—Brigitte Holt

Humans have a much slower life-history pattern than their relatives, the great apes: chimpanzees, gorillas, orangutans. Human infants mature much more slowly and thus face higher mortality risks. A human mother must invest more time and energy than a chimpanzee mother on each child. This higher investment increases reproductive costs, since it can potentially lower their fertility. Compared to great apes, though, humans have relatively high fertility. The interval between births in many human societies is two years, versus five to seven years for chimpanzees and orangutans.

How can human females afford to have children so closely together? Somewhere far back in their history, humans evolved two adaptations that formed the basis of the human family: pair-bonding and high levels of paternal investment in raising children. While for males the cost of these changes is a decrease in fertility (less access to females), this loss is balanced by several important benefits. The added paternal investment of resources and energy lowers the risk of the vulnerable offspring dying before adulthood, before they can reproduce. Males who form permanent bonds with one females invest more time and resources in their offspring, thereby reducing mortality rates of these offspring and  enhancing their chances of passing on their genes.

When did these traits emerge? One theory proposes that pair-bonding co-evolved with bipedal locomotion as early as 4 million years ago. Being able to walk upright on two feet allowed males to travel efficiently in the increasingly dry and open savanna of Africa, and thus to provide more resources to female and offspring. Females in turn selected males that were better providers, and the genes for bipedalism got passed on, because caring males who were able to provide more resources and therefore had more offspring survive left more copies of themselves. 

Arguing against this theory is that human fossils from 3 to 4 million years ago show that males were larger than females and had bigger canine teeth. This is characteristic of polygynous primates like gorillas, not monogamous ones. There is no advantage to monogamous males being bigger or having bigger teeth because they do not have to fight each other to compete for females.

More likely pair-bonding evolved as brain size began to increase, about 2 million years ago. There is good evidence that larger brain size in turn led to the slow-down in maturation. These two changes resulted in increased costs for human mothers. The larger-brained infant required more food to grow and took more time to become independent. Having a male that contributed to raising this expensive offspring would have been extremely adaptive. Around that time, the fossil record indicates, body size and canine size differences between males and females decreased. Apparently, the evolution of the human social system, and pair-bonding in particular, led to a reduction in male aggression.