Cooked food may be the reason why the human brain got as big as it did.
Raw food, say researchers in Brazil, doesn't cut it when it comes to feeding the brain the calories it needs to function and grow.
Indeed, the brain represents 20 percent of the resting metabolic rate, despite being only two percent of body mass, said the Guardian.
The researchers from the Instituto Nacional de Neurociéncia Translacional in São Paulo, Brazil, pointed out that gorillas, despite their massive body size, have smaller brains than humans.
This also despite the fact they eat a great deal more than people do - yet they spend a lot more time and energy doing it.
The researchers point to the gorilla's raw food diet cannot possibly provide the energy to develop a bigger brain.
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They also looked at the relative brain-to-neuron count over various other primates, which pointed to the notion that the bigger the brain, the more neurons it contained, said the Los Angeles Times.
This did not hold true for body size, however.
The study suggests that a human could not have developed a brain that holds 86 billion neurons without cooking their food and thus having the spare time to not continually eat all day as other primates do.
"Why are the largest primates not those endowed with the largest brains as well?" asked study authors Suzana Herculano-Houzel and Karina Fonseca-Azevedo, reported the Guardian.
"Rather than evidence that humans are an exception among primates, we consider this disparity to be a clue that, in primate evolution, developing a very large body and a very large brain have been mutually excluding strategies, probably because of metabolic reasons."
The findings were published in the Proceedings of the National Academy of Sciences.