why do living things have to die?
In: Biology
Well they don’t.
Some things are [biologically immortal](https://en.wikipedia.org/wiki/Biological_immortality). They simply don’t suffer from getting old. Lobsters, Jellyfish, some yeast and flatworms. And some forms of cancer cells keep going on for as long as they have food. It’s not common. And while they could live forever, something will get them eventually.
Most living things have a built-in timer. Human DNA snaps off the end bit every time it divides and there’s a long string of end-caps call telomeres. Once you run out of telomeres to snap off, it starts snapping off important code your body needs to function. That’s why old people have thin skin, and have health issues.
This is probably by design. Evolution finds it’s better if the previous group makes way for the new generation, who gets to try new things. Hopefully they’re better and the species makes progress. If we never changed, we’d still all be bacteria.
Many ways to answer this. Of course you got the philosophical aspect, but you tagged biology. But even in biology, you got the evolutionary perspective and the mechanistic perspective, so sort of the ultimate vs proximate “why.” Not to mention that even the latter has sub sections like physiological (whole organism so organs and such) vs cellular (cells dying without being replenished) vs molecular (like accumulation of epimerized amino acids and misfolded proteins).
On the evolutionary platform, death is necessary for evolution to take place at all, but that’s talking about complete invincibility. Otherwise you’d quickly saturate a given region in terms of space and resources leading to a cap on reproduction and bam (not to mention that having all species being invincible means you break the carbon cycle and that’s a whole other issue). However when talking about natural death being out of the equation but other deaths like accidents, murder and predator attacks, then the cap wouldn’t be on reproduction per se but on reproductive rate, so the progress of a species would be much slower. So while I cannot say species evolved death itself because that’s another topic (mechanistic platform), I can say they evolved different rates of deaths (average life spans) that are optimal to to their species’ environment.
On the mechanistic platform (I’ll focus on humans but this applies to a lot of species), we die because we age or because we are not invincible, as organic materials are fragile and our whole body has systems that are very fine tuned and dependent on each other, and the failure of one leads to the failure of the rest. Aging is a complicated process with several factors at play. The ends of each chromsome, called telomeres, gets degraded progressively each time the cell divides. Eventually the degradation gets to the actual sequence you care about and the cell is no longer viable. This is a very basic biological clock for the life of a cell. But aging affects the whole organism. Cells get damaged due to all sorts of reasons, like UV from sunlight and oxidative stress from metabolism alone. And when damaged a cell often commits suicide. We don’t have an infinite reservoir of cells, some specialized cells have only a limited number of stem cells. And when the we run out, renewal runs out. Besides that, you have many many processes that affect us gradually at the big picture level. Like nephrons (filtering units in the kidney) get less and less over a life time. These are relatively big structures made of many many cells so if they’re gone they’re really gone. Then you have things like atherosclerosis, plaques accumulating in your vessels, hardening them and narrowing them. You get strokes and ischemia and die from heart or brain issues. Or you may get proteins aggregating just due to random chance of misfolding, this happens in the brain and almost everywhere else. And we’re horribly bad at dealing with aggregates so they end up killing cells and you eventually. Then you got random mutations that are bound to once hit critical genes that then cause cancer. I barely even scratched the surface here, fixing a cell by allowing it to renew its telomeres has been investigated before. But the risk of cancer is too high. And even if it wasn’t, we got a million other things that are bound to go south over a lifetime. We’re simply not built to forever, if you want to be immortal you have to change your body altogether.
Edit: added some info
The design of living things has been shaped, over billions of years, by evolution, and there is no evolutionary pressure for immortality. There is evolutionary pressure to procreate. While some more complex species (such as humans) can have non-procreating members still contribute to the overall survivability of the species as a whole, it is still generally better to have older members to eventually die off, leaving resources for younger members so they can survive to procreate.
There is one known immortal species: the immortal jelly fish. But jelly fish are a relatively simple organism. It attains immortality by basically reverting to an earlier stage in its life cycle. This is not something possible for more complex organisms and, even if it were, it would still be a kind of death as you would retain no memories of your past “life.”
Lots of good explanations, but none of them really ELI5.
You have a copy machine and you make a copy. Then, later on, someone makes a copy of the copy. Later, a copy of the copy of the copy… and so on.
Along the line, the copy gets damaged. Someone tapes it together, and then makes a copy of that. And a copy of that…
Eventually, after decades, you can no longer recognize what was on the original.
The same thing happens to cells in your body. They make copies of themselves as they divide. After 80 or so years, defects from copying make them not work as well.
Eventually, the body can’t work at all and… death.
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