Just like reptiles, who die from the accumulation of damage, at a time depending on the rate of damage, but their point is that mammals don't merely die from accumulation of damage but also have a built-in clock.
Why assume there is a clock, rather than assume the damage is at the metabolic level? None of the predominant forms of human chronic disease that lead to most instances of death today (artherosclerosis, cancer, neurodegenerative disease, metabolic dysfunction like diabetes) seem like an intentional function. They seem like unintended consequences of other functions, like lipid transport or DNA replication, that don’t get selected against because they fall beyond the natural reproductive lifespan of most people. I suppose you could say that the biological clock in question is the number of eggs a woman has, but a simpler explanation for limitation could be that eggs are just very energetically expensive to produce.
Correct me if I'm mistaken, I was under the impression a human female is born with all the egg she will ever have, so the expensive production bit you're talking about happens during gestation of the human embryo.
Yes. Although, technically, no. In humans there is a massive change as the result of the first time air touches the lungs. It inflates the lungs. It causes the baby's immune system to engage, disconnect and start working.
And it causes the ovi to start the cell division that will create the next generation, form the "yellow body" and then effectively disconnect from the baby girl's body (obviously that cell division doesn't complete until a sperm combines with an ovi, hopefully multiple decades later)
So normally, this happens immediately after birth, together with dozens of other big changes.
Because you can reset the clock, and even sabotage it. In fact, that's how we produce a certain class of medicine (by now probably 10 classes of medicine, but ...)
Also, there Henrietta Lacks, died in 1951 of metastasized adenocarcinoma, but "still alive":
By that token normal reproduction qualifies as immortality as parents share a lot more DNA with their kids than those cell lines with their originator. Biologically those cell lines are generations of new organisms not the same individual living indefinitely.
Further those cells can’t compete out in a forest somewhere. Going multicellular means optimizing in ways counterproductive to survival on their own. But failing to cull off defective cells means you can’t have a functioning multicellular life form, thus making this a dead end.
So no in nature there isn’t some switch that means those cells get to survive indefinitely. The closest viable option is as an infection shared among very closely related organisms, but that’s not a stable option which means it’s really rare in nature.