Evolution Explains Why Exercise is Essential, Especially as We Age.

High levels of physical activity may become even more important for health as one gets older, as suggested by a new paper on something biologists call the “active grandparent hypothesis.”

In his recent book “Exercised”, the well-known evolutionary biologist Daniel Lieberman coins a term — exercists— which he broadly defines as people who espouse the benefits, joys, and importance of regular exercise, sometimes to a fault. And we all know those people (or perhaps are those people, not to turn the mirror on myself). While generally well-intentioned, exercists can sometimes give exercise a bad reputation, making it out to be some type of exclusive panacea for all that ails us — a club for the highly motivated and genetically gifted among us. While exercise is generally fantastic for all-around health, there are limits to what it can and can’t do, admittedly.

Not to completely negate the last sentence, but if there is one single thing that anyone can do to improve their health, it is to get more physical activity (assuming one isn’t already doing a sufficient amount). Interestingly, this might both have to do with the direct benefits of physical activity on our body and the direct harms of sedentary behavior. Just as movement is good, lack of movement is bad for humans (and all animals).

Evolution (and Dr. Lieberman himself) would tell us that in some sense, we are selected against movement. In other words, we are naturally lazy. As a species, we would have wanted to minimize excess energy expenditure by any means necessary, since traditionally food (energy) was in short supply. The bare minimum of physical activity — enough to hunt and gather and forage — is likely all that our bodies are “wired” to engage in.

Honesty, this “naturally lazy” hypothesis makes sense from an energy conservation perspective but does not quite mesh with observations. Millions of individuals now engage in high amounts of physical activity with seeming enjoyment. There is clearly a rewarding aspect to physical activity from a neurobiological perspective. Not only that, if evolution wired us to be lazy, then why do moderate to high levels of physical activity and aerobic fitness correlate with the best health outcomes? Up to a point, increasing levels of PA improves health outcomes. If activity was counterproductive to our success as a species, this might not be so.

Sure, we no longer live in an environment where energy is scarce (quite the opposite) and we don’t even have to hunt or gather our own food. We don’t “need” to do any physical activity to survive from day to day, yet many individuals opt to exercise for hours or more per day for various reasons.

All of this is to say that there must be a reason why physical activity is both rewarding and health-promoting in our modern environment, and why it produces such robust health outcomes into middle and old age for those who regularly engage in it.

In a new paper, Lieberman and colleagues propose a hypothesis on why physical activity promotes such large increases in healthspan and lifespan. They don’t seek to explain the mechanisms by which PA reduces disease risk factors, but go a layer deeper to posit an evolutionary explanation underlying why physical activity stimulates health-promoting, adaptive processes in the first place. Why did activity evolve to be beneficial?

Central to this discussion is something called the “Active Grandparent Hypothesis.” With the origin of hunting and gathering among our recent ancestors came an increase in physical activity — primarily aerobic physical activity (i.e. walking). This may have introduced a selection for increased physical activity, on the one hand.

At the same time, human lifespans began to increase significantly — well beyond the postreproductive years — the point at which many other animals begin the decline into disability and death. Grandparents thus became an invaluable source of knowledge and wisdom which they could impart to younger generations.

But as this new hypothesis suggests, it was physically active grandparents who may have been the most valuable asset for the community and therefore, a tendency for high levels of PA among middle-aged and older adults was selected for. 

A highly active grandparent would have been able to spend hours per day collecting food, which they could then provide to the younger generations for nourishment — sparing them of this energetically costly task.

The evidence for high PA levels among hunter-gatherer “grandparents” is strong — with post reproductive women in some hunter-gatherer tribes foraging for around 6 hours per day. They are much more active than younger women and, along with high daily walking and foraging levels, also engage in cooking, clearning, and digging. This contrasts sharply with how the elderly are treated in our society — generally discouraged from lifting a finger and typically thought to be too weak or unable to complete physical tasks.

Levels of physical activity across the lifespan in US males/females and Hadza hunter-gatherer males and females.

A high level of physical activity keeps “active grandparents” in robust health well into their post reproductive years. While walking speed, grip strength, and aerobic capacity (measured as VO2 max) decline precipitously in modern-day elderly individuals, there is considerable evidence that these metrics are retained to a much higher degree in hunter-gatherer civilizations with age.

A lot of what we take for granted as a “natural” process of aging is all but natural, but rather a case of “use it or lose it.”

Active individuals who were able to provide sustenance and conserve energy for their offspring and younger generations had their genes selected for, while sedentary individuals, who were unable or unwilling to do so, would be knocked out of the gene pool.

Evolution selected for those who were more physically active, and physical activity in turn was selected to increase healthspan and lifespan in order that those physically active individuals stay healthy and continue to impart their benefits on society. These genes are still with us today.

Now to briefly discuss the proposed reasons for why physical activity increases healthspan and lifespan. What physiological mechanisms make moderate to high levels of PA so favorable, especially in our modern environment?

