Infants aren't initially much more than wet noisy pets. They can't survive without continual adult nurturing, and they tend to get it. Elevated levels of the neurochemicals that enhance bonding in infants and their parents help to initiate and maintain the fascination that parents and children have for each other.

Our upright stance and consequent narrow female birth canal led to a human birth brain that's only one-third its adult size. A four-legged mother cat, has a relatively wider birth canal, and so her kittens are born with an almost completely developed brain. Feral cats can therefore survive independently when weaned.

Our brain's post-birth maturation takes about twenty years, divided into a 10-year sheltered childhood, during which basic survival systems mature, and the child learns how to identify the central elements of a challenge; and a 10-year more independent adolescence, during which young people begin to bond with each other, and develop the cultural knowledge and skills necessary for a productive autonomous adult life.