Scientists think they know why humans live so long: Moms
But some researchers worry the findings could be harmful for mothers if taken out of context
Why do humans live so long? A new study suggests a mother's care could be a major part of it.
The study, out of Cornell University, says that the reason humans and other primates live so long can be at least partly explained by the mother-child relationship.
Maternal care leads to the evolution of "long, slow lives," noted the study, published Friday in the peer-reviewed journal Proceedings of the National Academy of Sciences.
"Humans are a primate and we live much longer lives than is expected based on our body size, even compared to other primates. We also show a very long period of intense connection between mother and child that is unusual among animals," lead author Matthew Zipple told CBC News Wednesday via email.
"We argue in this study that these two facts are causally related, such that the intensity of the mother-child relationship partially explains our long lives, and those of other long-lived social mammals," said Zipple, a postdoctoral fellow in neurobiology and behaviour in the College of Arts and Sciences at Cornell University.
But as the authors link maternal care with human and animal longevity, some researchers worry the findings could be harmful for human mothers if taken out of context.
The study authors were looking at demographic tendencies — at a species level — and did not draw parallels to modern parenting or make suggestions that mothers should spend more time with their children.
But the link between long lifespan and maternal care may create temptation in some people to think that way, said Amanda Watson, a senior lecturer in sociology and anthropology at Simon Fraser University, who was not involved in the study.
Already, at least one science news outlet has pointed to the study as evidence that "a mother's love is a powerful force, shaping not just our childhood but potentially our entire lifespan."
"Might mothers nonetheless feel pressure and role strain when we read the [study] headline, and might this headline be weaponized by some who wish to foreclose freedom for women? Of course, but they shouldn't," Watson said.
No pressure, though
The researchers used modelling and empirical data to look at factors such as lifespan, maternal survival and offspring fitness in mammals. They made predictions based on Zipple's research on baboons and other primates.
They were able to show that in species where offspring survival depends more on the longer-term presence of the mother, such as primates (including humans), they tend to live longer and reproduce less often.
And it's not just primates — the authors said the model extends to other social mammals such as elephants, hyenas and whales.
But it's important to keep in mind what the authors mean when they say "maternal care" when interpreting the results, said Andrea O'Reilly, a professor in the School of Gender, Sexuality and Women's Studies at York University, who was also not involved in the study.
Care, in this case, refers to the long, intense period of connection between mother and child that can influence the offspring's survival to have kids of their own.
Not the "hyper-vigilant, helicopter, enrol your kids in three programs by age six months kinds of stuff" that so many mothers already feel pressured to perform, said O'Reilly, who is known for establishing the academic discipline of motherhood studies and maternal theory.
"A lot of what we do in Western culture, at least in the last 30 or 40 years, is designated as care. Things mothers should and must do. But I would suggest that a lot of that has nothing to do with the care of children. It's all about performing good motherhood to mark you as a good mother," O'Reilly told CBC News.
Her worry is that, interpreted incorrectly in this way, the study could be harmful for mothers who already face "impossible standards." And she notes it wouldn't be the first time research has been guilt-inducing for mothers, pointing specifically to attachment studies saying that the first hour of a baby's life is the most important for maternal bonding.
"So many mothers couldn't bond instantly with their child for various reasons — health or psychological — and they lived with such shame and blame. So I just worry that this study in the wrong hands is a way to regulate control and backlash with mothers," O'Reilly said.
"The last thing mothers need is yet another study that says 'oh my god, you need to do more for your kid.'"
No 'right' way to be a mother: study author
It's important to keep in mind that the correlations found in the data don't explore the nature of the caring relationship, Watson noted.
"We are talking about evolution of multiple species over millions of years, not about who ought to be reading the bedtime story," said Watson.
Zipple, the Cornell study author, emphasized that his study is about evolutionary modelling, which helps us better understand our place in biological history.
"We all have mothers. Our mother is our first social relationship, and for many people it remains one of their most influential social relationships across their lives," he told CBC News.
"For better and for worse, we are also animals, with complex social behaviours and relationships that have been strongly shaped by historical evolutionary forces."
Evolution doesn't distinguish between 'good' behaviour and 'bad' behaviour the same way that we do, he added.
"And so we should never assume that selective pressures from our evolutionary past can tell us about the 'right' way to be a mother or father or friend or spouse or citizen."
The mother and grandmother hypotheses
The Cornell study builds on what are known as 'mother and grandmother' hypotheses, which are also used to explain menopause and why females live beyond when they can produce more offspring.
Essentially, the theories posit that it's advantageous for older females to stop reproducing, because they gain more evolutionary success by helping their children raise their grandchildren than having more babies of their own, and offspring are more likely to survive if their mother and grandmother are in their lives.
Researchers have previously applied the theory to killer whales and four species of toothed whales, the only mammals other than humans that are known to experience menopause. They've even found that grandmother orcas stick around to help mothers raise their calves.
(Not for nothing, but another recent study found that mother killer whales who raise male offspring are half as likely to successfully breed again than those who have female calf. The researchers suggested this may be because the male offspring are needier, requiring more food.)
In this study, the authors note they extended the logic of the mother and grandmother hypotheses "to a more general form that we expect to have relevance across the mammalian taxonomy."
Watson, with Simon Fraser University, says the mother and grandmother hypotheses should also be interpreted with "extreme caution." She explains that scientists and anthropologists disagree on how to make meaning of correlations between the presence of grandchildren, advanced age, and reproductive fitness.
All in all, studies like this provide "exciting clues" about highly social species, including humans, Watson said.
"But they are humorous at best when taken out of this context."
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