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.
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.