October 23, 2012
New Evidence That Grandmothers Were Crucial for Human Evolution
Anthropologists argue that the presence of grandmothers has been crucial in driving human evolution. Image via Flickr user Mrs Logic
For years, anthropologists and evolutionary biologists have struggled to explain the existence of menopause, a life stage that humans do not share with our primate relatives. Why would it be beneficial for females to stop being able to have children with decades still left to live?
According to a study published today in the journal Proceedings of the Royal Society B, the answer is grandmothers. “Grandmothering was the initial step toward making us who we are,” says senior author Kristen Hawkes, an anthropologist at the University of Utah. In 1997 Hawkes proposed the “grandmother hypothesis,” a theory that explains menopause by citing the under-appreciated evolutionary value of grandmothering. Hawkes says that grandmothering helped us to develop “a whole array of social capacities that are then the foundation for the evolution of other distinctly human traits, including pair bonding, bigger brains, learning new skills and our tendency for cooperation.”
The new study, which Hawkes conducted with mathematical biologist Peter Kim of the University of Sydney and Utah anthropologist James Coxworth, uses computer simulations to provide mathematical evidence for the grandmother hypothesis. To test the strength of the idea, the researchers simulated what would happen to the lifespan of a hypothetical primate species if they introduced menopause and grandmothers as part of the social structure.
In the real world, female chimpanzees typically live about 35 to 45 years in the wild and rarely survive past their child-bearing years. In the simulation, the researchers replicated this, but they gave 1 percent of the female population a genetic predisposition for human-like life spans and menopause. Over the course of some 60,000 years, the hypothetical primate species evolved the ability to live decades past their child-bearing years, surviving into their sixties and seventies, and eventually 43 percent of the adult female population were grandmothers.
How would grandmothers help us live longer? According to the hypothesis, grandmothers can help collect food and feed children before they are able to feed themselves, enabling mothers to have more children. Without grandmothers present, if a mother gives birth and already has a two-year-old child, the odds of that child surviving are much lower, because unlike other primates, humans aren’t able to feed and take care of themselves immediately after weaning. The mother must devote her time and attention to the new infant at the expense of the older child. But grandmothers can solve this problem by acting as supplementary caregivers.
In the hypothesis—and in the computer simulation—the few ancestral females who were initially able to live to postmenopausal ages increased the odds of their grandchildren surviving. As a result, these longer-lived females were disproportionately likely to pass on their genes that favored longevity, so over the course of thousands of generations, the species as a whole evolved longer lifespans.
But why would females evolve to only ovulate for 40 or so years into these longer lives? Hawkes and other advocates of the hypothesis note that, without menopause, older women would simply continue to mother children, instead of acting as grandmothers. All children would still be entirely dependent on their mothers for survival, so once older mothers died, many young offspring would likely die too. From an evolutionary perspective, it makes more sense for older females to increase the group’s overall offspring survival rate instead of spending more energy on producing their own.
Hawkes goes one step further, arguing that the social relations that go along with grandmothering could have contributed to the larger brains and other traits that distinguish humans. “If you are a chimpanzee, gorilla or orangutan baby, your mom is thinking about nothing but you,” she says. “But if you are a human baby, your mom has other kids she is worrying about, and that means now there is selection on you—which was not on any other apes—to much more actively engage her: ‘Mom! Pay attention to me!’”
As a result, she says, “Grandmothering gave us the kind of upbringing that made us more dependent on each other socially and prone to engage each other’s attention.” This trend, Hawkes says, drove the increase in brain size, along with longer lifespans and menopause.
The theory is by no means definitive, but the new mathematical evidence serves as another crucial piece of support for it. This could help anthropologists better understand human evolution—and should give you another reason to go thank your grandmother.
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This is a very interesting possibility, and your reporting of it was pleasantly clear.
