Many have heard about the fabled "immortal" jellyfish, Turritopsis dohrnii, which doesn't die from aging (senescence) and can revert the aging process indefinitely. It is rather remarkable that only one, or very few species show this type of behavior.
I wonder if at some earlier point in evolution, when life was predominantly simpler and sea based, there were more species with this feature, but this "immortality" caused overpopulation and/or didn't allow for proper action of natural selection, meaning that "immortal" species didn't evolve much and got wiped out almost entirely by whatever phenomenon.
In this scenario, aging and dying from old age would be an important evolutionary trait which would allow for populations to maintain reasonable sizes and would allow for natural selection to impart changes.
I was wondering how plausible all of this is, since I'm not an expert in the area and know little about the appropriate technical concepts behind these things.