At first, we mainly ate fruit. Or rather, the most recent relative we share with our closest cousins, the chimpanzees, did — about four million years ago. Their vision could distinguish the colors of these sources of sugar, minerals, and fiber, probably to tell when they were ripe and sweet. From that time, and throughout the evolution of the genus Homo, the proportion of sugar in daily energy intake is estimated to have fallen by about three quarters, in favor of the greater diversity of an omnivorous diet. Paradoxically, during that period the organ most dependent on glucose — the brain — continued to grow relative to body size. Now, at about 2% of our weight, it consumes 20% of the energy required for our vital functions.

A new review in the journal Science examines years of scientific literature to try to trace sugar through the evolution of our lineage, a history we have often known better from arrowheads than from remains of rice stews or fruit feasts, which barely survive the passage of time. Perhaps for that reason we associate meat more than carbohydrates with the humans we are today. But the study reaches a curious conclusion: we have never needed less sugar than we do now and yet it has never been so essential. Our brain, red blood cells, kidneys and reproductive organs (mammary glands, the placenta and the fetus it carries) retain a substantial demand for glucose (simple sugar). That is why it is important to ask about sugar’s role throughout the journey to the present.

“In 2015 we published on the role of starches, complex carbohydrates, in the human diet and how we managed to incorporate more of them by cooking grains and tubers once we controlled fire, but that happened, at the earliest, about 1.5 million years ago,” explains Jennie Brand-Miller, emeritus professor at the University of Sydney (Australia). In this new study the authors look even further back, to before fire, when fruit and other sweet foods were the main source of calories and the brain — of apes more similar to chimpanzees — weighed about 300 grams.

The authors, analyzing a wide range of data — diet composition, metabolism, fossil isotopes, dental morphology and genetic changes — seek to specify how we sustained the growth of this organ to the current 1,500 grams on an omnivorous diet. “By combining sugar needs, increases in brain size and the special requirements of reproduction, we offer a more detailed perspective on human need for and desire for sugars and starch-rich foods,” says Karen Hardy, an archaeologist at the University of Glasgow (United Kingdom) and another author of the study.

An evolutionary model of our sugar needs

It is not an easy task: to estimate how much sugar was demanded by the body size and metabolism of different members of the human evolutionary line, the authors had to squeeze the scarce archaeological and genetic information available. They combined that with another difficult to determine proportion from the present: the plant-to-animal food ratio in the diets at each evolutionary stage. The result is a model with six stops on the journey from the common ancestor with chimpanzees to modern humans — passing through Australopithecus afarensis (the famous Lucy), Homo habilis and two stages of H. erectus — with progressively larger brains and an increasing share of animal foods rich in protein and fat.

Carbohydrate availability in the diets across the model ranges from 400 grams a day in the chimpanzee-equivalent, with more than 65% of energy from fruits and other sweet items, to 230 grams a day for modern humans, who on average obtain 19% of energy from protein, 25% from sugars, and 25% from starches. The estimated plant-to-animal food ratio moves from 5:95 (plant:animal), the known dietary composition of chimpanzees, to two possible ratios in present-day sapiens, reflecting our wide range of niches: 35:65 in diets lower in animal products and 50:50 among temperate-climate hunter-gatherer groups.

Color vision, the presence of cavities, and isotope studies of teeth suggest an early hominin frugivorous (fruit-based) diet. Meanwhile, reductions in gut size, teeth and jaws reflect the progressive introduction of meat. But these latter two traits, together with other modern features such as sweet taste receptors or salivary enzymes for digesting starch, also point to genetic selection driven by carbohydrates. The authors emphasize our high metabolic rate, especially because of the large brain size and the physical activity that characterized our development as persistence hunters. They estimate total glucose demand for men at 150 to 200 grams a day and 125 grams for children. But the highest demand is for women of reproductive age, at 200 to 250 grams a day.

High glucose requirements of reproductive-age women

That demand in women is due to the primary glucose requirements of the placenta, the fetus, and lactating mammary glands, which go beyond those of the brain. The study refers to this stage as the most critical for evolutionary success and says this exacting glucose demand is often under-recognized in anthropology. “Even today it is difficult for women to meet their needs through a typical diet; instead, recommendations focus on vitamin and mineral supplements,” Brand-Miller laments.

In fact, the study’s model shows that the carbohydrate balance — the gap between daily intake and the body’s glucose demand — can more easily become negative in the later stages of our lineage and especially today. In other words, our ancestors could tolerate lower carbohydrate intakes than we can without entering a deficit, which can put pregnant and breastfeeding women at particular risk. On the other hand, the authors warn about another danger: the high availability of sugar in today’s diet. “Today’s refined sugar sources — sweets and snacks — are very different from foods like fruit and are causing health problems,” Hardy warns. Moreover, those foods “are devoid of micronutrients,” Miller insists.

Despite the reflections prompted by the study, models based on simulations from data as hard to obtain and validate as those used in this review should be treated with caution. Marina Lozano, a researcher at the Catalan Institute of Human Paleoecology and Social Evolution (IPHES-CERCA), acknowledges that the model reinforces an already accepted hypothesis: “that brain evolution is a consequence of an omnivorous diet, in which both animal protein and fats and plant-derived glucose have been key.”

However, she questions the claim of such ancient control and use of fire. “For example, we know that Homo antecessor, about 800,000 years ago, or pre-Neanderthals, about 400,000 years ago, did not use fire to cook. Therefore, there are ancestral species with large brains (1,000–1,200 cubic centimeters) that could not make starch-rich foods digestible. Widespread control and use of fire does not occur until much later, with the Neanderthals,” notes the associate professor at Rovira i Virgili University.