Faculty of Biology and Chemistry, University of Białystok
Academia: What is old age, from the standpoint of evolutionary biology?
Marek Konarzewski: A kind of side-effect of natural selection. As time passes, various sorts of damage accumulates in the body, but given the limited resources the organism has at its disposal it is unable to repair itself and at the same time to perform the most important biological function, namely reproduction. Because passing one’s genes on to subsequent generations is the priority, such an organism here plays the role of a used-up carrier of reproductive cells, aptly described by Tom Kirkwood in the 1970s as a “disposable soma.” This theory today forms the basis for the evolutionary approach to aging, and there is a widespread conviction that it is an inevitable process. However, we can consider how to slow it down and try to prolong our lifespans.
Given that our priority task is to bring offspring into the world, how can we explain the fact that people live quite a long time after their reproductive period has ended?
The time after menopause (in women) or andropause (in men) long posed a riddle to biologists. For me it is best explained by the “helpful grandmother” theory, although it could be extended without any trouble to a “helpful grandfather” as well. The hypothesis is that women who have themselves stopped reproducing are very effective at helping raise their own grandchildren.
In life there always comes a point when ceasing to reproduce makes evolutionary sense. Children born to older mothers stand smaller chances of survival and are in poorer condition, and so an older woman can propagate her own genes more effectively by taking care of her grandchildren than by having more children herself. In fact this strategy is evident not only in humans, but also in other long-lived species, such as the elephant.
Like people, elephants are social animals. How does old age manifest itself in species that live in isolation? Is there also a role to play for older individuals of such species, or do they die before they reach old age? In other words: Does old age require social bonds?
Old age, in the biological, physiological sense, occurs in all animals. But if we are talking about a long post-reproductive life, then yes, there must be social bonds and relations that extend beyond the parent–offspring connection. But we must bear in mind that length of life is also affected by other factors. For example, a safe environment with a relatively sparse food supply. A good example can be found on the floor of the oceanic trenches, where invertebrates live phenomenally long lives, even several hundred years. But we might ask: “What kind of life is that?”
Pace of reproduction and lifespan typically depend on the pace of an animal’s metabolism. Given limited resources, an organism may invest either in its own growth, or in reproduction.
Yes, that is one part of this jigsaw puzzle. In the oceanic depths, which offer a very safe environment yet at the same time are sparse in food, long life is possible albeit at the price of slow reproduction. It is not that an organism can freely choose between reproduction or its own development – that choice is always determined by the circumstances. In evolutionary biology, we call these dependencies “life histories.” The fundamental factors that determine how long an organism’s lifespan lasts are mainly how safe its environment is and what kind of resources it offers.
A beautiful example of differing life histories can be found in the social insects. In some species, fertile queens may live upwards of a decade, whereas their workers live very short lives of a few months. Such a queen lives in an extremely safe environment, with essentially the whole social structure devoted to feeding her and raising her offspring. As a result, unlike in the oceanic depths, she can live a long time and reproduce intensively. However, that takes very special conditions, with essentially the whole colony serving as an extended digestive tract to this single reproducing individual.
From the ecological standpoint, therefore, does old age depend in large part on external factors? Of course there are physiological circumstances, but there are not many old animals out there in nature, since they die of various causes before they have time to reach old age.
That is true, but not entirely so. When I was first starting out as a biologist, it was considered an axiom that under natural conditions there were no old animals. But later there appeared data about birds that had been observed in the United Kingdom and Scandinavia for many years. It turned out that even among small species, such as titmice and flycatchers, there are certain individuals that live five, six, seven years, even though the average is two. There is a quite a wide spread, therefore.
We can also see that when we compare animals that live in the wild vs. those in captivity. Housecats sometimes live to be 20, whereas their feral counterparts live just a few years.
That is true, but please note that if they sometimes reach such an age under the “greenhouse” conditions at home, then some individuals should also live to a similar age under natural conditions. Otherwise the selective pressure would end up capping the maximum lifespan. Such a mechanism has been demonstrated in a series of experiments on guppies. Animals that are kept in the presence of predators grow to smaller size and reproduce more quickly than those not subject to such pressure.
In other words, under good external conditions an animal would be able to live endlessly, if it were not for the fact that at a certain point certain internal, physiological mechanisms gain the upper hand?
Definitely not endlessly – judging by what we know today, that appears impossible. A lot of attention has been paid in recent years to the naked mole-rats. Certain species of these desert-inhabiting rodents are very long-lived and have a slower metabolism than other mammals of their size, but most interestingly, they exhibit very effective mechanisms for recognizing and combating cancer cells. That is possible because they live in a very safe world of underground burrows, where the main threat to their reproductive success is not predators or a shortage of food, but diseases, including cancer. Rodents living in other environments, such as mice and rats, contract cancer very often.
Many diseases of old age are also observed in people, for example Alzheimer’s. It is sometimes claimed that they are the result of our ever-longer average lifespan.
I would describe them as the “shadow of evolution.” Natural selection operates on genes, and so individuals are “visible” to it only as long as they are able to pass their genes on to the next generation. All traits that manifest themselves in the post-reproduction period, therefore, are not subject to selection, so the repair mechanisms grow weaker with time. To outsmart that mechanism, all of us humans would have to agree to reproduce later in life, and to continue doing so as late as possible. Then the period when our health is subject to natural selection would become prolonged. But of course that is an unlikely scenario.
We can also imagine a situation in which the healthiness of grandparents has a significant impact on the grandchildren they help raise.
Yes, that should also work. However, we have to bear in mind that this is speculation, because there is a debate over whether modern humans are actually subject to natural selection. It is visible in certain aspects of small populations, but it seems doubtful to me that it operates globally. Apart from that, people’s length and quality of life depend on economic and technological development. Over the past 25 years, the average lifespan in Poland has increased by 5-6 years, and in my opinion that is one of our greatest achievements. Of course that is a consequence of broader and better access to medical care, but also changes in lifestyle. We can change a lot in a short time, therefore, because only 20% depends on our genes, whereas the rest hinges on the environment and how well we treat ourselves… or, unfortunately, how badly.
Interview by Agnieszka Kloch
Further reading:Bourzac, K. (2012). Live long and Prosper. Nature, 492. S18-20.
Flatt T. (2009). Diet and longevity in the balance. Nature, 462. 989-990.
Kirkwood T.B.L. (2005). Understanding the odd science of ageing. Cell, 120. 437–447.
© Academia 3 (43) 2014