You might see ageing described as a universal, inescapable process to which all living things are inevitably bound.
Indeed, king of evolutionary biology, William Hamilton – best known for kin selection – argued that all organisms must age (even theoretical organisms, like aliens). This was based on the idea that selection pressures are weaker for older individuals within a population – leading to the accumulation of genes that are ineffective or even harmful for older individuals.
How could a harmful gene be selected for?
Think about a gene that’s downright rubbish for an older individual – but which helps the younger individual succeed (and therefore ‘pass on’ their genes to the next generation). You can see how this could work – natural selection favours those traits that increase the propensity for genetic information to reproduce, after all. It doesn’t much care if your alive or dead after this has happened.
But you’ve probably seen the problem here already. What if individuals normally reproduce late in life?
It turns out that the argument about inevitable ageing isn’t as ‘slam dunk’ as Hamilton had thought. More recent mathematical frameworks have shown that, in theory, selection pressures may not always decline with age. For instance, think about an organism that becomes better and better at reproducing as it grows older – in this case, it’s easy to see that selection pressures would be high at this ‘old age’, since so much reproduction is occurring. So, out the window goes that previous argument about the accumulation of mutations that aren’t any good for the elderly – not if they’re getting funky, baby.
To note, missing so far from our discussion has been the assumption that an individual is only ‘useful’ to reproduction after completing said reproduction – but we know that this isn’t quite true. Thinking back to the key theory championed by Hamilton himself – kin selection – traits that encourage individuals to help the shared genes in the bodies of their relatives propagate are also selected for. So, if an ‘old’ individual is particularly important for helping its offspring or relatives to survive, you might imagine selection pressure to be high. But let’s not go down that rabbit hole of evolutionary theory, eh?
To answer your obvious question at this stage, can some organisms be immortal? Yes – or, at least, we do have evidence to support this idea, and you won’t even need to look far to find examples of possibly immortal creatures. Hydra, tentacled animals that live in your local pond, may not age. In an experiment that lasted around 8 years, Hydra were found to have a low, constant mortality – and the authors predicted that they would need to run the experiment for over 3000 years to observe the last Hydra die! But while Hydra may hold the secret to an immortal lifestyle (potentially linked to their remarkable regeneration abilities), we might first look a little closer to home for a ‘cure’ for ageing. To understand what this is, we first need to take a look at structures called ‘telomeres’.
Telomeres are the ‘ends’ of chromosomes that get progressively shorter following each DNA replication event; cells typically gradually lose their telomeres to varying degrees in different organisms, leading to senescence. ‘Aha! Let’s protect our telomeres, and then we won’t age!’ I hear you proclaim. The situation is complicated, though, by the fact that different cell types use telomerase enzymes to maintain telomere length – for example, sperm, egg, embryonic stem, and some immune cells.
Regardless, it’s been recently proposed that telomerase enzymes could be used as therapeutic agents against diseases associated with the ageing process. There is a fatal catch, though (of course there is) – while telomerases do not tend to be active in most of our body’s cells, they are activated in most cancers. While we might be able to prolong the life of cells using such a strategy, we may well run the risk of activating some serious nasties in the process.
Hm. Suddenly immortality doesn’t sound so great.
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- Vaiserman A, Krasnienkov D. Telomere Length as a Marker of Biological Age: State-of-the-Art, Open Issues, and Future Perspectives. Front Genet. 2021;11:630186.