So, is this suggesting that humans, as an animal species, are basically "engineered" by evolution to have a 38-year flat part of their MTBF bathtub-curve?
Where I mean "engineered" in the sense that evolution only can only really steer toward adaptive fitness of an organism up until a certain point in its lifetime, that being the point when the organism has done all the breeding they're going to do. Past that point, most things the organism does won't impact the evolutionary cohort of its species, so, turning that around, there will be no adaptations in its genome to help it survive past that point (since, where would they be sourced from?)
That time coming at 38 years for humans would explain a lot, I think. (It makes sense; that's about the oldest average age that humans are willing to continue to have children at.)
Wouldn't evolution steer animals even after breeding if the parents continue to contribute to the success of their children?
If having long lived parents was beneficial to their offspring, evolution will select for longevity well past breeding age. OTOH, if an organism laid eggs and then vanished from it's offspring's lives forever, then it would be hard to see how evolution would select for longevity past the egg laying phase.
You're only 1/4 related to grand children. A strategy of living a long life to help raise a grandchild means that 3/4 of your effort is promoting someone else's genes. Now what if the strategy of those other people is to have many more children, possibly with many different partners, with a relatively diminished per-child investment, reliant on someone like you to help raise those children. Who's going to out-compete whom?
I'm not saying the second strategy is better, just pointing out that the first strategy is self-limiting. Which means we can't even begin to understand the benefits and viability without answering more complex questions; questions we don't yet have answers to.
Also, humans are the only known species, extant or extinct, which exhibits significant non-kin altruism.[1] There's no strong theory for how this emerged. Which means there are some very important dynamics to human evolution (and evolution in general) that we're completely ignorant to--we don't even know what the questions are, let alone the answers.
[1] The most popular mammals used for comparison to humans, bonobos and naked mole rates, are organized as matriarchies--the females are sisters, somewhat like ants and bees. The above-average altruism they exhibit is easily explained by basic Darwinian genetic evolutionary theory.
> You're only 1/4 related to grand children. A strategy of living a long life to help raise a grandchild means that 3/4 of your effort is promoting someone else's genes.
But you are helping to raise 100% of your descendants. I'm not sure it is relevant that your descendants have less genetic material in common with you as you move down the tree.
> Now what if the strategy of those other people is to have many more children, possibly with many different partners, with a relatively diminished per-child investment, reliant on someone like you to help raise those children. Who's going to out-compete whom?
The premise of Idiocracy! In this scenario, though, there could still be benefits of some per-descendant investment. I'm not sure "high investment in few offspring vs. low investment in many offspring" debate really matters to the question of "why stay alive longer than (age of infertility)+(maturation time of offspring)".
I think the answers lie in 'group selection'. Not sure how accepted a theory that is though. An example I've heard (but can't find reference to now) is that the tonsils may serve to kill off sick individuals before they can infect others.
Emphatically yes! I believe that may even be the reason humans grow feeble when old - so that the grandparents sit around the fire and nurture the young, teaching and telling stories.
Right. Think of “breeding” as the complete act of making autonomous organisms. If your children can’t survive without you, then in the evo-bio sense, you haven’t finished “breeding” yet, because the ability of your genes to be passed on still depends on facts about your genome rather than solely on facts about your child’s genome.
Humans take a pretty long time to “breed” compared to most animals, since humans are born pretty non-autonomous compared to most animals. But there still comes a point where the probability of breeding success of a child becomes entirely dominated by the factor of that child’s genes, over the factor of the parent’s.
Also note that, just because this effect exists, doesn’t mean it guarantees that it will be an effect in the direction of longer health-span in every organism. In fact, in many organisms, the organism is wired to die as part of the breeding process, since dying better guarantees success for the offspring! (Some because they no longer compete with their offspring for food; some because their body decays and fertilizes the spawning grounds, leading to a bountiful harvest for their children; etc.)
You know, a man who was young indefinitely would have a lot of time to spread his seed...I'd expect such a trait to be strongly selected for if it arose. Unless it had a stiff penalty, maybe more competitive among men, or interbreeding problems?
That's why we see menopause in us and in other species like Orcas. Grand parents contribute more food to their grand children without competing to have more kids. I can't find the reference, Google is spammed with diet websites.
when the organism has done all the breeding they're going to do
You could argue that having older and more experienced individuals around might increase chances for survival for everyone. You'd predict that that effect would be especially pronounced in a species that highly relies on culture, like us humans. Indeed the hypothesis was found true for orcas: https://www.pnas.org/content/early/2019/12/03/1903844116
It can be true, but for a gene to reach fixation, it has to be selected for, i.e. organisms without it have to die before breeding.
So this effect might work out if you have a lot of isolated sub-populations, and the ones that live less long suffer extinction, while the ones that live longer go on to eventually reconnect and breed. But it doesn’t really work in a “liquid market”, because free riders without the longevity genes will be just as successful at breeding (due to the existence of the long-lived species members) as the ones with those genes.
Where I mean "engineered" in the sense that evolution only can only really steer toward adaptive fitness of an organism up until a certain point in its lifetime, that being the point when the organism has done all the breeding they're going to do. Past that point, most things the organism does won't impact the evolutionary cohort of its species, so, turning that around, there will be no adaptations in its genome to help it survive past that point (since, where would they be sourced from?)
That time coming at 38 years for humans would explain a lot, I think. (It makes sense; that's about the oldest average age that humans are willing to continue to have children at.)