Just as mammals before us got brainier relative to their ancestors, we primates got brainier, too, compared to the other mammals. At the same time, gestation length expanded—babies cooked for longer inside mom before being born. Litter size fell, so mothers had fewer children at a time to tend to. The period of parental investment after birth lengthened and intensified, and sexual development happened later and later, giving ever more time for young primates to learn how to feel, how to think, how to be.
