Speak to professionals from various disciplines and you will notice something funny: Even when they are off duty, they tend to view the world through the lens of their professional background. For example, a psychiatrist at a dinner party might pause to think a bit about the possible neuroses of the guests. A police officer might scrutinize an ordinary scene — and the people present — for any sign of illegal activity. It’s not that they will do anything about it or even speak their mind, but I bet that’s what goes on in their heads — it’s the same with evolutionary biologists.

A dinner party is a rich source of observations of animal behavior and a biologist automatically categorizes human activity according to what we’ve learned from animals. Try it next time you’re at a party or in a bar. One of the most obvious behaviors shown by men is what biologists call “mate-guarding.”

This occurs when a man (or a male of any species) has reason to believe their mate may attract the attention of a rival. Females have many reasons why they might actually want to interact with other males but, for now, let’s consider the male point of view.

Men have various tactics for discouraging the attention of other men. You might see a man standing close to his partner, his arms folded in order to display his muscles. Sometimes you’ll see men staring down potential rivals who approach, or drawing their mate away from areas where single men are clustering. In this way, the man is attempting to enforce a buffer zone around the woman to prevent her from interacting with other men.

Until recently, the evolutionary explanation for this behavior has been straightforward: Once you’ve gone to the effort of attracting and securing a mate, you don’t want some other male to come along and steal her. This guarding happens in various ways in animals as diverse as dung flies, dogs, birds and humans. Now, however, it seems there is a more nuanced explanation.

Kristen Hawkes at the University of Utah developed an idea that grandmothers help increase our life span by helping women have their next babies sooner. The reasoning — borne out by computer simulations — is that a grandmother helps feed the offspring and this frees up women to get pregnant again. This then leads to an evolutionary increase in life span, since long-lived grandmothers can help feed more grandchildren.

OK, you ask, but how does this lead to mate-guarding? The answer is simple: As life span increases, you get a surplus of older men. This leads to stronger competition for young, fertile women. And it means men have to guard their mates more closely.

“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 says. “The more males there are, the more their average reproductive success goes down.”

Hawkes says that this is also the reason that humans form pair bonds, when our closest relatives, chimpanzees, don’t.

“Human pair bonds have the characteristic of male proprietary claims on females,” she says.

That’s not all. Hawkes says that when women started having babies closer together, the mother naturally became less committed to each new infant.

“Where before infants had mom’s full commitment as a birthright, now infants (that are) better at actively engaging their mothers and grandmothers were much more likely to survive,” she says. This means there was natural selection on infants to become sensitive to their social environment, to become more willing to interact and more responsive.

So when grandmothers started helping raise their own grandchildren, the argument goes, certain key human traits — including longer life span and pair bonding but even increased brain size, cooperation and empathy — started to evolve, too.

It’s certainly an ambitious idea.

“We’d hypothesize all that follows from ancestral grandmothering,” Hawkes says.

The new study that has led Hawkes to this conclusion is published in the journal Proceedings of the National Academy of Sciences. Hawkes and her team ran computer simulations of human evolution with and without grandmothering.

The simulations showed that societies gradually became male-dominated. This is not the case in real-life populations of chimps, which have more fertile females than fertile males. The authors say the excess of older males competing for mates is a likely source of men’s preference for young women.

“This is different than what you see in chimpanzees, where males prefer older females,” Hawkes says.

As the “grandmother factor” kicked in, human longevity increased. That leads to lots more old guys. “So you have an increasing number of males in the paternity competition, and the only way you can become a father is with a fertile female, which means younger females,” Hawkes says. “So males who had preference for younger females were more likely to leave descendants.”

The idea of the grandmother effect originated from observations of the Hazda people in Tanzania. I saw these people myself once on an unforgettable trip to Africa. They are hunter-gatherers, living without livestock and without farming, living an extremely basic life similar to the one Western societies lived some 100,000 years ago.

Anthropologists noticed that in Hazda society, the older women dug tubers and fed the young in the group. Infants and young children aren’t strong enough to dig their own tubers.

“Human infants — blobs that they are — are nevertheless remarkably socially precocious,” Hawkes says.

Grandmothers are incredibly helpful — I can testify firsthand to that — and it’s fascinating to think that they might have had such an influence on our evolution.

Rowan Hooper is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human”). Follow Rowan on Twitter @rowhoop.

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