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If you are in a special relationship with another person, thank grandma - not just yours, but all grandmothers since humans evolved.
University of Utah anthropologist Kristen Hawkes is known for the "grandmother hypothesis," which credits prehistoric grandmothers for our long human lifespan. Now, Hawkes has used computer simulations to link grandmothering and longevity to a surplus of older fertile men. Which may explain their tendency to guard a female from other males and form a "pair bond" with her instead of mating numerous partners.
"It looks like grandmothering was crucial to the development of pair bonds in humans," says Hawkes, senior author of the new study published online in the Sept. 7 edition of the journal Proceedings of the National Academy of Sciences (PNAS).
"Pair bonds are universal in human societies and distinguish us from our closest living relatives. Our hypothesis is that human pair bonds evolved with increasing payoffs for mate guarding, which resulted from the evolution of our grandmothering life history," explains Kristen Hawkes, anthropologist at the Univerity of Utah, and her colleagues. The quote is taken from the published study.
That conclusion contradicts the traditional view that pair bonding "resulted from male hunters feeding females and their offspring in exchange for paternity of the kids - so that those males have descendants to pass on their genes," Hawkes adds.
The new study focuses on one of the results of grandmothering — an excess of older males competing for mates, looking for young women to bear their children. "This is different than what you see in chimpanzees, where males prefer older females," says Hawkes, a distinguished professor of anthropology and National Academy of Sciences member. As human longevity increased, there were "lots more old guys, so you have an increasing number of males in the paternity competition. As the only way you can become a father is with a fertile female, this means younger females are sought after. Males with a preference for younger females were more likely to leave descendants."
The grandmother hypothesis and pair bonding
The grandmother hypothesis was first published in 1997-1998, based on observations that began in 1984 amongst the Tanzania's Hazda people. The Hazda still lived by hunting and gathering food as had all our ancestors. Researchers noticed the importance of older Hazda women digging tubers to feed youngsters not strong enough to dig tubers themselves. Female chimps rarely live past their childbearing years, usually dying in their 30s and sometimes 40s, whereas human females often live decades past child-bearing, which may have begun 2 million years ago with our early Homo relatives.
Previous research by others has also shown a link between "mate guarding" - in which various male animals guard their female mate against competing suitors when the male to female ratio is high - adding to the development of pair bonding in humans.
Pair bonding includes but doesn't require an exclusive relationship - polygamists can have multiple pair bonds - but it does mean "a special and persistent relationship between a male and female. Even something like two people going together for a couple of months - that's a pair bond," Hawkes adds. "Copulation alone doesn't count. In humans, there's emotional weight to social relationships, certainly to pair bonds."
By contrast, chimps have no persistent, special pair bond relationships between a particular male and female. A female chimp in heat mates with multiple males. Species from dung flies to primates guard their mates to ensure others don't mate with them.
Simulating the evolution of grandmothering
As human lifespans grew longer, women's fertility continued to end around age 45, while older men remain 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 says.
For the study, researchers ran computer simulations of human evolution — 30 simulations with grandmothering and 30 without. The simulations showed that male-female sex ratios changed over time to become increasingly male-dominated. This is unlike actual nonhuman great ape populations with more fertile females than fertile males.
The ratio of males to females in fertile ages rose from 77 males per 100 females without grandmothering to 156 males per 100 females with grandmothering in 30,000 to 300,000 simulated years.
Unlike humans, most mammal species have more fertile females than fertile males.
In the simulations, researchers showed male-female sex ratios matched closely those of living populations — and principally in chimps who lack grandmothering. Chimps are the only other great ape with good demographic data, along with that from four human hunter-gatherer societies in Africa and South America.
The study then cites previous studies from both living animals - dung flies to mammals - and computer simulations, which show that when the ratio of fertile males to females is high, mate-guarding is likely.
"Mate-guarding and pair bonds are not necessarily the same. But, they have in common the tradeoff — paying attention to the current partner or seeking another," Hawkes adds. There is also previous research showing that, like mate-guarding, "human pair bonds have the characteristic of male proprietary claims on females."
The lengthening of adult lifespan via grandmothering involves evolution in prehistoric time. Increasing average lifespans in recent centuries is largely attributed to huge reductions in infant and child mortality as the result of clean water, sewer systems and other public health measures.