FROM THE STANDPOINT of biology, males have nothing to do after copulation. “It's literally wham-bam thank-you-ma'am,” says Kermyt G. Anderson, an anthropologist at the University of Oklahoma–Norman and co-author of Fatherhood: Evolution and Human Paternal Behavior.
What made the first father stick around afterward? He was needed. At some point in the six million years since the human lineage split from chimpanzees, babies got to be too expensive, in terms of care, for a single mother to raise. A chimp can feed itself at age four, but humans come out of the womb essentially premature and remain dependent on their parents for many years longer. Hunters in Amazonian tribes cannot survive on their own until age 18, according to anthropologist Hillard Kaplan of the University of New Mexico–Albuquerque. Their skills peak in their 30s—not unlike income profiles of modern men and women.