Our ancient ancestors had to make a tradeoff in order to combat the extremely high mutation rate of their microbes, by increasing their own rate of genomic diversity (through sex, and through adaptive immunity) including in the receptors which detect microbes.
Sexual reproduction https://en.wikipedia.org/wiki/Sexual_reproduction
V(D)J recombination https://en.wikipedia.org/wiki/V(D)J_recombination
Evidence of G.O.D.’s Miracle: Unearthing a RAG Transposon https://www.sciencedirect.com/science/article/pii/S009286741...
However, nothing is free, and with the ability to generate this extreme genetic diversity comes a higher risk of making mistakes including un-programmed DNA breaks, etc.. All these little mistakes add up as aging, and sometimes cancer, when the repair pathways are not able to keep up with the damage.
Also, by having an adaptive immune system you're constantly breeding tougher and tougher microbes, which could be considered harmful.
The acquired immune system: a vantage from beneath https://pubmed.ncbi.nlm.nih.gov/15539148/
Mucus acts as both a barrier and a food source with an extremely diverse O-linked glycans which decorate its backbone. ~800k permutations of the 5 possible O-linked if you consider a glycan chain of length 5, and 3 possible linkages types. In actually the diversity is likely greater given chain branching patterns, randomized expression of glycosyl hydrolases which generate the mucus, etc..
>I, too, used to think that we had our immortality in the wrong place, but I don’t think so any longer. I think it’s in the right place. I think that is the only kind of immortality worth having -- and we have it.
If you take that line of reasoning, you might as well go further. The protons of the hydrogen atoms in your body are from seconds after the Bing Bang. They were there at the formation of gas clouds, and galaxies, and stars, and novae, and supernovae, and, the formation of our sun and planets. They were present even before the single celled organisms in the water, the methane, the precursors of nucleic acids and organic molecules. And, with an estimated half-life of 10^34 years, these protons will be around long after the all life on Earth is gone. Compared to them, the germline, is but a drop in the bucket.
As far as I know the proton decay is just a hypothesis - they may be able to exist forever. Usually particles decay into lighter particles, but there is no lighter particle that a proton can turn into while keeping it's electric charge.
Rather, the point is that at some point the immortal germ line found a weird trick: "let's make in each generation a bunch of cells that will help the germ line reproduce, but that won't themselves reproduce". Like sterile worker bees. And our body, including the brain, is a pile of just such disposable cells.
