Why did hominin brains triple in size over the past 6 million years?

Why did hominin brains triple in size over the past 6 million years? Discover magazine says it's because humans needed bigger brains to be able to accurately throw rocks.

Only an increase in brain size could make throwing–especially over long distances–accurate, he says. That neural circuitry was able to handle other complex matters: keeping track of social relationships, planning for the future, and developing language.

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