I think the real issue is that when it comes to complex animals, it's much easier to build a brand new one than to repair existing ones. Our genes encode how to grow a human from scratch, but now how to deal with the myriad of problems that can occur as a result of normal wear and tear. Repairing an ageing body is a completely different problem from growing one.
We do have some DNA that deals with maintenance processes, but I think the issue is that these processes inevitably can't cope. It's like trying to patch up an old house. You can keep doing little fixes here and there, but at some point, serious renovations are needed, and we're just not genetically equipped to do that.
You have to wonder what it is nature could do anyway, if humans living to be extremely old had been selected for. Old human bodies are so broken, we'd need to be able to grow entirely new ones and shed body parts, even regrow brain tissue. We'd need a metamorphosis on the level of what butterflies go through.
Matt Ridley's The Red Queen also talks about the effects of parasites, which also maps in to why there are genders (more accurately sexual reproduction). The problem is the mismatch between lengths of generations. A parasite that has one generation every week gets 52 shots a year at an unchanging host. As time passes the probability of success for the parasite increases. The human with ~20 years per generation has to withstand that long before making a new one that is different and (partially) resets the clock.
Gender comes in because asexual reproduction would produce offspring virtually identical to their parent which means a successful parasite would have a large number of hosts to exploit. By combining DNA from two different individuals (note how much we avoid individuals from the same family with substantially similar DNA) things are mixed up enough to give a head start against the parasites.
I think the real issue is that when it comes to complex animals, it's much easier to build a brand new one than to repair existing ones. Our genes encode how to grow a human from scratch, but now how to deal with the myriad of problems that can occur as a result of normal wear and tear. Repairing an ageing body is a completely different problem from growing one.
We do have some DNA that deals with maintenance processes, but I think the issue is that these processes inevitably can't cope. It's like trying to patch up an old house. You can keep doing little fixes here and there, but at some point, serious renovations are needed, and we're just not genetically equipped to do that.
You have to wonder what it is nature could do anyway, if humans living to be extremely old had been selected for. Old human bodies are so broken, we'd need to be able to grow entirely new ones and shed body parts, even regrow brain tissue. We'd need a metamorphosis on the level of what butterflies go through.