Why do we age?

Ageing and death are a part of life. Every animal on Earth is shaped by them. In his new book, Death on Earth, zoologist Jules Howard explores the never-ending cycle of death and the impact it has on the living. Here, in an exclusive article for How It Works, he explains some of the common theories about why life must come to an end…

Ming
Ocean quahog Ming was the world’s oldest animal when it died aged 507

If you want a new perspective on the human condition, my advice to you is to stand for a few moments next to the world’s oldest animal. I must confess, it’s good for the soul. I did it a couple of years ago, in fact. In my 35 years of human life, during which I had gone from baby to bearded, the lowly clam that I had ogled in its display case had barely aged at all. It was an Arctic quahog and its name was Ming. Ming had been 507 years old when it had died. Ming was, and is, a record-breaker.

Seeing Ming in real-life (as part of my zoological journey into the evolution of death for my new book, Death on Earth) was strangely… life-affirming. Perhaps twenty human generations had lived and died during its long life. Centuries of human history had played out. A scientific world had blossomed and bloomed while it lay stationary there on the bottom of the sea. It was, and is likely to always be, the only time I have ever been emotionally moved by shellfish. Ming opened up a host of questions in me about death, and I owe that clam a great deal, for piquing my interest in death, I guess…

So why don’t animals evolve to shake off death? Why is true immortality in nature so elusive? Why must we age? And why do we die? I was to learn, during my journey, that these have become some of the most interesting questions in evolutionary biology, not least because of the numerous hypotheses that such questions generate.

So why do we age? And why must all life die? Here goes…

Perhaps the three most commonly cited reasons for why we age have to do with cells and the ways that cells work. You can think of these hypotheses of ageing as The Good, The Bad and the Ugly (a sci-comm turn-of-phrase I have mercilessly stolen from Carl Zimmer for the purposes of this How it Works piece). It goes like this…

The first hypothesis then. The UGLY hypothesis has it that the ongoing rigmarole of cell metabolism generates problematic ‘free radicals’ that build-up in cells. These are highly reactive oxygen atoms that, when they cumulatively fill-up during an individual’s life, have the potential to cause havoc upon important cellular proteins. According to the free radical theory of ageing, you and I see the result: it’s a build-up of this damage. We call this ageing. That’s one idea behind why we age. Then there’s the Bad…

The BAD hypothesis has it that we age because of telomeres. Telomeres are rather like the plastic caps on the ends of shoelaces: pull apart one’s chromosomes and there, at each end, you will see the telomeres.  Many experiments suggest that, for each successive division of a cell, the telomeres shorten, and this puts a finite limit on cell division, and thus, life. According to the telomere theory of ageing, we only have limited capacity for survival. Our cards are marked from the beginning…

And then there’s the third hypothesis: The GOOD. This hypothesis has it that genes that lead to an individual’s survival or reproductive effectiveness will flourish, regardless of the consequences of those genes further down the line. The idea –pleiotropy- comes from the evolutionary biologist George C. Williams, who deemed that some of the problems of old age might come about as side-effects of things, well, that help animals have better (or more) sex earlier in life. And, in gene-pools, sex, rather than longevity can (and often does) hold the balance of power. Williams chose a hypothetical gene to illustrate his point. Imagine a gene that alters calcium metabolism in a way that both strengthens bones in youth and occludes arteries in old age; natural selection would favour such genes, because they lead to sex and the survival of those genes, regardless of the consequences twenty or thirty years down the line. According to Williams’s hypothesis, we age and die because we have evolved to live so… productively… earlier in life. (It’s GOOD then. Ish.).

Each hypothesis has its own suite of evidence (and there are other hypotheses out there, of course) but The Good, The Bad and The Uglydemonstrate the breadth of factors that might influence ageing: genes, metabolism, DNA and the blossoming relationship that all animals have evolved with sex and death over the last 1,000 million years and probably longer. Death is part of our life, and we may never be able to change this. Unless. Unless…we look to the clams. For in them, we might discover the secrets to ageing… but that, dear readers, is another story entirely…

Death on Earth, published by Bloomsbury Sigma, will be released on 10 March 2016. 

Death on Earth
Death on Earth by Jules Howard

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