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Why We Age

Why We Age Mitch Kirby


Going deeper into this topic, interesting trends and patterns have been observed with regards to how long different species live. For one, the larger the organism, the longer it tends to live. This makes sense intuitively, bigger animals take longer to grow and reach maturity and thus should have longer lifespans.  On the other hand, animals with faster heart beats tend to have shorter lives. The thought here is that the basal metabolic rate of organisms with faster heart beats is higher, which leads to faster damage accumulation, and ultimately aging and death.

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So you're saying there's a chance we could determine a way to slow the aging process?

Theories of why we age have been answered over the course of scientific history. The first modern theory of aging was formalized in the wake of the publication of Darwin’s On the Origin of SpeciesThis theory, posited by August Weismann, proposed that we age as a part of a genetic program that causes the old to die out in order to leave more resources for the young. Though Weismann presented a good idea with a lot of intuitive appeal, the problem is that evolution doesn’t really work this way. Life does not “decide” to do something – rather, certain genes, arising from innate genetic variation in a population, render greater fitness to an organism, leading to the propagation of the gene. Re-framing the aging debate in this context was necessary for a more complete, and accurate understanding.

It was not until 60 years later that a better theory was proposed. Peter Medawar, a British biologist, hypothesized that the pressure of natural selection fades rapidly after an organism reaches sexual maturity. Genes only get propagated through reproduction, so after an organism is expected to reproduce, there isn’t much benefit in keeping it alive any longer. Basically, in a simplified sense, after we reach our prime reproductive age, evolution essentially gives up on us. We live as long as we do because our genes promote our growth and health up until reproduction, but after that they abandon us to the ravages of time. (Pretty selfish huh?) 

Medawar’s theory was further developed five years later by George Williams. At this time, biologists started identifying genes that were tied to the aging process and were surprised to find that they were shared among nearly all organisms. This got Williams thinking. What if the genes that make us weak in old age are actually the same genes that make us fit and strong in our youth? This theory became known as the antagonistic pleiotropy theory of aging (a mouthful I know). A generally accepted theory today, antagonistic pleiotropy means that in the fight to reproduce, our genes sacrifice longevity for virility. The beautiful thing is, this theory not only accounts for why we age, but it also provides an explanation for how these aging genes could arise in the population to begin with – they bestow us with greater fitness.

We're screwed by our own youth?!

Yes. We sacrifice longevity for virility. 

Some common factors that promote a longer lifespan across organisms include increased reproductive success, low adult mortality (permitting more reproductive events per lifetime), and high juvenile mortality (making it necessary for adults to live longer so they can  compensate for such loss). 

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