"Our Ancestors Were Babysitters"

"why would infant sharing be an evolutionary advantage? For one possible answer, just look at human development, says Chris Kuzawa, an anthropologist at Northwestern University. In a 2014 study in the Proceedings of the National Academy of Sciences, he and his colleagues used brain-imaging data to calculate how much glucose (energy) the brain needs from birth to adulthood. They found that the brain demands the most sustenance between ages 3 and 7—a crucial period of brain development.

By this time, Kuzawa points out, a paleolithic mom probably would have been pregnant again, or already nursing another kid. So how did her firstborn get the nourishment he needed to grow a big, healthy brain? The child bonded with other clan members, mothers as well as non-mothers—a skill we still use today, Kuzawa argues. “Whoever is a part of our lives and cares for us is who we attach to. It’s pretty flexible.”"

http://nautil.us/blog/our-ancestors-were-babysitters