Why could the body not keep making healthy, young cells forever?
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Aging is extremely complicated and we don’t yet fully understand it, but we do know it’s not as simple as just needing to keep making healthy young cells. After all, there’s more to a human than just the cells. For example, in the nervous system, what’s important isn’t just that cells are there, but the way in which those cells are connected. Replacing cells would damage or destroy those connections, so deterioration would occur even with a constant supply of new cells. There’s also the collagen lattice, which is like a big scaffold that holds all our cells in place, and that degrades over time too (damage to the collagen is one of the main components of skin wrinkling).
Even cells themselves have a problem – cancer. To make new cells, a special type of cell called a stem cell has to divide. Stem cells aren’t immune to cancer – in fact, it’s easier for a stem cell to become a tumour than it is for a regular cell, because stem cells leave turned on special mechanisms that other cells don’t, and one of the many things a cell needs to do to become cancerous is have those stem cell mechanisms turned on. Cancer is the great inevitability. It *will* happen at some point, it can only ever be delayed, and the more you replenish cells, the more opportunities you give stem cells to become cancerous.
The best way of explaining why we age though I think is the evolutionary angle. Y’know how humans start out as weird tiny featureless mini-people small enough you could throw and catch them if you wanted? And then over 18-21 years, grow to be like, a hundred times bigger with special new features like hair and language? Well, the reason that happens is because there are a whole bunch of genes telling the body how to grow up. And this is great, up until we reach physical maturity – having an adult body and the ability to do maths is much more productive than having stumpy little limbs and only being able to drink milk.
However, for a lot of these genes, we never evolved an off switch, so they just keep going, well past the point that we don’t need them anymore. And because at this point, the effort is no longer properly coordinated, the continuing activity of these genes ends up causing damage rather than building towards an even better stage. Much of biology is about making trade-offs for reproductive success, and there are a lot of genes that damage us for the sake of improving the success of offspring, and those mechanisms don’t get turned off after we’re done having kids, so the damage just keeps accumulating.
The reason that we never evolved an off switch is because natural selection works by affecting the frequency of certain genes in each generation. If a certain gene becomes more common in the new generation, then natural selection is favouring it, and the species will evolve in that direction. If a gene makes you less likely to reproduce successfully, then it will be less common in the new generation, and over time may disappear completely. If a gene doesn’t affect your chance of successfully reproducing either positively or negatively, then overall it tends to remain about the same frequency.
Humans only start deteriorating after most of us are done having kids (or at least, have had enough kids to pass on our genes reliably). So, if a mutation occurred in one that made aging slower, that wouldn’t affect their chance of reproducing successfully – that gene effectively doesn’t exist until they’ve already reproduced, so it’s only passed down at the same frequency as the ‘fast aging’ genes. In fact, even a gene that literally said “You instantly die at 40” would not be reproductively disadvantageous, given most people across history will have been reproducing in their late teens and early twenties. In evolution’s eyes, there’s no benefit to aging slower.
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