Carbohydrates are a rich source of energy. That's exactly why some of us may feel a bit conflicted about them, since several recent studies and diets have suggested we should cut them to lose weight. (The latest study concluded that total calories matter most if you want to shed pounds.)

Our ancient ancestors, however, gathered carbohydrates greedily for their energy. And by looking at past work on human evolution, a group of British researchers has a hunch that once ancient humans learned to cook, starchy foods like root vegetables or grasses could have given them a calorie bump that fueled the evolution of the human brain.

Researchers studying Paleolithic diets have previously suggested the early human brain began getting big at least 2.5 million years ago after early humans learned to butcher and process meat with stone tools. "But I don't think that's the whole story," says Mark Thomas, an evolutionary geneticist at University College London. "Around 800,000 years ago, the brain truly accelerated in increasing its size."

In a new paper in The Quarterly Review of Biology, Thomas and colleagues argue that the brain suddenly began evolving faster because our ancestors discovered an even better brain food. "At that point, we develop the use of fire and start consuming more carbohydrate-rich foods," Thomas says. "[That] was critical to the expansion of the brain."

Carbs, particularly long chains of the simple sugar glucose or starches, are an ideal food for fueling the brain, says Thomas. "The brain has an absolute requirement for glucose," he says. And with carb-rich food, the body doesn't need to spend extra energy converting other nutrients, like those found in meat, into glucose to feed the brain.

Once humans began cooking vegetables, starch-digesting enzymes in their guts called amylases could work much more efficiently than they could on raw vegetables. "In potatoes, for example, you could digest the starches about 20 times faster if [they're] cooked rather than uncooked," Thomas says. "And the earliest evidence for fire are also around the 500 to 800,000-year period." Cooking turned carbs into a major source of energy at the same time our brains began expanding in earnest, the team suggests in the paper.

To figure out if humans actually were eating more starches at this time, the team looked at the genes that make amylase. Thomas says genetic analyses suggest these genes started multiplying around the same time people began cooking, meaning our saliva was evolving to carry higher concentrations of the enzyme. "There's more starch to digest and therefore it's an advantage to increase the amount of salivary amylase genes," he says. That would mean early humans could get more glucose out of a mouthful of potato, providing more energy to grow a hungry brain.

But other researchers aren't swayed by the hypothesis. "It makes sense that if you eat more starch you'll have more amylase, and that's a beautiful suggestion. But the fact is the data don't support it yet," says Richard Wrangham, a biological anthropologist at Harvard University.

To support their hypothesis, Thomas and his colleagues cited work from anthropologists at Arizona State University that suggested certain groups of humans with high-starch diets had more amylase in their saliva, while humans with low-starch diets had less.

Wrangham says that study analyzed each population's diets incorrectly. "Based on the data I found, it seemed as though they got it exactly opposite. Their high-starch populations all ate less starch than any of the low-starch populations." According to Wrangham, that means there's no evidence that eating more carbohydrates will make saliva evolve to digest starches better. (Thomas also acknowledges the study is flawed.)

Other critics say that the timeline in Thomas' version of the brain's evolution doesn't fit neatly together, something Thomas admits is true, too. When amylase genes began multiplying, for one, is uncertain. "It's basically sometime in the last million years, but we don't know when," he says. "We want to have more, much more evidence for when [it] started to increase."

And when exactly humans began cooking is also up for debate. "The best evidence we have for fire from 700, 800,000 years ago," says Shara Bailey, a biological anthropologist at New York University. "If they're arguing starch cooking was needed for large brains, well, that's fine," she says. "But large brains were already established by the time we have any good evidence for cooking."

What specific food lead to the rise of the modern human brain might be a moot point. Cooking makes everything easier to eat, and being able to cook at all is probably more important than what early humans cooked, says Suzana Herculano-Houzel, a neuroanatomist at Federal University in Rio de Janeiro, Brazil.

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