-year-old children, the brain’s share climbs to around 66%, or two-thirds of the energy the body burns at rest. Researchers at the University of Sydney and the University of Glasgow, writing in the journal Science, modeled how much glucose early human ancestors would have needed at each stage of brain development over roughly 4 million years, then compared that demand to what the diet could realistically have supplied. What they found pushes back hard against the longstanding “meat-first” view of human origins.

Before our ancestors mastered fire and cooking, wild fruits and honey were the primary sources of the glucose their growing brains demanded. Animal foods provided protein and energy but contain almost no carbohydrates. Plants, particularly sweet fruits, were the ones reliably delivering glucose. According to the researchers’ modeling, natural sugars likely provided between 20 and 35% of total dietary energy across much of human evolution, though the researchers emphasize this is a model-based estimate rather than a measured historical average.

How Much Sugar the Human Brain Really Needs

To build their case, the research team combined several streams of evidence: the known nutritional makeup of foods eaten by chimpanzees and modern hunter-gatherers, fossil records, chemical signatures preserved in ancient teeth and bones, and the physical structure of teeth and jaws across different ancestral species. They also calculated how much glucose different body tissues require each day.

Adult males were estimated to need roughly 160 grams of glucose daily. Reproductive females, including those who are pregnant or breastfeeding, needed about 230 grams per day. Young children needed around 125 grams, despite their smaller bodies, because their brains are disproportionately large and burn energy at a remarkable rate.

Human bodies cannot manufacture all the glucose they need from protein alone. The liver can produce some glucose from protein building blocks, but this process is inefficient and has clear limits. According to the researchers’ estimates, relying on that conversion alone would leave adult males roughly 30 grams short of their daily glucose needs, and reproductive females potentially 105 grams short. That gap had to be filled by food, and in a world without cooking, sweet fruits and honey were likely the main way to fill it.

Fruit First, Then Fire

Using chimpanzees as a baseline for what the earliest human ancestors likely ate, the researchers worked through six stages of dietary evolution. Chimpanzees, whose diet consists of roughly 95% plant foods, get about 66% of their daily energy from sugars in fruit and other sweet sources. Early human ancestors were likely eating in a similar way. As ancestral human species developed more sophisticated tools, began scavenging and hunting animals, and eventually mastered fire, the proportion of animal food in the diet gradually increased. But in the researchers’ model, sugars from fruit and honey remained a major source of energy throughout much of this journey, even as the overall diet shifted toward more animal food.

Once fire and cooking became possible, starchy foods like roots, tubers, and seeds became far more useful as a source of glucose. Cooking breaks down raw starch and makes it digestible, effectively opening up a new reservoir of brain fuel. Before that point, raw starch provided very little usable glucose. In the researchers’ model, modern humans consumed roughly 19% of their energy as protein, 26% as sugars, and 23% as starch, with carbohydrates together making up the largest share of the energy supply.

Why Sugar Mattered Most for Mothers

One of the more telling elements of the analysis involves reproduction. Pregnancy and breastfeeding push glucose demand to its highest levels. A human fetus and placenta together use around 70 grams of glucose per day. At peak breastfeeding, human milk produces roughly 70 grams of the milk sugar lactose daily, equivalent to about 40% of an infant’s energy needs. Stack those demands on top of the mother’s own requirements, and the total glucose need for a reproductive female becomes substantial.

When carbohydrates are scarce, the body shifts into a fat-burning state that produces compounds called ketones as an alternative fuel. Ketones can partially power the brain in adults, but the researchers flag concerns about sustained ketone production during pregnancy. Some studies have linked high maternal ketone levels to potential effects on fetal brain development, though the authors note the evidence in this area is not fully consistent and further research is needed to establish causality. Women who eat the lowest amounts of carbohydrate have also been associated with higher rates of a type of birth defect affecting the brain and spine. From an evolutionary standpoint, the researchers argue, females who could reliably access sweet, carbohydrate-rich foods would have had a meaningful reproductive advantage.

Sugar Written in Our Genes and Teeth

Beyond the modeling, the researchers point to biological clues suggesting our bodies were shaped by a long history of sugar consumption. Humans have sweet taste receptors not just on the tongue but throughout the small intestine and in the brain’s appetite center. Our preference for sweetness is highly heritable, meaning it runs in families and has a clear genetic basis.

Fossil teeth from ancestors dating back millions of years show signs of decay, which the researchers treat as one line of evidence for sugar in the diet. Drawing on earlier research, the authors note that populations that historically ate more starch carry extra copies of a gene that boosts the saliva enzymes that break it down. Arctic populations with little access to carbohydrates show a notably high prevalence of sucrase-isomaltase deficiency, an inherited condition that impairs the digestion of certain sugars and starches, a pattern the researchers link to the near-total absence of these foods from the traditional Arctic diet.

Taken together, these details form a pattern of genetic selection driven by dietary carbohydrates, running alongside the well-documented adaptations tied to meat eating.

None of this research dismisses the role of animal foods in human evolution. Meat provided dense protein and energy. Bone marrow and fat were among the most calorie-rich foods available. These resources almost certainly played a role in allowing early humans to shrink their digestive tracts and redirect energy toward a larger brain. But the researchers argue that the heavy emphasis on animal foods in standard accounts of human evolution has obscured an equally important part of the story.

Swings in fruit availability, tied to shifting climates and seasons over millions of years, may have pushed early humans to forage more cleverly, travel farther, and remember more about their environments, all demands that could have driven brain development forward. By the same logic, the researchers propose that periods of sugar scarcity may have created enough reproductive stress to contribute to the disappearance of some early human lineages.

Sugar was not a dietary villain lurking in the background of human prehistory. If the researchers’ model holds up to further scrutiny, sweet foods would look less like a luxury and more like an essential ingredient in the story of how human brains got so large, a relationship built over millions of years and still written into our biology today. That conclusion remains tentative, grounded in models and estimates rather than direct measurement, but the evidence the researchers have assembled makes it more difficult to tell the story of human evolution while leaving sugar out of it entirely.

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