Astounding Question #4 - Why do we age?

Why getting old makes no sense

Let’s start with why it makes sense asking the question. As any other physical or biological phenomenon, if it happens it means that it can be explained by the laws of physics. That just means that aging is natural and that doesn’t really answer our question. Just for the fact that we’re used to it, it doesn’t make it universally true. If aging is a thing we should be able to explain it or at least we should try to.

I say that we need an explanation not only because this phenomenon happens to everybody (and thus affect us very closely, making it very interesting to study) but because it doesn’t make sense in an evolutionary perspective as well. Let me be clearer: Evolution always promotes traits that improve fitness of an individual, such as its reproductive success during the lifetime. Hence, reducing the lifetime of an organism, by aging and dying, seems counterintuitive according to evolutionary standards.

Here’s is why aging might make sense

Here we see some hypothesis that have been raised to explain this Darwinian’s puzzle.

The rate of living theory

A pure mechanistic theory has been called “the rate of living theory” based on evidence that molecular processes, cells and more unclearly organisms have an intrinsic error nature in every metabolic process. This means that just for the fact that we’re living our lives and that we live according to the second law of thermodynamic, we can’t be 100% efficient and thus we accumulate errors throughout our lives. The higher our metabolic rate the higher the error rate will be. Thus, we expect that organisms at a very old age (relatively to the species) got to the limits of their error-resisting threshold. From that point on they will die due to irreparable error accumulation. This will give 2 predictions:

1. Individuals should have a lifespan proportional to their basic metabolic rate
2. Individuals shouldn’t be able to evolve longer lifespans since they already got to their limits. We won’t go into details, but unfortunately there’s not a lot of evidence supporting these predictions at the organism’s level, the only reason this theory is still around is because it moderately holds for cell cultures.

The mutation accumulation theory

Two more evolutionary hypotheses have been advanced to explain aging. The first one is called “The mutation accumulation hypothesis”. Similar to the previous idea, this theory predicts that individuals accumulate mutations during their lives. But this time, these mutations don’t have an effect on the body until old age. Almost like a switch. We see examples of this in tumors that are genetically determined by come out only at old age. This theory says that since these mutations have an effect only at old age, they won’t be negatively selected very strongly simply for the fact that at old age the reproductive success is very low anyway. A relatively mathematical model, which we won’t go into, can simply explain this. What we need to understand is that if these late-acting mutations are not selected against, they will stay in the population and will keep causing aging and death. Apparently, aging could be just a consequence of weak selection against aging itself.

Intuitive explanation of the mutation accumulation hypothesis. The reproduction probability of an individual decreases with age. In the selection shadow, the selective pressure has no effect as the accumulated mutation appear only late in life, when the reproductive success is already very low. Source: Wikimedia Commons

The antagonistic pleiotropy theory

The last theory we will go through is “The antagonistic pleiotropy theory”. I know: what a mouthful! But it can be simply explained. Assume a mutation arises that will cause an individual living less but becoming reproductively active earlier in life. This is defined as antagonistic pleiotropy because a single mutation causes 2 effects (aging and becoming sexually mature earlier) and because the effects on fitness (reproductive success) are opposite, or antagonistic.

So if such a mutation can arise in the population, we can prove, through a mathematical model, that this mutation will become more and more popular simply because it’s advantageous. Intuitively you can understand that if I become sexually mature 2 years earlier I can more offspring even if I die 10 years earlier. This is just due to the fact that young organisms are very sexually active and don’t mind dying young if they can propagate their genes in their younger years. So, this theory predicts that having this mutation is actually good for organisms and so that aging and dying is actually favored.

Pretty cool huh? There’s no need to say that we still don’t know why we age and consequently die, but we can at least speculate. Different theories can give you different perspectives on life itself. These are powerful, astounding questions. Let me tell you another story…