Fetal Timeline      Maternal Timeline     News     News Archive    Sep 18, 2015 

A Hazda couple and child in northern Tanzania in 1985. Research on the Hazda led anthropologist Kristen
Hawkes and colleagues' to formulate the original grandmother hypothesis'. The theory says that grand-
mothering among early humans led to our human lifespan extending much longer than that of other apes.
Now, a new study credits grandmothering for the human tendency to form couples or pair bonds.
Image Credit: James F. O'Connell, University of Utah

The grandmother hypothesis

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 grandma hypothesis:

"The key to why moms can have their next baby sooner is not because of dad bringing home the bacon but because of grandma helping feed the weaned children. That act favored increased longevity as longer-lived grandmothers helped more."

Kristen Hawkes PhD, Department of Anthropology, Univerity of Utah, and colleagues.

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.

The grandmother hypothesis contends that before 2 million years, few females lived past their fertility. But drying climates led to eating food such as buried tubers which hold moisture in their swollen roots. However, newly weaned children can't dig up such food by themselves, so older females helped feed the kids, allowing their daughters to have their next baby sooner.

By allowing their daughters to have more kids, grandmothers' longevity genes became increasingly common in the population and our human lifespan increased. A 2012 computer simulation study by Hawkes and colleagues supported this hypothesis, finding that without grandmothers, simulated lifespans reach equilibrium when they match those of great apes, but with grandmothering, computed lifespans get longer like those of humans, often into the 70s or 80s.

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.

"The male bias in sex ratio within mating ages makes mate-guarding a better strategy than trying to seek an additional mate. There are too many other guys in the competition. The more males there are, the more their average reproductive success goes down."

Kristen Hawkes

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.

Many anthropologists argue that increasing brain size in our ape-like ancestors was the major factor in humans developing lifespans different from apes. But Hawkes' 2012 study ignored brain size, hunting and pair bonding, and found even a weak grandmother effect led to human longevity.

She believes the shift to grandmothering was the foundation for several important steps in human evolution: longer adult life spans, increased brain size, empathy, cooperation and pair bonding.

Significance
Pair bonds are universal in human societies and distinguish us from our closest living relatives. They characteristically involve men’s proprietary claims over women—mate guarding—which in animals generally is both predicted and observed to be more frequent when sex ratios in the fertile ages are male-biased. A marked male bias in the fertile ages evolved in our lineage as longevity increased without an extension of female fertility. We compare the sex-ratio shift in simulations of the grandmother hypothesis to demographic data from chimpanzees and human hunter–gatherers then connect the expanded proportions of older men to benefits for mate guarding, the evolution of pair bonds, and the long recognized importance of male alliances in human social life.

Abstract
The evolution of distinctively human life history and social organization is generally attributed to paternal provisioning based on pair bonds. Here we develop an alternative argument that connects the evolution of human pair bonds to the male-biased mating sex ratios that accompanied the evolution of human life history. We simulate an agent-based model of the grandmother hypothesis, compare simulated sex ratios to data on great apes and human hunter–gatherers, and note associations between a preponderance of males and mate guarding across taxa. Then we explore a recent model that highlights the importance of mating sex ratios for differences between birds and mammals and conclude that lessons for human evolution cannot ignore mammalian reproductive constraints. In contradiction to our claim that male-biased sex ratios are characteristically human, female-biased ratios are reported in some populations. We consider the likelihood that fertile men are undercounted and conclude that the mate-guarding hypothesis for human pair bonds gains strength from explicit links with our grandmothering life history.

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