The hypothesis of "active progenitors"
Long life was explained by the mobility of grandparents and vice versa
Polina Loseva, N+1
Physical activity could be not only a cause, but also a consequence of human longevity. This conclusion was reached by scientists who compared the life history of humans and other primates. In their opinion, the fact that people began to develop longer required the participation of grandparents in the upbringing of offspring. Consequently, they needed to be physically active — and this, in turn, helped them to start living longer. The researchers collectively called their findings the "hypothesis of active progenitors." The work was published in the journal Proceeding of the National Academy of Sciences (Lieberman et al., The active grandparent hypothesis: Physical activity and the evolution of extended human healthspans and lifespans).
A healthy lifestyle and, in particular, physical exercise is considered one of the surest ways to prolong this life or, at least, not to shorten it. And, there are many mechanisms that explain this relationship. However, it is still not entirely clear where this relationship came from in the course of evolution.
For example, in rodents, everything is arranged differently: additional physical education does not allow them to live longer. And in the closest relatives of humans, large primates, everything is arranged differently: the average American (who is usually reproached for a sedentary lifestyle) passes the same amount per day as the average chimpanzee. And orangutans and gorillas move even less. This means that physical activity can be seriously useful not for all animals, and maybe only for people.
With age, people, unlike chimpanzees, begin to walk less. But they still walk more than other primates. Drawings from the article by Lieberman et al.
To explain why this happens, Daniel Lieberman from Harvard University and his colleagues turned to the long-standing "hypothesis of grandmothers". According to her, people owe their long lives (compared, at least, with other primates) to grandmothers: as soon as women began to live to participate in the upbringing of grandchildren, they began to save the resources of mothers and allowed them to reproduce more often. Thus, it became profitable for ancient people to live longer, and selection supported those who succeeded.
Lieberman and colleagues noticed that grandmothers (and with them grandfathers) are useful not only because they can look after their offspring. At least in modern hunter-gatherer tribes, the older generation remains active even when they are out of reproductive age, continues to go in search of food for six hours a day — and supplies their children and grandchildren with extra calories. This means that simply living for a long time was not enough for ancient people to succeed — they still had to remain full of energy, and selection acted not so much on life expectancy (lifespan) as on the duration of a healthy life (healthspan).
It turns out that a long life in itself entailed physical activity — because selection maintained longevity only when the individual retained the ability to move. But then, the authors of the article suggest, physical activity could, in turn, affect longevity — and those who moved more began to live longer.
However, this second relationship is not at all obvious. The theory of life history assumes that the amount of resources available to the body is limited. Consequently, in any period of life, the body spends it only on one activity — it can be physical activity, growth, reproduction, maintenance of strength — or it stores it. And if people began to spend a lot of energy on physical activity, then the ability to maintain strength should inevitably suffer — and, consequently, prevent an increase in life expectancy.
The authors of the article propose to solve this paradox with the help of two hypotheses. The first of them has been repeatedly expressed by their predecessors: it suggests that physical activity prevents the deposition of fat in tissues.
Humans differ from other primates in that they have a stretched childhood, so they generally spend more energy on long-term gestation, feeding and raising offspring. As a result, humans are much more efficient at storing fat: up to 25 percent of hunter-gatherer body weight versus 9 percent of chimpanzee body weight. And physical activity does not allow the body to invest in fat reserves (and at the same time in reproduction) — and thus counteracts obesity and reduces the risk of many diseases that it brings with it.
The second hypothesis, with the help of which researchers explain the benefits of physical activity, is related to the fact that such activity inevitably serves as stress for the body. This stress manifests itself at different levels: tears occur in muscle fibers, pressure acts on the walls of blood vessels, inflammation increases in tissues, free radicals accumulate in cells. All this triggers an anti—stress response - mechanisms begin to work that must cope with the effects of stress, and at the same time with other breakdowns that have accumulated in cells and tissues during less costly activity.
The authors emphasize that physical activity is more stressful for cells than aging processes. But it also launches more powerful recovery processes. At least, after physical exertion, the human body continues to consume an increased dose of oxygen for some time — and it is believed that this energy is just going to repair breakdowns. And since during most of the existence of mankind, elderly people did not lose activity until the end of their lives, then, the researchers suggest, they did not have mechanisms that would trigger the repair of breakdowns at rest.
Oxygen uptake increases dramatically during physical activity and gradually decreases for a long time after it has ended.
The authors of the article gathered all these considerations into the hypothesis of "active grandparents" (active grandparent hypothesis), according to which physical activity has become both a cause and a consequence of a long life. Humans became slower and took longer to develop than primates, and they needed grandparents — but they were only useful in populations when they moved a lot. And the selection favored those who are better able to use physical activity to burn fat and repair breakdowns. Therefore, the fact that they moved a lot allowed them to live longer.
Researchers admit that this effect is not very noticeable against the background of the development of medicine: modern hunter-gatherers, on average, do not live longer than office employees. Therefore, the authors of the article consider the fact that in terms of the number of chronic diseases and the duration of a healthy life (healthspan), hunter-gatherers are still ahead of those who move a little.
However, the researchers note that it is quite difficult to verify the hypothesis that people had selection for more efficient fat burning and repair processes. The ideal model, in their opinion, would be two genetically similar populations, one with an active, the other with a sedentary lifestyle, while both should not have access to modern medicine. And if the first one is not difficult to find, then the second one does not seem to exist now. Therefore, the authors of the work hope that it will be possible to obtain some data by comparing humans with other animals - although not all are suitable here, but only species with a similar life history.
Earlier, biologists have already confirmed the "grandmother hypothesis" in killer whales. And they also found out that elephants have menopause — that is, they also spend quite a lot of time between the last birth and death.
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