[derektank]

Aging exists because the human body is optimized to survive and reproduce in a resource constrained environment with many threats. Our predecessors eked out just enough calories to survive to the age of 15, when we could begin reproducing. Any traits that made it more likely for us to survive until that point, even if those traits resulted in damage that would eventually accumulate and wear us down after our reproductive window, was selected for. We are all basically running the biological equivalent of overclocked CPUs without investing in proper cooling.

We no longer exist in a resource constrained environment and have access to massive amounts of energy from the sun which makes entropy a negligible concern. There is no good reason to not at least try to prevent or reverse senescence.

[hakfoo]

I don't think "live to 15, have kids, and die" makes sense as a model for humans, even if you're modeling them solely as child generating machines.

Humans are a pretty damn care-dependent species. They're not going to defend or feed themselves without years of support, so if they aren't surviving en masse into their 30s and 40s, the next generation is probably going to have a severe die-out.

Beyond that there's probably ongoing marginal benefits to species fitness with longer lifespans. If you can keep a few generations in circulation at once, you probably have greater resilience to things like disease outbreaks (the 50-year-old cohort might have some past related immunity to a disease that rampages the 20-year-olds).

This also completely ignores any value of intelligence and ability to pass down knowledge, which is definitely a fitness factor just for being able to prevent future generations from poisoning themselves as easily.

[derektank]

Having kids takes a long time, especially when many of them die of infection or malnutrition. So yes, you're absolutely right that evolution "cares" about human beings up until they hit ~40. At that point, human beings enter the selection shadow, when improvements to survivability have a negligible impact on selection. I was eliding this point for clarity but I don't think it really alters my broader argument.

This is all pretty obvious when you look at one of the main diseases of aging, cardiovascular disease. There are natural genetic variations that result in very low free LDL blood levels, which basically prevents artherosclerosis from every developing and which inspired the development of the PCSK-9 inhibitors. However, there's no evolutionary benefit to not dying of cardiac arrest when you're 60, but there are marginal benefits to the liver binding to fewer LDL particles when people are in a starvation environment, so these variations never became widespread.

The grandparent hypothesis is certainly compelling and might explain some marginal improvements in human lifespan relative to e.g. chimps (who live about 10-20 years less than us even in captivity). But if anything, that reinforces my broader point, which is that aging did not develop as some kind of culling agent by evolution and that we should be looking for ways to extend human longevity now that we have the resources to.