I have one remaining confusion–I don’t quite understand the jump to say grandmothering would have facilitated the development of bigger brains. You seem to suggest that chimpanzees, gorillas, and orangutans have only one baby in their lifespan. Is that really true? It just surprised me to see that. And assuming it’s true, is their argument then that having more children made larger brains (at least executive function/attentional processes housed in the PFC) adaptive, whereas there would be no such need among single-offspring species?
Thanks!
You’ve made a good case for grandmothers. So why are there grandfathers?
To the first commenter:
The jump to the larger brain size is a bit of a leap and a lot of conjecture, but here is the hypothesis: It’s not that other apes only have one offspring, it’s just that the offspring learns to fend for itself before the mother has another (hence the ‘your mom is thinking of nothing but you’ quote, because by the time she has another baby to think about, she doesn’t have to worry about you anymore, you’re already perfectly capable of fending for yourself).
Then the jump to larger brain size is that since you as the older offspring still need taking care of, you have had evolutionary pressure to have a larger brain that is capable of engaging your mother (or grandmother) to force her to pay attention to you. I’m not sure I buy that, but that’s the model the authors are putting forth.
I think that another aspect to grand mothering after menopause is the first and second sleeps that many post-menopausal women experience. It has been my belief that the two hours of non-sleep is designed to allow grandmothers to take care of baby’s and to tend to the fires.
I could have told you that !
As a grandmother who has two grandaughters living with me, for almost their entire lives, this article was a blessing. Although I still work, the time I spend with my granddaughters allows my daughter, who also works (both of us are single parents)peace of mind that her girls receive nurture & guidance when she’s not home. If she was an at-home mom, my purpose would be the same- to give them love & attention when she can’t, and to take over with house duties so she can spend time with them. I can see the same situation in caves ages ago. As for grandfathers- obviously, they teach the children life skills whilr the fathers are busy providing shelter and food… same as today.
Born into a dysfunctional family and later reared by a stepfather who beat and belittled me, I regarded my nearby grandmother as my port in a storm. But I have to thank my grandfather and his wife for showing me the museums, concerts, plays art galleries ever summer. I hate to think how I or my offspring would have turned out without either. The buck has stopped with me.
The brain responds positively to exercise and food, just like muscles; with better nutrition and more personalities to interact with – naturally it would grow and expand!
This ties-in with a study published 40-odd years ago by the Szold Institute in Jerusalem. In a study on child development based on ethnic background, the surprise results were that development was directly linked to the number of children per family and the amount of time/attention received from BOTH parents.
This in turn depended on the education level of the parents, NOT their ethnicity!
Put very crudely: from age 0-2 years infants in larger families developed faster than their counterparts in smaller ones, but the trend reversed drastically from then on. The reasons: in larger families the infant received more attention until around age 2, when displaced by a new arrival; in smaller families the mother was primary caregiver, with the father becoming more active in the child’s development from age 2 onwards.
In short: the more quality parenting time, the faster and better the development!
It has been my 80 year observation that among dogs and many other animals, all older female in the pack, including older sisters, aunts, grandmothers, etc. pitch in and help, even with baby sitting and training. I had a litter of pups and kittens born on the same day. The mothers even allowed the other’s baby’s to attempt nursing. This research is stretching.
Don Spohn
I loved my Granny. She was an anchor in the storm of life! I was lucky to live next door to her and my grandfather. He taught me to fish and she taught me tolerance, both, through example. I lost me Grandaddy at 7 and my Granny at 28, I still miss them dearly. I feel my Grandaddy on a spring morning fishing trip with my son and my Granny when I hold my children. My Granny showed me love when no one else did. I still remember her hugging me and the smell of her “old Lady” powder. God I miss them.
I’d love to understand your data. Or, any data that you put into your “simulations.”
Are you associated with those who believe that a simulation is as good as data?
As we used to say, “In God we trust, all others bring data.” One last point, the data I’d like to see is uncensored data, i.e., the raw data. BET YOU DON’T HAVE ANY!? SURPRISE ME!
