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The modern brain is an energy hog. The organ accounts for about 2 percent of body weight, but it uses about 20 percent of the oxygen in our blood and 25 percent of the glucose (sugars) circulating in our bloodstream, according to the American College of Neuropsychopharmacology.
These energy requirements have spurred a debate among anthropologists about what fueled the evolution of big brains in the first place. Many researchers credit meat, citing evidence of hunting in our early ancestors. But meat would have been an unreliable food source, say other scientists. A 2007 study published in the Proceedings of the National Academy of Science found that modern-day chimps know how to dig for calorie-rich tubers on the savanna. Perhaps our ancestors did the same, boosting their brainpower with veggies.
As for what motivated the brain to balloon in size, there are three major hypotheses: climate change, the demands of ecology, and social competition.