In 2017 researchers studying primate brains made a curious discovery: across more than 140 species, primates that ate fruit had larger brains, on average, than their meat- or leaf-eating peers. The findings suggested that increases in brain size in primates were driven by food—or perhaps foraging—rather than complex social interactions, another leading theory.
For Jennie Brand-Miller, it was “a light bulb moment.” Fruit is full of sugar, and our brain relies on glucose, a sugar, as its primary energy source. That led Brand-Miller, an emeritus professor of human nutrition at Australia’s University of Sydney, to wonder whether sugary food might have had a hand in human evolution.
Now, in a new analysis published in Science on Thursday, Brand-Miller and her colleagues argue that sugar played a “central” part in human evolution, particularly early on. “No matter what period of human evolution, sugars were playing an important role,” she says.
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Brand-Miller and her colleagues relied on existing data about the size of various hominin species and information gleaned from the fossil record about their diets to model the breakdown of sugars, protein, fiber, starch and fat in the hominin diet over time. The analysis spans approximately four million years—starting with Australopithecus afarensis, or “Lucy,” one of the first hominins believed to walk on two legs, and ending with the diets of modern humans in foraging communities.
The findings suggest that early hominins may have gotten as much as 66 percent of energy from sweet food such as fruit and honey. Over the span of human evolution, hominins accumulated 20 to 35 percent of their energy from these foods, the analysis found.
The results don’t mean animal-based products such as meat or fish didn’t matter, Brand-Miller emphasizes. “We believe that this is two parallel stories, that a balanced diet means you get sufficient protein, fat and carbohydrate,” she says.
The analysis has “a lot of value,” says Matt Sponheimer, a professor in the department of anthropology at the University of Colorado Boulder, who was not involved in the study. “It lets us play in conceptual space, and that can be really important for developing new ideas and better understanding old ones.”
“But it is crucial to remember that such work is built on uncertain ground,” he adds. Indeed, the analysis inherently relies on assumptions. The authors assumed, for instance, that A. afarensis ate about the same diet as that of a chimpanzee. That might be true, Sponheimer says, but who’s to say? In fact, there is fossil evidence to suggest Lucy’s diet could have been more like that of primates “that eat very different foods,” he says.
Ultimately, the takeaway fits with what most researchers think early humans ate—which what popular ideas about meat-rich Paleo diets might suggest. “Almost no one who studies hominin diet seriously believes they were carnivores, although we have friendly debates as to the proportions of plant and animal foods they consumed,” Sponheimer says.
To Brand-Miller, the results add to a growing body of evidence pointing to the importance of fruit and sugar in our diets: For instance, humans depend on vitamin C, a nutrient found in fruits, for bodily functions such as producing collagen or supporting the immune system. We have sweetness receptors throughout the small intestine. And we like the taste—that didn’t come from nowhere.
“I think we should recognize that sweetness is something really important to humans, and that desire should be satisfied on a regular basis,” she says.