Nichelle Nichols as Lo's Grandmother in the 2008 drama film, 'Tru Loved'.

Washington - Grandmothers: They feed you, they spoil you, they constantly needle you about your relationship status.

And, according to anthropologist Kristen Hawkes, they might be the driving force behind the evolution of much of human society.

Hawkes, an expert in human evolution and sociobiology at the University of Utah, is the author of several studies on the “grandmother hypothesis”, which asserts that many of the characteristics that distinguish us from our ape ancestors are thanks to the thoughtful care of our mothers’ mothers.

In the latest, published last week in the Proceedings of the National Academy of Sciences, she and her co-authors explain how grandmothering is a crucial factor behind the spread of monogamy.

The ancient evolutionary explanation goes like this: When grandmothers started to help out with child rearing, they freed up mothers to have more children, more quickly. Those longer-lived grandmothers end up having more grandchildren, each of whom carried their genes for longevity, helping to increase the human lifespan.

Longer lives and larger kin networks also made it more advantageous for men to mate with and protect a single women, so humans relationships became monogamous.

No wonder grandma is always asking why you aren’t married yet.

The chain of events that connects the existence of grandmothers to monogamous relationships is convoluted but compelling. It starts, Hawkes explains, with the Hadza people of northern Tanzania.

Hawkes began studying the Hadza in the 1980s. They are among the last hunter-gatherers in the world, and their way of life has persisted unchanged for tens of thousands of years. The fact that they eschew agriculture and continue to hunt and forage as their ancestors did makes their community a rare window onto humans’ prehistoric past, Hawkes explained.

One of many things she and her colleagues noticed was that the older Hadza women were “these amazingly productive tuber diggers”, she said. “They gathered this really important food resource that little kids are just too little to be good at finding”, and then fed the tubers to the children.

The sight of grandmothers feeding children might seem mundane to those of us whose own grandmas insist on plying us with food whenever we visit. But for Hawkes, it sparked a “eureka” moment.

The presence of post-reproductive women is something of an anomaly in the natural world, where, as anyone who has watched a nature documentary can attest, the prime directive is to find food and a mate. Among primates, humans are the only species that continue to live beyond menopause. Since having children is what drives evolution, there’s no good evolutionary reason for women to live past their ability to reproduce – at least as far as nature is concerned.

Unless, as Hawkes suggests, it’s so those women can become grandmothers.

In a 1997 study in the journal Current Anthropology, she and fellow anthropologists James O’Connell and Nicholas Blurton Jones argued that long post-menopausal lifespans evolved as older women began to play a greater role in caring for young.

If a grandmother was around to help out with her daughter’s children, the daughter was able to have more babies more quickly, rather than waiting until their older children were capable of caring for themselves (humans are also the only primates who give birth to a second child before the first is fully mature).

The longer a woman lived, the more grandchildren she was able to care for, meaning that the longest-lived grandmas had the most descendants and were best able to pass along their longevity genes. Over the course of millennia, this resulted in human lifespans that lasted decades past the point at which women were able to have children.

In 2012, Hawkes worked with an Australian statistician to develop a mathematical model for this process. They found that grandmothering could roughly double human lifespans from those of our closest ape cousins over the course of about 60 000 years.

Now Hawkes had humans living for far longer than they ever had before. She knew what the older women were doing – taking care of their grandchildren – but what about the men?

Unlike women, male fertility doesn’t decline around age 40. This meant that human societies had far more fertile men wandering around, and not enough women for them to mate with. This was a big shift from the grandmotherless societies of our primate relatives, where fertile females usually outnumber males. (Biologically speaking, male creatures tend to spend more time hunting and fighting, and are therefore at a much higher risk of an untimely death.)

There are three ways that male animals, driven by biology to ensure the furtherance of their genes, can maximise their number of offspring: they can attempt to mate with as many females as possible, they can stick with one female and prevent other males from mating with her, and they can invest time and resources in the offspring they already have.

Among most species, males opt for the first option, as a lady can be pregnant and responsible for only one batch of offspring at a time. That’s why bonobos, our closest relatives, have astronomical rates of sexual interaction.

But when the ratio of fertile adults is skewed towards men, playing the Don Juan can be a risky proposition.

With increased competition for mates, “the advantage ends up going to guarding the woman you’ve got”, as Hawkes put it. Among the newly longer-lived humans, it became evolutionary advantageous for men to mate for life, devoting their time and energy to protect a single woman and her children. Thus, Hawkes and her colleagues argue in their PNAS study, the monogamous relationship was born.

Hawkes and other proponents of the grandmother hypothesis believe a whole host of other unique human qualities might stem from grandmothers: bigger brains (because longer lives result in bigger pay-offs for time spent learning skills); complex communities (because of the switch from independent to co-operative child rearing); big game hunting (enabled by those big brains and big communities); even empathy (because large kin networks required that we evolve to respect and understand one another).

“It’s amazing, when you start to take grandmothering into account, how much it can tell you,” Hawkes said. “It’s such a rich likely source for a lot of other activities.”

However, not everyone agrees. The grandmother hypothesis is controversial in the world of anthropology. Many studies have argued that the evolutionary pay-off for grandmothering is insufficient to account for the dramatic increase in human longevity. Others say Hawkes’s emphasis on grandmothers disregards the fact that in many hunter-gatherer societies, including the Hadza, men provide the vast majority of food for their young.

A competing theory on menopause argues that it stems from intergenerational conflict among women. In other species, like elephants, younger females suppress their fertility while older females reproduce, so as not to compete for help and resources. Menopause achieves the same effect, but in reverse.

But Hawkes believes that one reason the grandmother hypothesis sits uncomfortably with some scientists is that it turns much of what is assumed about human societies on its head.

The “standard textbook story” for monogamy, as Hawkes puts it, starts with nuclear families and long-lasting pair bonds. If they were in it for the long haul, women were more likely to mate with good hunters, so larger, smarter communities developed, complete with kin networks,

That’s the story that anthropologists take for granted, said Hawkes.

But the grandmother hypothesis suggests that the monogamy isn’t quite so innate. Rather than being something fundamental to humans, Hawkes’s narrative has it developing over time – a response to the circumstances at hand, just like any other evolutionary adaptation.

It’s a less romantic account, perhaps, but it might be a more interesting one. Try asking your matchmaker grandma about it next time she calls.