Researchers have long suspected that the evolution of a bigger brain is what caused humans to live longer than other Great Apes. However, a new study has challenged this belief and proposed that the introduction of grandmothers into the nuclear family was not only the major factor behind our long lifespans, but also may have played a key role in the evolution of romantic relationships between mates.

In 1998, University of Utah anthropologist Kristen Hawkes first introduced us to her “Grandmother Hypothesis,” which theorized that prehistoric grandmothers were behind longer human lifespans . Now, in her most recent study published in the Proceedings of the National Academy of Sciences , Hawkes had added her to original theory and presented compelling new evidence to further stress the importance of ancient grandmothers.

"It looks like grandmothering was crucial to the development of pair bonds in humans,” explained Hawkes in a statement .

According to Hawkes pair bonds or relationships between male and female mates are “universal in human societies” and help to distinguish us from other Great Apes, our closest living relatives. The new hypothesis supposes that grandmothers enabled other women to have more babies at a faster rate by helping to take care of children who no longer relied on breastmilk. This allowed women to have more children who could survive into adulthood. The grandmother’s importance in helping to raise families led to an evolutionary preference for women who could live longer, and thus look after grandchildren longer. As a result longevity genes became more prominent in the human population and in time increased the lifespan of all humans.

Women were not the only ones to benefit from this increased lifespan and soon there became an abundance of older males. This increasing population of older adults had long-lasting effects that are still seen today. In humans, a female’s fertility significantly drops at around 45 but male fertility remains well into old age. As a result, the number of fertile men soon overpassed the number of fertile women. Due to the scarcity of fertile females, males began to pay more attention to their mates and take up the practice of mate-guarding. According to Hawkes, this practice soon led to the distinctly human practice of pair-bonding.

“Mate-guarding and pair bonds are not necessarily the same, but they have in common the trade off between paying attention to the current partner and seeking another," Hawkes explained. "Copulation alone doesn't count. In humans, there's emotional weight to social relationships, certainly to pair bonds."

Pair-bonding is also reflective of the idea that females are male’s property, an unfortunate yet realistic characteristic of human societies . Hawkes used a computer simulation to prove her hypothesis and ran 30 simulations of evolution with grandmothering and 30 without. Results showed that the simulations with grandmothers closely matched the male-female sex ratio of humans. The simulations with grandmothering more closely matched that of chimpanzees, a Great Ape species who lack grandmothering in their societies. Along with lacking grandmothers, chimpanzees also have shorter lifespans (with women rarely living past childbearing years) and generally have more fertile females than males in their communities, thus further proving the validity of Hawkes' claim.

Source: Hawkes K. Proceedings of the National Academy of Sciences . 2015