Dads in early human species would have aided in carrying children, as well as in their bathing, feeding, playing and teaching them the lessons of prehistoric life, says Gettler. They traded these services with the females for access to mating, allowing for monogamy and the modern family structure to develop.
Gettler’s hypothesis aims to explain a mystery anthropologists have long explored. When the Homo genus branched off from other ape descendants, it grew larger by increasing caloric intake and reducing energy expenditures. The largest energy expenditures found in primate species is child-bearing and rearing.
Yet even as our pre-human ancestors grew larger, the amount of time between pregnancies — known as the interbirth interval — actually grew shorter than their smaller ancient counterparts. Females began having more, rather than fewer, children.
Gettler says that the shorter interbirth interval and the long period of child rearing characteristic in modern humans could have only happened with ancient dads lending a hand.