There is no question that our large brains have provided humans an extraordinary advantage in the world. Still, the human brain is an incredibly expensive organ, taking up only about 2 percent of the body’s mass yet using more than a fifth of the body’s energy, and until about 2 million years ago none of our ancestors had a brain larger than an ape’s when compared to body size. So what kicked off the push for a larger brain? One possibility is that increased smarts helped our ancestors make better tools. Another is that larger brains helped us interact better with each other. Perhaps radical changes in the environment also demanded that our ancestors deal with a shifting world.
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