Why could the body not keep making healthy, young cells forever?
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Nothing is perfect. As cells replicate errors creep in. Eventually this causes decay. Even copying a digital file enough times will eventually cause corruption to the data since the copying process isn’t 100% perfect.
Essentially, copying errors. Our bodies aren’t 100% perfect at copying DNA. As you age, small mistakes creep in, and these mistakes accumulate over the decades.
Imagine making a photo copy of a document. There’s 3 inches on the margins full of nonsense and all the important text is right in the middle.
The copier is a little imperfect, the edges get a little blurry, but it’s okay. They’re not important.. yet. Just zoom in a little and trim them away.
But eventually you run out of margins to cut away, so things that are important need to start getting messed with in the middle. Or maybe you drip a little coffee in the middle on accident and a smudge happens, but you think that G is a C so you copy it in with a pen.
This is similar to issues copying DNA.
It’s imperfect but has extra at the end to compensate, eventually those run out and the imperfections hit the important bits. There are repair structures in there but those are also imperfect. Or something like a coffee drip, which would be like radiation or some other issue that alters part of the DNA leading to an imperfection.
We do not know quite what advantage the aging mechanism have. It could easily just be a mistake of evolution, an artifact of how our cells work creating this defect. However it is more likely to be a distinct advantage, we do see animals which do not age and the mechanisms they use is also present in humans, but not as active. The most likely benefit to aging is reduced cancer rates. Cancer is when a cluster of cells divide too fast. However this also means they age faster and stops dividing much faster. And when we compare animals to each other there is a strong correlation between aging, the length of the telomeres and cancer rates. This is a big issue in the effort to create anti-aging drugs. If they can not be controlled then they tend to cause cancer and premature death. It is likely that evolution have found a pretty good balance between dying of cancer and dying of old age. We might be able to improve on this if we can control the aging process though.
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.
It’s a mechanism we developed through evolution. Through the process of evolution advantageous traits are strengthened and/or multiplied generation by generation, meaning every generation is more fit to survive in its environment than the last. This is especially important when our environment changes, because while old generations might have been great at living in the old environment, they might no longer be optimized for the new one.
By killing off the previous generation through the process of ageing, new resources become available for the next generation and the process of evolution can continue. If people lived forever, procreation could come to a halt, because the world can no longer support more people. With that, evolution would come to a halt too and we would be stuck at an old version of ourselves and be unfit to deal with a new situation.
In a broad sense of the term we are still undergoing evolution. Social evolution in our case. The mindset of the people also evolves. Just think about how helpless many old people are with electronic devices. Many also fall for scams that most young people would instantly look through. Meanwhile a lot of young people wouldn’t be able to handle problems that were completely normal for the previous generation.
You might be interested in this presentation and Q&A from scientist Andrew Steele on the biology of aging: [https://youtu.be/87VOwAtyl-A?t=222](https://youtu.be/87VOwAtyl-A?t=222)
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