Grandmoms helped humans pair up and live longer: study

If you have found the love of your life, thank all the grandmothers since humans evolved, as they helped us bond with a single partner instead of mating with numerous people, a new study suggests.
University of Utah anthropologist Kristen Hawkes is known for the "grandmother hypothesis," which credits prehistoric grandmothering for our long human lifespan.
Now, Hawkes has used computer simulations to link grandmothering and longevity to a surplus of older fertile men and, in turn, to the male tendency to guard a female mate from the competition and form a "pair bond" with her instead of mating with numerous partners.

"Our hypothesis is that human pair bonds evolved with increasing payoffs for mate guarding, which resulted from the evolution of our grandmothering life history," the researchers said.
That conclusion contradicts the traditional view that pair bonding resulted from male hunters feeding females and their offspring in exchange for paternity of those kids so the males have descendants and pass on genes, Hawkes said.
The grandma hypothesis holds that "the key to why moms can have next babies sooner is not because of dad bringing home the bacon but because of grandma helping feed the weaned children. That favoured increased longevity as longer-lived grandmothers helped more."

As human longevity increased, there were increasing number of males in the paternity competition. However, to become a father these males needed younger, fertile females.

"So males who had preference for younger females were more likely to leave descendants," the researchers said.
Human females often live decades past their child-bearing years. Changing environments led to the use of food like buried tubers that weaned children could not dig themselves.
So older females helped feed the kids, allowing their daughters to have the next baby sooner.
By allowing their daughters to have more kids, grandmothers' longevity genes became increasingly common in the population and human lifespan increased.

A computer simulation showed that with grandmothering, the computed lifespans get longer like those of humans, often into the 70s or 80s.
As human lifespans grew longer, women's fertility continued to end by about age 45, while older men remained fertile. The new study indicates the ratio of fertile men to fertile women increased over time.
"That's what made it advantageous for males to guard a female and to develop a pair bond with her," Hawkes said.
The researchers ran computer simulations of human evolution - 30 simulations with grandmothering and 30 without.
The simulations showed how male-female sex ratios changed over time to become increasingly male-dominated.

"This male bias in sex ratio in the mating ages makes mate-guarding a better strategy for males than trying to seek an additional mate, because there are too many other guys in the competition," Hawkes said.
The study was published in the journal PNAS.