Eating a raw food diet is a recipe for disaster if you're trying to boost your species' brainpower. That's because humans would have to spend more than 9 hours a day eating to get enough energy from unprocessed raw food alone to support our large brains, according to a new study that calculates the energetic costs of growing a bigger brain or body in primates. But our ancestors managed to get enough energy to grow brains that have three times as many neurons as those in apes such as gorillas, chimpanzees, and orangutans. How did they do it? They got cooking, according to a study published online today in the Proceedings of the National Academy of Sciences.
"If you eat only raw food, there are not enough hours in the day to get enough calories to build such a large brain," says Suzana Herculano-Houzel, a neuroscientist at the Federal University of Rio de Janeiro in Brazil who is co-author of the report. "We can afford more neurons, thanks to cooking."
Humans have more brain neurons than any other primate—about 86 billion, on average, compared with about 33 billion neurons in gorillas and 28 billion in chimpanzees. While these extra neurons endow us with many benefits, they come at a price—our brains consume 20% of our body's energy when resting, compared with 9% in other primates. So a long-standing riddle has been where did our ancestors get that extra energy to expand their minds as they evolved from animals with brains and bodies the size of chimpanzees?
One answer came in the late 1990s when Harvard University primatologist Richard Wrangham proposed that the brain began to expand rapidly 1.6 million to 1.8 million years ago in our ancestor, Homo erectus, because this early human learned how to roast meat and tuberous root vegetables over a fire. Cooking, Wrangham argued, effectively predigested the food, making it easier and more efficient for our guts to absorb calories more rapidly. Since then, he and his colleagues have shown in lab studies of rodents and pythons that these animals grow up bigger and faster when they eat cooked meat instead of raw meat—and that it takes less energy to digest cooked meat than raw meat.