WHY are our brains so big?

WHY ARE OUR BRAINS SO BIG? 

     Our brain is twice as heavy as it should be for an ape of our size. A single mutation may have cleared the way for this to happen. The strong jaw muscle of other primates constrains the growth of their skulls. But 2 million years ago a mutation weakened this grip in the human line. A brain growth spurt began soon after. What drove this spurt is a matter of debate. The difficulties of navigating a larger social network probably played a part, as did nutrition. A big brain is incredibly hungry (the human brain makes up just 2 per cent of our body weight, but devours 20 per cent of our energy intake) so early humans needed to change their diet to support it. The transition to eating meat and seafood would have helped, as would the invention of cooking.