Humans typically hang around for a few decades after we’re done with reproducing. We’re not the only creature that does this — orca whales do it, too — yet very few other animals live beyond their reproductive prime as we do, a distinction that has perplexed scientists.
In a recent perspective article titled “The Active Grandparent Hypothesis,” published in the Proceedings of the National Academy of Sciences, evolutionary biologists and biomedical researchers from Harvard University lay out two theories as to why the physical activity may fuel this longevity.
The work suggests humans can capitalize on this evolutionary benefit by staying active throughout their lives and taking regular exercise.
“Many people in high-income countries don’t have to do manual labor,” lead author Daniel Lieberman tells Inverse. “We have this thing called retirement that’s only been around for a few years. We push physical activity. We push it for children, but we don’t push it as much for older people even though we know it’s healthy.”
SCIENCE IN ACTION — Human’s nearest animal relative, the chimpanzee, is both lazier and less long-lived than humans. Adult chimps in the wild move two kilometers a day and typically die around age 30. Comparatively, hunter-gatherer humans in groups like the Hadza walk between nine and 15 kilometers a day and can expect to live until 70.
“Similar or higher levels of activity characterize nonindustrial farming populations, indicating that until recently most human adults engaged in moderate levels of daily [physical activity],” the paper states.
The article presents two hypotheses as to how physical activity uniquely helps humans keep up their long lifespan.
HYPOTHESIS 1: PHYSICAL ACTIVITY MAKES GOOD USE OF THE ENERGY WE NEED FOR REPRODUCTION
Paleolithic parents needed a lot of calories. The hunter-gatherer lifestyle constituted a CrossFit-level daily workout of traveling, foraging, digging, hunting prey animals, and outmaneuvering dangers. Compare that to the easy, breezy life of our evolutionary cousins, chimpanzees, sleeping in nests in trees and plucking fruit and seeds.
Then there is the energy required for human mothering.
“[A] typical hunter-gatherer mother struggles to satisfy not only her own high energy needs but also obtain extra calories for the substantial costs of nursing (circa 500 [calories a day]),” the paper reads, “as well as feeding and caring for several older but still immature juveniles — all with large brains that ceaselessly require as many as 400 calories a day.”
Essentially, humans are built to store fat. Hunter-gatherer males had body masses of 10-to-15 percent fat and females 15-to-25 percent, compared to 2-to-9 percent in other primates.
Reproductive hormones are very sensitive to these fat levels, particularly in women. Excess fat increases estrogen levels. The effect is less in men but active men do have less basal testosterone than inactive.
Physical activity seems to keep all these factors in check.
“But in modern conditions of sustained energy abundance along with low [physical activity], these tradeoffs can lead to mismatches, most critically excess fat storage,” the article notes. This can give way to obesity, which can, in turn, lead to chronic inflammation, which is associated with a host of serious health conditions.
HYPOTHESIS 2: PHYSICAL ACTIVITY HELPS REPAIR AND MAINTAIN THE BODY
This one is simpler: Part of the energy costs of physical activity is to do with the recovery. The article states that “varying doses and types of [physical activity] can generate numerous forms of stress and damage at the molecular, cellular, and tissue levels.”
This repair-and-maintenance process may ultimately have enriching benefits.
WHY IT’S A HACK — Until recently — in the evolutionary biology sense of time — there was not a sample of physically inactive human beings to help scientists understand the evolutionary benefits of exercise. “No one was physically inactive at any age, except maybe the Pharaoh,” says Lieberman.
But the modern world of cars, office jobs, and home theater systems has created a new kind of human, for whom exercise is optional and often difficult to schedule. If this sounds familiar, know that it could cost us our evolutionary edge.
The average U.S. adult takes 4,774 steps per day and engages in less than 30 minutes of moderate-to-vigorous-physical activity (which can be as low-level as a brisk walk), the article notes. In contrast, adults in hunter-gatherer populations average 15,800 steps a day and 135 minutes of moderate-to-vigorous physical activity, according to the paper.
HOW THIS AFFECTS LONGEVITY — It’s no medical mystery that being physically active across the years helps increase people’s longevity. Moderate exercise reduces the risk of hypertension, type 2 diabetes, heart disease, stroke, cancer, and several other ailments. Exercise also benefits mental health, aids with sleep, and emotional regulation.
The authors of this paper say that some of these diseases of old age are exacerbated by physical inactivity and one reason is that we are stuck in bodies built for a different level of energy use and consumption.
“All animals need to engage in regular [physical activity] but, at some point, hominins were selected to engage in significantly more [physical activity] than their comparatively sedentary ancestors to make possible our unique life-history strategy in which parents and grandparents gather and hunt surplus energy which they transfer to their children and grandchildren,” the article notes.
“A key component of this energetically intensive strategy is for post-reproductive adults to stay healthy for several decades as they continue engaging in [physical activity].”
In other words, it might be best to keep up the gym routine.
Abstract: “The proximate mechanisms by which physical activity (PA) slows senescence and decreases morbidity and mortality have been extensively documented. However, we lack an ultimate, evolutionary explanation for why lifelong PA, particularly during middle and older age, promotes health. As the growing worldwide epidemic of physical inactivity accelerates the prevalence of noncommunicable diseases among aging populations, integrating evolutionary and biomedical perspectives can foster new insights into how and why lifelong PA helps preserve health and extend lifespans. Building on previous life-history research, we assess the evidence that humans were selected not just to live several decades after they cease reproducing but also to be moderately physically active during those postreproductive years. We next review the longstanding hypothesis that PA promotes health by allocating energy away from potentially harmful overinvestments in fat storage and reproductive tissues and propose the novel hypothesis that PA also stimulates energy allocation toward repair and maintenance processes. We hypothesize that selection in humans for lifelong PA, including during postreproductive years to provision offspring, promoted selection for both energy allocation pathways which synergistically slow senescence and reduce vulnerability to many forms of chronic diseases. As a result, extended human healthspans and lifespans are both a cause and an effect of habitual PA, helping explain why lack of lifelong PA in humans can increase disease risk and reduce longevity.”
