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.

Oddly enough, bird families also tend to have stay-at-home dads. In more than 90 percent of bird species, both parents share the care of their young. This arrangement probably began, at least for most birds, when males started staying around the nests to protect helpless babies from predators. “A flightless bird sitting on a nest is a very vulnerable creature,” explains evolutionary biologist Richard O. Prum of Yale University.

Some birds, though, might have inherited their particular form of fatherhood from dinosaurs. Male theropods, a close relative of birds, seem to have done all the nest building, just as male ostriches do today. That doesn't mean everything was on the up and up. A female ostrich will lay an egg in the nest of her mate, but usually a different male fertilizes it. “There's a loose relationship,” Prum says, “between paternal care and paternity.”