I do agree with you that the benefit attenuates with time, but we seem to disagree a great deal about steepness of that attenuation. You're talking about great-grandchildren. I believe that in a hunter-gatherer society the benefit of having parents goes to zero within 20 years after reaching adulthood and there is never any benefit to having gandparents.
You severely underestimate the value of having grandparents. In traditional societies, children are raised by grandparents and great-grandparents at least as often as by parents.
The reason is simple: hunting and gathering is extraordinarily difficult when you've got a baby on your knee. If you reproduce at 20 and now you have to take care of a baby, then the people who are in peak hunting-and-gathering condition are suddenly unable to find food. Solution: leave the baby with the 40-year-old grandparents, and go off to hunt/gather.
Problem is, a lot of 40-year-olds are still in pretty good hunting-and-gathering condition, and won't be maximising the group's survival by sitting on their duffs doing baby-guarding duty. The solution: leave the baby with the 60-year-old great-grandparents, and go off to hunt/gather.
This isn't conjectural: this is how traditional societies actually work.
Now, the 60-year-olds definitely aren't in particularly great hunting/gathering condition, so taking care of children is a good way they can contribute to group survival. The children can grow up and become self-sufficient under their tutelage. There's little evolutionary need another generation beyond them, so you'd expect mortality to increase rapidly after 60. Which of course is exactly what we see.
>There's little evolutionary need another generation beyond them, so you'd expect mortality to increase rapidly after 60
That's exactly my point. Hunter-gatherer diet didn't need to work very well against cancer, heart disease or Alzheimer's and hence we shouldn't expect it to promote what we consider longevity today.
But I think you're overestimating the value of grandparents for breast feeding toddlers. Population growth wasn't very rapid back then because most kids died at birth or very soon after. So there weren't actually that many kids around I think.
I wasn't saying it couldn't attenuate faster; I was just saying it clearly attenuates. I think that clearly one could conceive of a hunter gatherer existence where the grandparents do provide help to grandchildren - I have vague recollection of something about a modern indigenous society where grandparents watch grandchildren while parents hunt and gather, but it's sufficiently vague that I'm not confident in asserting anything about its reliability beyond mention. Again, though, when discussing evolution we're talking about human-design-space, not specifics about individual human cultures.
>Again, though, when discussing evolution we're talking about human-design-space, not specifics about individual human cultures.
Yes, in principle, and that is definitely a good point. But with hindsight we can observe that the part of the "design space" that includes 90 year olds was never before explored, and hence evolution tells us nothing about whether or not hunter-gatherer diets are good for longevity.
