Early Humans Likely Ate Carbs and Sugary Foods

Compared with other primates like chimpanzees and gorillas, humans have very big brains.

Nearly 4 million years ago, Lucy—a member of one of the earliest human species, Australopithecus afarensis—had a brain that weighed 380 grams, the same as an adult chimpanzee today. Modern Homo sapiens, however, which emerged around 300,000 years ago, boast a brain weighing a whopping 1,500 grams.

How did humans evolve to have such large and useful brains? A lot of attention has been given to meat-eating and the advent of cooking, which made meat far easier to chew and digest. Because animal proteins are so calorie-dense, the explanation goes, all of the excess energy from meat could be used to power our growing brains.

But missing from the discussion has been the importance of sugars and carbohydrates in the early human diet, says Jennie Brand-Miller, lead author of an August 2026 paper in Science claiming that sugary fruits and honey provided between 20 and 35 percent of energy intake throughout most of human evolution. Another 25 percent of the early human diet came from carb-heavy starches, the study says, while less than 20 percent of dietary energy came from proteins.

“Meat is energy dense in ways that fruits and plant foods are not, and we needed that energy and those calories,” says Brand-Miller, emeritus professor of human nutrition at the University of Sydney, “but you can’t get everything from one category of food. We have billions of neurons in our brains, and those neurons run on glucose—they run on sugar.”

Millions of years ago, the diets of Lucy and other early humans looked a lot like a chimp’s today. They ate a ton of fruit.

“If you look at a chimpanzee diet, 75 percent of their calories come from carbohydrates,” Brand-Miller says, “and for the most part, those carbohydrates are sugars from fruit and honey.”

In modern nutritional science, sugar gets a bad rap because it’s often consumed as refined sugar added to high-calorie, low-nutrient foods like soft drinks and candy. But natural sugar found in fruit is packaged with fiber, vitamins and protective antioxidants. To our early human ancestors, those natural sugars likely played a critical role in fueling an increasingly energy-hungry brain.

While meat was definitely a caloric bonanza for early humans, recent primate studies offer evidence that sugary fruits continued to be a critical “brain food.” A 2017 paper looked at 140 primate species and compared their brain sizes with their diets.

“What [that study] showed very clearly was that the fruit-eaters, the frugivores, had 25 percent bigger brains, on average, than the folivores, the leaf-eaters,” Brand-Miller says. The fruit-eaters also had bigger brains than the omnivores, which ate meat.

A clear indication of the importance of sugar in human evolution is our ability to see colors. Evolutionary biologists believe that the primary function of color vision was to identify the ripest, sweetest and most nutrient-rich fruits.

“The fruits that are the deepest red and the deepest orange and the deepest yellow are the ones with the most sugars,” Brand-Miller says.

There’s even a theory that human intelligence leveled up through the search for ripe fruit. In a tropical forest, different fruits ripen at different times of the year. There is a distinct survival advantage for primates—including early humans—that could create a mental map of the forest and find those ripe fruit trees before the competition. In the foraging race, the smartest species gets the most sugar.

“You can see how this would be a stimulus for cognition, for a bigger brain and for a bigger memory,” Brand-Miller says.

Another clue that early humans and other ancient primates ate a lot of natural sugar in their diet is that most of them had cavities. Brand-Miller says there’s fossil evidence that primates living 30 million years ago got cavities from eating sugary foods. Human cavities really exploded after the invention of cooking, because that’s when people started eating starchy tubers and other carb-heavy foods that were previously indigestible.

Most of human evolution took place in Africa, where ripe fruit and honey were abundant and available year-round. But as humans migrated out of Africa and into colder, drier climates, sources of dietary sugar were harder to come by. In response, Brand-Miller believes human evolution continued to select for genes that improved our ability to access sugars and carbs.

A good example is milk consumption. No other animal species drinks milk as an adult, and not all humans carry the genes that allow them to digest lactose, the sugar found in dairy. But for ancient humans who lived in cold climates where fresh fruit was scarce, there was a distinct evolutionary advantage to being able to drink and digest milk.

There are even more recent genetic adaptations that greatly improved humanity’s ability to access dietary sugars and carbohydrates. Starting with the Agricultural Revolution 10,000 years ago, the human diet began to include far more carbohydrates in the form of grains and starches like rice, corn and potatoes.

There’s an enzyme called salivary amylase that kickstarts the digestion of starchy foods in the mouth, releasing their sweet taste. Brand-Miller says chimpanzees have just one copy of the gene that encodes for salivary amylase in their DNA, whereas humans have as many as 20 copies. Evolution has repeatedly selected for additional copies of the salivary amylase gene, because they allow us to efficiently break down complex carbohydrates into glucose, our favorite brain food.

“These genetic changes tell us how important carbohydrate calories were to human beings,” Brand-Miller says.