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.

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.

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 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.

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.

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.

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.

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.

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.

So then it's fine for young hipsters to practice?

That seems very hard to believe. Americans are rather tall on average.

http://books.google.com/books?id=p3liL6fAjrcC&pg=PA83&lpg=PA...

I suspect you are protecting some cherished preconceived notions.

American diet is not exactly a cherished notion of mine either. Neither is it my diet of choice nor am I American.

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?

- I don't know, Bog. My parents have always eaten meat raw, and they lived healthily to the old, ripe age of 26.