>Stuff our ancestors used to do is likely to be neutral or good for you, due to the way evolution works.
That's not true when it comes to age related diseases like cancer or heart disease. Through most of mankind's evolution these diseases did not affect our procreation rate at all.
Evolution optimizes for procreation not for longevity. It is very likely that an ample supply of burgers, fries and fat cheese (especially during winter) would have led to higher birth rates not lower ones.
It could well be that much of our ancestors' lifestyle is poison for anyone over 50.
> Evolution optimizes for procreation not for longevity.
Incorrect. Evolution optimises for propagation of genes. Procreation is one but not the element of that process.
Think about this way: if you are living by yourself in the forest and have octuplets, then you are very good at procreation; if you die immediately afterwards, then you are evolutionarily unfit despite your procreative abilities, because none of your young will survive to pass on their genes. For creatures like humans -- born with helpless young -- the person who has two children and lives long enough to ensure that they can pass on their own genes is more evolutionarily fit than the person who procreates like mad but lets all their children die.
In our evolutionary environment, post-procreative individuals played vital roles in ensuring the survival of the young: taking care of children while their parents were off hunting and gathering; preparing foodstuffs; passing on lore about which plants were dangerous to eat, etc. In this way, the presence of elders facilitated the propagation of genes -- right up to the point where the care and maintenance of said elders becomes such a drain on resources that it begins to diminish rather than enhance the survival and procreation prospects for the young. That's quite a long time, however, so generally speaking, genetic lines that produce long-lived healthy individuals who enhance the survival prospects of the young will be more successful than genetic lines which don't.
If most people have children in their 20s, there is very little evolutionary advantage to living beyond, say, 60. Until then, they will have passed on all their valuable wisdom.
I think the key point is that ability to help ensure survival and reproduction of great-grandchildren matters (evolutionarily) half as much as the same of grandchildren, which matters half as much as the same of children. Though in principle there's nothing preventing continued ability to have children in later years...
I understand the argument, I just don't believe that surviving elders were actually helpful for the procreation and survival of their offspring during most of evolution. Not beyond an age where cancer and heart disease start to play a role.
Older men can have children in principle, but older women can not. So those men would have to compete with younger men for the remaining fertile women. The birth rate of a woman would probably not change just because there are more old men available to her.
"I understand the argument, I just don't believe that surviving elders were actually helpful for the procreation and survival of their offspring during most of evolution. Not beyond an age where cancer and heart disease start to play a role."
I don't think I was disagreeing with you, there. My point was that regardless of how much benefit an ancestor can give to their descendants, if you have declining fertility beyond some age the benefits of doing so attenuate with time.
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.
"Older men can have children in principle, but older women can not."
I didn't mean "ignoring social factors" - I meant "in theoretical human-design-space". Women don't produce new ova, but why does that necessarily have to be the case? Most cells in the body divide regularly.
That's not true when it comes to age related diseases like cancer or heart disease.
Well of course! Whether it's "good" or "bad" is kinda hazy and contextual as well. A gene that might encourage heart disease at age 50 might also make us better at spearing ferocious animals at age 19.
It is very likely that an ample supply of burgers, fries and fat cheese (especially during winter) would have led to higher birth rates not lower ones.
I know of someone from the pacific island of Yap, and his entire generation was stunted in their growth from his culture's encounter with American junk food.
It could well be that much of our ancestors' lifestyle is poison for anyone over 50.
He told us unequivocally that his generation was noticeably shorter. Some searching turned up lots of mentions of malnutrition on pacific islands when local diets were disrupted, though it's not so easy to find mentions of Yap specifically. Here's one mentioning the Marshalls.
What I'm saying is that food can have bad consequences in terms of cancer or heart health but not necessarily stunt growth. The unhealthy American diet (tons of sugar, salt, meat and animal fat) is not known for stunting growth, and your evidence for it is more than thin: One sentence in a book written by someone with a degree in political science.
American diet is not exactly a cherished notion of mine either. Neither is it my diet of choice nor am I American.
Stunting affects about 147 million children worldwide.
This long term under nourishment is very common. The WFP (world food program) gives estimates of 227 million for Africa and 553 million people in Asia and Pacific.
What happened when local diets were disrupted? Were local foods bought in return for much lower quality food? Was malnutrition made worse? Or did we just start documenting it better?
That's not true when it comes to age related diseases like cancer or heart disease. Through most of mankind's evolution these diseases did not affect our procreation rate at all.
Evolution optimizes for procreation not for longevity. It is very likely that an ample supply of burgers, fries and fat cheese (especially during winter) would have led to higher birth rates not lower ones.
It could well be that much of our ancestors' lifestyle is poison for anyone over 50.