Why is death a thing - a scientific answer
Living things don’t necessarily need to die. There are of course external circumstances that can lead to death, but what if we eliminate them? There are some lifeforms that can theoretically live forever. We all heard of incredibly old trees for example. There are also some species of birds that do not measurably age. And of course most single-cell organisms don’t have to worry about a ticking clock in their genome.
But then why is this pre-programmed dying, this ageing so prevalent in most species? Since evolution leads do the development of advantageous treats and elimination of disadvantegeous treats, that means that somehow immortality must be disadvantageous for the continued existence of a species. At first glance this seems counterintuitive, so how does that work?
Take a single indivduum: it is born, it grows up, it learns things, acquires experience, gets better at finding food and generally at surviving. Take a population of immortal individuals: at some point they’d all be at the prime of their skills and experience.
What happens to new individuals when they are born now? As long as there are enough resources they grow up and join the ranks of the other immortals. If the limit of the resources is reached, they are at a disadvantage and famish. They will not be able to replace their elders, because if there is no ageing in this scenario youth is no advantage. So these immortals would only produce few offspring and probably wouldn’t spend much energy on parenting either, because there is no evolutionary pressure to procreate.
Now this hypothetical species of immortals would be fine like this as long as their environment is constant. But as their environment changes, as pathogens mutate, as resources fluctuate, they are unable to adapt to it. And this means that sooner or later they die out. And that was that for immortality.
tl;dr: Ageing and natural death is an evolutionary mechanism to give new individuals an advantage over older individuals, so that the driving forces of evolution, variation and selection, can work and make the species adapt and survive. Immortality is not impossible, it is just not advantageous for a species under most conditions.
