Before the pot, bread and flour there was already a rather cumbersome problem: a brain that was growing and demanding glucose. To provide it, our ancestors may have relied much more than we have previously reported on ripe fruit, honey, and, later, starchy tubers.
It is the reconstruction proposed by a study published in Science by Jennie Brand-Miller, Karen Hardy, David Raubenheimer, and Les Copeland, who modeled approximately four million years of evolution of the human diet. The research debunks one of the most resistant narratives about our origins: that in which increased meat consumption almost alone provided the energy necessary for brain growth. Animal foods remain important. Fruit, honey and starches, however, enter the story through the front door.
A huge brain presents a huge bill
Among the first hominins e Homo sapiens the brain went from roughly 300 to 1,500 grams. In the adult body it represents just 2% of the weight, yet it can absorb about 20% of the basal metabolism. In five-year-old children the share can even reach 66%. A small organ compared to the rest of the body and decidedly not parsimonious.
Its main fuel is glucose, which is also necessary for red blood cells, kidneys and, during pregnancy and breastfeeding, the fetus, placenta and mammary glands. Our body can also produce it through other metabolic processes, but the authors wondered how much of this demand could be met directly by the carbohydrates available in the diet of our ancestors.
They then built six scenarios, starting from a diet similar to that of chimpanzees, with just 5% of energy coming from animal foods, up to ratios observed in some contemporary hunter-gatherers in warm climates, where animal foods reach 35-50%. The model includes metabolism, physiology, fossil isotopes, teeth, genetics, primate nutrition and food composition.
So no one found Lucy’s lunchbox. We are talking about an evolutionary model, built by matching different clues and metabolic needs: it serves to estimate which diets were plausible, not to reconstruct the Tuesday lunch of an australopithecus.
At first, glucose came mainly from fruit
In the first scenario, the most ancient hominins are highly dependent on carbohydrates: according to calculations, over 65% of the energy could have come from the sugars contained in fruit and other sweet foods. Honey was another concentrated source of energy, when of course you could get to it without turning the meal into an argument with thousands of bees.
Overall, the model suggests that sugars provided more than 25% of dietary energy for long periods, enough to contribute to the obligatory glucose demands of an ever-larger brain. It is a quota that makes it rather difficult to relegate vegetables to the role of prehistoric side dish.
The authors also recall other clues: color vision suitable for identifying ripe fruit, tooth morphology, stable isotopes, dental caries and genetic adaptations linked to carbohydrate metabolism. None of these elements, taken alone, deliver the menu of our ancestors. Put together, however, they are compatible with an early hominin diet very rich in sweet vegetable foods.
Then we learned to cook the tubers
At a certain point fire enters the scene and changes a lot. Raw starch is difficult to digest and makes the glucose it contains much less accessible. Cooking and mechanical processing break this barrier and turn tubers and other underground parts of plants into much more convenient energy sources.
The University of Sydney, where lead author Jennie Brand-Miller works, lists wild yams, water lily rhizomes and African potatoes as plausible foods. Cereals and flours entered history firmly much later.
Thus the source of glucose slowly changes face: first mainly sugars from fruit and honey, then more and more cooked starches. In one scenario for anatomically modern humans, with 35% of energy coming from animal foods, sugars and starches each provide about 25% of total energy.
Cooking, from this point of view, has done something much more interesting than making tubers less sad: it has made the glucose trapped in starches available and enormously expanded the range of foods useful for supporting an energy-intensive brain.
Meat loses its monopoly on storytelling
This study doesn’t send meat out of prehistory. The same researchers define animal foods as important in human evolution: they offered concentrated proteins, fats and micronutrients and their weight probably increased along with the ability to hunt, recover carcasses and process food better.
But the photography changes. The evolution of a big brain seems much less like a linear march toward steak and much more like a story of meat, fruit, honey, tubers, fire, and tools, with proportions shifting over millions of years.
Pregnancy and childhood also find space within this reconstruction, often quite marginal when evolution is told through hunters, spears and mammoths. The model estimates an overall obligatory glucose requirement of approximately 150-200 grams per day in men, 200-250 in women of reproductive age, and approximately 125 in young children. Fetus, placenta and milk production further increase the female energy bill.
Before dealing with the snacks, there is one detail missing
However, “Sugar” risks taking us straight to the wrong section of the supermarket. The work talks above all about the sugars naturally present in fruit and honey and the glucose subsequently obtained from starches. He’s not arguing that industrial cookies, sodas, and sweets have suddenly been given an evolutionary certificate of merit.
The University of Glasgow, where co-author Karen Hardy works, makes just this distinction: refined sugars and the modern abundance of rapidly available carbohydrates have very different characteristics from the foods that accompanied much of human evolution. The fruit came with fiber, vitamins and minerals. And above all we had to find it.
Perhaps this is also why the vegetal part of our history has remained more blurry. A butchered bone may survive long enough to end up under the eyes of an archaeologist millions of years later. An eaten fig hardly grants the same favor.
