LOS ANGELES — 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 with 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 Neurociencia Translacional in Sao Paulo, Brazil, decided to try and help explain why modern humans’ brains were able to grow so large compared with 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.

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 with their body size. Great apes’ brains (including gorillas and orangutans) make up something like 0.4 percent to 0.6 percent of their body mass, while humans’ brains take up a full 2 percent – even though it drains some 20 percent of the body’s resting metabolic rate. (In non-human primates, the brain burns roughly 9 percent of the body’s energy.).

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 percent the size of its body mass – as a human does – it would have to add 122 billion neurons, corresponding to an extra two hours, 12 minutes of feeding – and a gorilla already spends 80 percent of a 12-hour day in feeding.

Humans, however, have clearly overcome this limitation – we have large brains, and we don’t spend nine 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.