Study links cooked food to early humans’ growing brains

Here’s something for raw-food aficionados to chew on: Cooked food might be a big reason humans were able to grow such large brains compared to their body size, scientists say.

If modern human ancestors had eaten only raw food, they’d have to regularly feed more than nine hours a day, according to a study published online Monday in Proceedings of the National Academy of Sciences.

A pair of researchers from the Instituto Nacional de Neurociéncia Translacional in São Paulo, Brazil, decided to try and help explain why modern humans’ brains were able to grow so large compared to their body size and why other primates’ brains did not.

They looked at the relative brain-to-neuron-counts of a host of primates, from owl monkeys to baboons. For the most part, these primates followed a linear pattern – the bigger the brain, the more neurons they had.

Things get a bit tricky with the brain-to-body ratio, however -- partly because humans’ brains are so large (and have so many neurons) compared to their body size. Great apes’ brains (including gorillas and orangutans) make up something like 0.4% to 0.6% of their body mass, while humans’ brains take up a full 2% -- even though it drains some 20% of the body’s resting metabolic rate. (In non-human primates, the brain burns roughly 9% of the body’s energy.)

The brain is the third most energy-expensive organ in the body, behind only skeletal muscle and the liver, according to the study authors. Researchers suspect this might explain why gorillas don’t have large brains, too -- there’s a limit to the energy they can take in to sustain a big thinker, because there’s only so much time in the day to eat.

Previous work had shown that adding neurons to the primate brain cost about 6 kCal per billion neurons. For a gorilla to have a brain that’s 2% the size of its body mass -- as a human does -- it would have to add 122 billion neurons, corresponding to an extra 2 hours, 12 minutes of feeding -- and a gorilla already spends 80% of a 12 hour day in feeding.

Humans, however, have clearly overcome this limitation – we have large brains, and we don’t spend 9 hours a day eating. (Or at least, we don’t actually need to.)

So how could humans have broken this pattern? A theory, as posited by Harvard University primatologist Richard Wrangham, is that humans began cooking their food, releasing nutrients locked in the raw food, saving time they spent chewing the stuff.

Had Homo erectus not cooked their food, they would have had to spend more than eight hours a day feeding their 62 billion neurons, the authors calculate, and Homo sapiens would have had to spend more than nine hours a day sustaining their 76 billion to 86 billion neurons.

Lucky for today’s raw-food devotees, there are blenders to do much of the work.

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