by James Morris
Lately, I have been thinking a lot about aging. Everything ages. This may sound obvious, but have you ever considered why this is so? Why do organisms age? Why is it nearly universal among living organisms? And what can we do about it?
First, let’s get some terms straight. Your chronological age is your actual age and simply reflects the passage of time. From this perspective, you started aging at the instant of fertilization and continue to do so throughout your lifetime, from childhood to adolescence to adulthood to old age.
Your biological age, by contrast, reflects your physiological health and may be more, less, or the same as your chronological age. It is often measured using certain biomarkers. For example, scientists recently studied a specific modification to DNA, called methylation, and found that it provides a measure of your biological age. This “epigenetic clock” may serve as a useful way to follow aging in patients and the effects of new anti-aging therapies.
Finally, senescence is what most of think of when we hear the word “aging.” It is functional decline.
Senescence, in spite its name, is not the same as senility or extreme disability. It just means that if you measure some function, like running speed or eyesight or strength or cognitive ability, there is a time when each of these starts to decline. You may still run very fast, but if you are slower than last year and the year before, it could be due to senescence.
You can see the effects of senescence among athletes. There is a peak age for short sprints, like the 100-meter dash, and another peak age for long distance running, like the marathon. After this age, whatever it is, runners slow down and running times increase. Similarly, you can see the effects of senescence by following the slugging percentage, on-base percentage, and batting average of former New York Yankees shortstop Derek Jeter over time, as shown here:
Senescence begins earlier than you might think (and earlier than you might want to believe). For many traits in humans, it begins in the early 30’s. So, I guess I am both aging and senescing. My teenage sons are just aging.
Much has been written about why we age, focusing on its immediate causes. For example, we live in a physical-chemical world, which has its hazards. Things break. They fall apart. They can be altered and broken.
At the level of our DNA, mutations occur, either spontaneously as DNA copies itself, or in response to mutagens in the environment, like cigarette smoke or ultraviolet light from the sun. In some cases, mutations can lead to uncontrolled cell division, or cancer.
We can also look at our chromosomes. We have 23 pairs of chromosomes, each of which is a long, linear strand of DNA. When chromosomes make copies of themselves during DNA replication and cell division, the ends of the chromosomes get shorter due to the mechanics of the process. Fortunately, there is an enzyme called telomerase that prevents this shortening. Over time, however, telomerase doesn’t work so well, our chromosomes shorten, and cells senesce.
But why is senescence nearly universal? Why did it evolve?
Every living organism, no matter how strong or healthy, has a declining chance that it will be here at some future date, simply because of the hazards of the world. Therefore, natural selection “rewards” early reproduction. This key insight was made by Peter Medawar, who described it in his 1952 lecture “An Unsolved Problem of Biology.”
Consider a harmful mutation. Natural selection will favor changes that delay its action. Delaying the action of a harmful mutation is the same, from a reproductive point of view, as the elimination of the mutation. By a similar logic, a mutation that has even a small advantage early in life will be selected for, even if it has a catastrophic disadvantage later in life.
Played out over time, in all organisms, these two processes lead to senescence.
These ideas were further explored and elaborated upon by George Williams and William Hamilton. Hamilton, for example, published a mathematical model of senescence and reasoned that it is an inevitable outcome of natural selection. As a result, it is likely a universal feature of life itself. That is, you can’t have life without senescence. They are intimately linked.
But I haven’t been thinking about any of this. Instead, I have been thinking about the Red Queen. The Red Queen comes from Lewis Carroll’s Through the Looking Glass. At one point in the story, the Red Queen tells Alice that she needs to run as fast as she can simply to stay where she is: “Now, here, you see, it takes all the running you can do, to keep in the same place.”
This idea is often invoked in evolutionary biology, where it is called the Red Queen hypothesis, to describe how organisms have to keep adapting in the face of a changing environment just to maintain their ecological place.
For example, certain bacteria infect humans. In response, we mount defenses against these bacteria. In turn, the bacteria evolve new ways to evade these defenses. And in turn, our defenses become more complex and robust. This is an example of an evolutionary arms race. Each player evolves rapidly (runs as fast as possible) to maintain its ecological niche (to stay in the same place).
The Red Queen not only describes antagonistic relationships, but also mutualistic ones, where each partner benefits. Some species of ants, for example, protect acacia plants from herbivores. In return, the plant provides food and nest sites for the ants. It turns out that even when both partners benefit, they both evolve rapidly. So, instead of thinking of these relationships as static, they are more like a dance, each one moving at just the right pace to avoid stepping on each others’ toes. Again, they both move quickly (evolutionarily) to maintain a stable partnership with each other.
What does this have to do with aging? Recently, I began a 7-day exercise routine, involving the elliptical, stationary bike, weights, core work, stretching, swimming, and the treadmill. My goal, however, is not to get stronger or faster, though that would be nice. Instead my goal is simply not to decline any further than I already have. My goal is simply to stay where I am.
I get on the treadmill, running as fast as I can, in order to stay in the same physical place. In fact, I don’t seem to be going anywhere, which is just fine with me.
© James Morris and Science Whys, 2016.