Contemplating Longevity Part 3: Evolution Based Theories of Aging

What was the first moment you realized you were getting older?

A memory for me was when I went into the training room at college after basketball practice. As a freshman, I wondered what all these guys were doing laying down on training tables, sitting in ice baths, and having things wrapped and taped. Four years later, I found myself appreciating the services of the trainers to soothe aching joints and muscles.

Now, the grind of competitive athletics certainly introduces some wear and tear, but even for non-athletes the progression of age seems inescapable.

There is a Norse myth that tells of Thor wrestling an old woman named Elli. The harder he struggles, the stronger she seems. Eventually, even mighty Thor submits, and finds out that this elderly woman is 'old age' herself.

Aging is not a uniquely human experience, although some organisms age in dramatically different ways. The naked mole rat, tortoises, redwood trees and the 'immortal' hydra come to mind.

Humans have made remarkable gains in life expectancy. More people are living for more years. But the rate of aging, think of the point in life we see wrinkles and gray hairs appear, has not changed very much at all. Why is that?

The scientists that are looking under the hood have a few ideas. From studying other creatures, and looking at various unique versions of homo sapiens, we are starting to build a knowledge base that can give us some insights.

From this knowledge we are starting to show that in some cases we can slow down or delay the aging process. At the very least, we get a better understanding of things that can speed it up (which most of us want to avoid).

It is important to note that there is not one universal theory of aging, rather there is a universe of aging theories.  In some ways we are like the blind priests holding different parts of the elephant.

Let's take a quick tour through the aging universe and see what's out there.

Evolutionary and Nature Based Theories

  1. This message will self destruct! Nature is brutal and competition is fierce. In order to avoid over consumption of resources there needs to be a way to purge excess baggage. Originally proposed as the Programmed Death theory, in order to preserve our society, we must have an internal termination protocol.

    To quote August Weismann (who gets the credit for the theory) 'The unlimited existence of individuals would be a luxury without any corresponding advantage." Since it's original proposal, this theory has gone out of style for an entire ogranism. That said there do seem to be some countdown timers at the cellular level. They will be explored in the next post "mechanistic theories of aging".
  2. We don't clean the deep end of our gene pool. A.k.a "Mutation Accumulation" For most of our evolutionary history we fell victim to disease, predators, or environmental hazards and this kept our lives relatively short. So traits that might have made us live longer never got that much attention. Furthermore we reproduce well before we get old, so any adaptations that may have been useful to extend our lives never get passed down future generations. More and more junk accumulates as we advance in years, explaining why we become more and more vulnerable as we age.
  3. Play now, pay later. It may even be the case that the things that help us in our growth years are in fact harmful in our later years, and we prefer to have help early in life. For example, think of calcium storage to get strong bones as youth vs. problems with too much calcium in bones for the elderly. This idea, is formally known as the Antagonistic Pleiotropy hypothesis.  
  4. Spawning salmon and suicidal bamboo - An outstanding question relates to the role of reproduction and longevity. In some species death follows shortly after reproduction. Salmon start to rot on their fatal journey to create the next generation. Bamboo has a 100 year reproductive cycle that results in immediate death! The link between human fertility and longevity is mixed. At a minimum, in our evolutionary experience, survival means more than just making more humans. Knowledge transfer is as important as genetic transfer. Also, we usually don't make a bunch of babies all at once, so hanging around a few more years post child helps us get to the next one.
Now it may be that evolution gives us some reasons why we age, but we may need to understand how a little better. So next time, we will look a little deeper at mechanistic explanations. In the meantime, I am going to get a massage.