I pondered the question why, at differing ages, human sleep patterns change.
Older folks often complain about waking up ‘too’ early in the day; teens often sleep late, and their parents complain about that, blaming it on late nights…
Children typically wake up early (most annoyingly on days when their parents want to sleep in), while adults seem to run a wide range of sleep patterns.
I feel confident in associating much of this with hormone activity, but also wonder if such disparity wouldn’t have been essential in pre-historic family groups.
After all, wouldn’t it be handy to have a few people awake at any given hour of the day or night? Especially with the possibility of attack from animals or rival groups – or other dangers, such as flood and fire.
Teens could probably be trusted to stay out of the bushes at night long enough to keep the fires from going out and perhaps remembering to bring in water for the next day. Grandparents, being awake early anyway, would be the logical breakfast and child-care tenders. The rest of the adults would sort themselves out better than if everyone was in bed by sundown. Wouldn’t they?
Just a thought.
I am grandpa. Considering what my wife has done and will do, she deserves to live one order of magnitude more than me. Yet I will have but a few less years. Will I contribute something to my grand children? Ray
I am a Grandma, mother of a single parent,
son, who is a great Dad. Son and Grandson live with us. Grandpa and I are both happy to be here as back-up and are very greatful for the connection/bond that we have with our Grandson. Please don’t exclude Fathers and Grandfathers when looking at the total well-being of children. The world dosen”t revolve around mothers…….It equally revolves around all family.
This premise is based on current cultural assumptions about the roles of women and child care. It may be that the tribal elders (yes, that means menopausal women too!) had other tasks that contributed to the well being of the group beyond that of just chasing after the grand children. There would have been a group mentality about raising the children simply because of the shortened life spans. Aunts, uncles, cousins and older siblings all would have had a role in taking care of the younger members. This type of “theory” neglects the passing of skills such as weaving that take time to master. This is also a patriarchal theory in nature – no where does it mention the extremely high attrition rate for males, who may not have been the center of tribal life, as these researchers assume. Kinda like elephants… females are the center of the universe and the males exist outside of that.
This has been floating around for awhile. A lot of untested assumptions; not very good science. Grandparents (yes, grandfathers were important, too) didn’t assume their role until people had settled down into fixed camps while the adults went hunting. The important factor here was the adoption of hunting and the cultural artifacts that come with that adoption.
There are few assumptions in how this evidence is presented in the article that I see as unfounded [giving some benefit of doubt to the actual theory].
There’a a primary assumption stated that human lifespans are actually more than 45 years of age. They aren’t. The archaeological record shows quite clearly that the vast majority of humans only lived between 35 and 45 years of age. Of course there are exceptions, but in 1900 the average lifespane in the US was 47. The reason we now live well into our 70s and 80s is primarily due to modern medicine and nutrition.
Additionally in early societys, as shown in any number of cultural anthrology studies over the last 100 years of tribal peoples, weaning of an infant typcially happens between ages 3 and 4, most often the later. At 4, a child can take care of itself to some degree. With higher primates like chimps and gorillas, there is a similar weaning age and these offspring still have a dependancy on the mother until several years later. There is strong indicators that siblings and older lineal females will tend the younger offspring/siblings. It’s most certainly not as cut and dries as present, with apes abandoning offspring once they are weaned.
I’ve never seen or heard of a study of apes in captivity — zoo or laboratory environments where nutrition and health care can extend their livespans — looking at the possibility of them also reaching menopause. In past human cultures, if probably very save to say very, very few women manages to reach it either.
Hi,
Interesting article.
Did people not live a lot shorter then they do now? Would women stop ovulating a lot earlier as well back then? If not, and the average age was around 35 (just guessing), then ‘young’ grandmothers would perhaps still be able to have children, and there would not be anybody to help with young offspring. Is there research that shows the start of menopause over time. And from what time do the researchers think grandmothers stepped in. I mean did the grandmothers from the species from whom we evolutionised also step in, or should I think more about a couple of thousand of years ago?