One hypothesis is that physical activity — which expends energy — diverts energy away from investment in fat storage and (unnecessary) reproductive processes. Let’s admit that we are faced today with an energy abundance — in most industrial societies the problem is caloric surplus and overweight/obesity rather than malnutrition or food scarcity. 

Energetic abundance is a wonder of modern society but a boon to our health — as excess calories get stored as fat and used to build up reproductive tissues. While an energy deficit can negatively impact hormone and reproductive function, a surplus of energy can cause an overabundance of growth-promoting hormones which predispose to certain types of diseases and cancers. And we all know the risks of overweight and obesity caused by caloric surplus and a lack of physical activity.

In short, as we expend more energy through physical activity, we thus have less energy available to divert to growth- and storage-related processes in the body which, if left unchecked, lead to high levels of obesity which predispose to disease. 

A model of the investment in repair in maintenance (expressed as calories) for an individual engaging in 20, 41, and 135 minutes/day of physical activity over 20 years.

The second hypothesis concerns the adaptive functions of physical activity on resilience and repair. While physical activity (especially high-intensity activity) is a short-term stress, in the long term (during and after recovery), it promotes growth, repair, and maintenance processes in the body that ultimately make us stronger and more resilient. 

During physical activity (exercise), heart rate increases, blood pressure spikes, cortisol and other stress hormones are released, and free radicals are produced that cause oxidative damage to DNA, proteins, and lipids. Seems harmful, but after exercise ends, our body mounts an adaptive response to “correct” this damage.

This response involves essentially the opposite processes occurring — heart rate and blood pressure are reduced (often below baseline levels), stress hormones decrease and parasympathetic nervous system activity increases (promoting rest and relaxation), and our body produces free-radical neutralizing antioxidants. All of these changes are long-lasting in the setting of regular physical activity — evidenced by the fact that physically active individuals have lower blood pressure and higher levels of antioxidants compared to sedentary peers.

In this way, the short term stress and damage caused by physical activity is rapidly repaired and furthermore, activates mechanisms that actually make the body stronger in the long run. On the contrary, in sedentary individuals, these mechanisms are never activated. So, while they aren’t getting the “stress” of physical activity, neither are they getting the benefits of these ancient repair and maintenance mechanisms that are activated by physical activity.

Cardiovascular benefits of regular aerobic exercise/physical activity. Source: Pinckard et al. 2019

These two hypotheses are placed in the context of physical activity’s robust effect on reducing the incidence of two major killers throughout the world — cardiovascular disease and cancer.

Both hypotheses (the energy allocation and repair/maintenance hypotheses) can mechanistically explain how and why people who are physically active experience lower levels of chronic disease well into old age, and also why sedentary individuals succumb to these diseases at a disproportionately high rate. Sedentary behavior is not just a lack of physical activity, but something much more actively destructive to health.

Coming full circle to our notion that humans are “intrinsically lazy.” As the above discussion would suggest, perhaps there are some flawed assumptions in this hypothesis, at least when it comes to middle-aged and older adults that should be addressed. 

The “Active Grandparent Hypothesis” would suggest that, rather than evolution selecting against energetically-costly physical activity, it “encouraged” high levels of activity in older adults, making them healthier in the process and more able to provide for younger generations. Both the group and the individual seem to be rewarded for engaging in physical activity.

This discussion emphasizes the importance of physical activity into old age. Not to say that younger people shouldn’t worry about being active, but it seems that maintaining or even increasing levels of physical activity as one enters their post-reproductive years is essential. This is typically the opposite of what happens — individuals are highly active as young adults, less so as they reach middle age, and laughably so as they enter their 6th, 7th, and 8th decades of life and beyond.

The data from hunter-gatherer populations make the case that high levels of activity are reasonable and more than achievable in older age. Engaging in 4, 5, and 6 hours per day of low-level physical activity does not seem to be uncommon among postreproductive men and women in these societies. It is only because of our modern day coddling of the elderly that we come to “expect” them to be inactive, senile, and lack physical robustness.

There are plenty of examples of “active grandparents” in today’s society, which leads one to conclude that these people can be the norm, not the exception. What seems to be common among these people (though admittedly lacking a formal analysis by myself) is that they either maintained activity levels with age or, as is often the case, adopted their rigorous physical activity regimens later into life.

Much of what we know about aging is being turned on its head with advances in modern science. Along with our deep understanding of molecular mechanisms of aging, just as important to our breakthroughs in healthspan and lifespan will be our changing ideas and limiting expectations about what aging is. This applies as much to physical activity as it does to career ambitions, learning, and cognitive capacity. 

The next time you go for a workout, remember that your hard-earned sweat probably makes sense, like everything, in light of evolution. If we aren’t there already, we can all strive to embody what it means to be the “active grandparent” — boosting our health and fitness while simultaneously imparting wisdom and energy to those who we care for.

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