There is simply no compelling reason we know of to explain why human brains got large,' says Tattersall. Huge brains are demanding organs: they make up only 2 per cent of the body's mass, but devour 20 per cent of its energy42. They are also comparatively picky in what they use as fuel. If you never ate another morsel of fat, your brain would not complain because it won't touch the stuff. It wants glucose instead, and lots of it, even if it means short-changing other organs.