Thanks, Cristyl
Why is there no mention of the grandmothers breastfeeding their grandchildren and many other babies? The bonding associated with breastfeeding (a particular person) could (and does) play vital role in interpersonal communication and cohesion, which in turn would support overall survival of any population.
interesting.
Isn’t it also possible though that the evolution of the lengthening of child bearing years just hasn’t caught up yet with the extension of human lifespan rather than the other way around?
I suspect the reason grandfathers are not included is that they simply are able to go on fathering children (though less potently) through most of their lives, and hence there’s no real change between father and grandfather stage.
Very interesting article indeed.
One particular aspect I fail to understand. Let us assume that the human species had two genetic mutations which evolved over the years
1) Human babies aren’t able to feed and take care of themselves immediately after weaning.
2) Female menopause (leading upto the concept of importance of Grandmothers, as specified in this article).
Now, evolution would support #2 primarily based on the facts that #1 already exists within the population. That is, female menopause had no reason to spred amongst the species, had the human babies were more capable of taking care of themselves much earlier like the babies of other primates.
However, it is not really clear, why #1 would spread amongst the population !! What kind of evolutionary pressure would support more and more dependant babies.
Any ideas/ facts on this topic?
Responding to Denise’s comment (#17), I just want to remind people that “average lifespan” includes infant mortality, which was historically quite high. If you made it out of your childhood in 1900, the average age you could expect to reach was around 60. I think the numbers are similar in earlier times.
How do we know that humans lived much past 40 in prehistoric times? That seems like quite an assumption to me. Can’t an infected impacted wisdom tooth kill, if left untreated?
responding to Arnab #22:
“However, it is not really clear, why #1 would spread amongst the population !! What kind of evolutionary pressure would support more and more dependant babies.”
From what I have read it seems to relate more to brain size. Human babies have to be born at such a helpless stage in their development because their massive brain size. They would not survive childbirth if their gestational development went on much further. The human pelvic structure has not even fully caught up with our incredible evolutionary advance in brain development, which is why of all animals only humans optimally require assisted childbirth. Our great big brains are the evolutionary payoff to the cost of being born so physically dependent.
Another hypothesis put forward as fact by those who do not understand the scientific method at all. “New Evidence”–whaaa?
The presumption is that menopause and long life has a biological function. Key phrase to note: In the real world, female chimpanzees typically live about 35 to 45 years in the wild and rarely survive past their child-bearing years.
Fact is people died long before anyone reached the age of menopause. And there are many other more plausible explanations for the increase in lifespan…i.e. agriculture, tool making, group cooperation, brain development that allows you to transmit complex knowledge to you and MOST importantly the advent of language which allows you to convey complex information like tool making and hunting strategies.
Another feel good story, but the fact is females in other species are able to provide the same ‘biological services’ without the advent of menopause….elephants come to mind.
Surly this is an argument simply for having older relatives than grandmothers per se. A grandfather could offer the same advantages?
For decades, Sarah Blaffer Hrdy (see Amazon) has been writing about alloparenting and other related topics. Her insights probably inspired the research that we’re talking about now. Her work is an excellent read. My personal take on this topic is about the elephant in the room. If menopause evolved for the survival of the species, we should consider the surprisingly high incidence of other non reproducing members of our species having a parallel effect.
I should say that grandmothers (and grandfathers) are crucial to human development. Our daughter-in-law has recently given birth, and our help is very much needed just to enable mother and baby to tick over. Besides the historical fact that without grandparents having produced the children in the first place, there would be nobody around to produce generation No. 3! Question: Do grandchildren contribute to the education, physique and mental development of grandparents?
Post menopausal women represent the only adult demographic of people whose brains do not exist entirely in their pants. Everyone else is way too influenced by the sex drive, other than children under 8. This is why we need a woman President in the U.S. :-)