“Some 2.5 million years ago, our ancestors’ brains expanded from a mere 600 cubic centimetres to about a litre. Two new studies suggest it is no fluke that this brain boom coincided with the onset of an ice age. Cooler heads, it seems, allowed ancient human brains to let off steam and grow. For all its advantages, the modern human brain is a huge energy glutton … [and] would have generated heat faster than it could dissipate it in the warmer climate of earlier times.”