I’m not going to say that Ron Howard is old or anything, but he isn’t Opie any more. (And, in fact, it has been fascinating and inspiring to watch his career, by the way.) Anyway, Howard produced a new documentary with National Geographic called “Breakthrough: The Age of Aging, which premieres Sunday, November 29 at 9 pm et on National Geographic Channel. And, pursuant to this, National Geographic’s web site is sponsoring a Roundtable on the topic. The roundtable addresses the question, “By treating aging as a disease are we just prolonging the inevitable or can we change the course of our lives?”
The short answer to this is, I’m not really sure, but I think it is helpful to put aging, and changes in human patterns of aging, in a broader anthropological and evolutionary perspective.People have long lived long, even hunter gatherers in the Stone Age, as to modern hunter gatherers. In fact, hunter gatherers may have had longer and healthier lives than some of their errand cousins who went and invented agriculture and animal husbandry. In some cases we know from archaeology that populations engaged in early experiments with agriculture experienced dramatic decreases in overall health, and presumably, life span. This may have been a combination of larger groups sharing more diseases, unsanitary conditions developing in a more settled lifestyle, and a diet based on a smaller range of foods one ends up when casting off the foraging way of life. Eventually, in regions where this has been observed, things got better, either as a result of cultural adaptation or genetic changes.
When we look into the past, it is too easy to compress our ancestry into a caricature of primitive humanity, and based that conception on the wrong model. For example, it is said that “people were shorter back then.” Often, that is true, but the shorter people were actually poor urban dwellers in late medieval European settlements where diet was poor and disease demanded more energy of the immune system than average, so growth was sacrificed. If we look at pre-agricultural foraging populations, we often see relatively tall people. This is a bit enigmatic because so many modern forager groups are short statured. The explanation for that is probably that forager groups who are still around today, or have been extant over the last century or so, eek out their existence in relatively marginal habitats, the better parts of the landscape taken over by farmers and herders.
So, we should expect that prehistoric lifespan varied across time and space, and as I noted, there were probably always elderly people, but just not too many of them, compared to today. It has become axiomatic to note in modern day conversations that many of our diseases, in the West, are “diseases of civilization.” This is a combination of health effects, but one of the most important is the lack of disease of the past because they have been addressed, at least for now, at least for a subset of the human population. Antibiotics alone probably allow a much larger proportion of the human population to survive long enough to experience age-related disease.
A good part of Howard’s documentary is about the science of aging. We want our scientists to figure out how to beat aging, or at least, slow it down. But this is not easy. Humans are primates, and primates are mammals. The very earliest mammals probably evolved to die young. That seems counterintuitive but it really isn’t. Life History Theory predicts that organisms will be selected to produce some sort of balance (or bias, imbalance) of three major energy shunting systems: growth, maintenance (including the immune system), and reproduction. Humans reproduce slowly, producing one (or two) offspring at a time, and putting a lot of effort into each one. This goes along with a long lifespan, because in order to produce a small number of high-quality offspring one must take some time. This, however, places additional demands on the immune system. In order to keep up with evolving microbes and the overall ravages of time, we need to spend a fair amount of effort on keeping from being too sick. And, we happen to be large, for a primate. That probably relates to predator pressure and a few other factors. So while we are selected to live a long time compared to the average primate (and certainly, the average mammal) we can only go just so far.
But perhaps more importantly, we (humans, and to a somewhat lesser extent, primates in general) are modified versions of mammals, and there are indications that mammals were never originally designed (by natural selection) to live long lives. Early mammals were probably small, and small goes along with a short lifespan in the mammalian world. Remember, those early mammals were living along side dinosaurs! (There were large early mammals but modern mammals, including all the more recent large one, probably evolved from a subset of them that were on the small side.) In a world where the smallest dinosaurs were larger than the largest mammals (or close to that) mammals were probably more often prey than predator. The best strategy if the most likely cause of death is being scarfed up by something larger is to live fast, have one or two litters of offspring, and do the whole “circle of life” thing really fast.
See: How Long Is A Human Generation?
One strong piece of evidence that a live fast and die young strategy applied to early mammals is the fact that mammal females are born already containing all the egg cells they will ever produce. This is the primary determinant of reproductive lifespan for human females. Organisms that are born ready to reproduce tend to have that strategy of rapid early reproduction followed by an early death. One of the more extreme examples of this is aphids. Aphids have two modes of reproduction, but in one of them, female aphids are born gravid. While human females are not born pregnant, they are born with the eggs ready to go.
Not only have humans (following the primate lead) extended their lifespan and slowed down their reproduction, but they ave added, apparently, another phase of life: Post reproductive. Human females in foraging societies around the world are productive members of their families after they have stopped being fertile. This seems to not make sense from a Darwinian perspective. Why not just keep reproducing until you die? Probably for two reasons. First, they can’t, because human lifespans are already extended to the limit of our phyolgeneticaly constrained abilities. Second, that post-reproductive period probably enhances Darwinian fitness. Studies have shown that elder women in foraging societies contribute significantly to the health and wellbeing of their own children’s offspring. Grandmothers are an adaptation!