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An Attenborough TV show suggested that the reason people live long compared to animals is that grandparents are needed to help raise the young. The grandparents raise the young while the parents work/hunt/farm/whatever.

So having healthy grandparents gives a survival advantage.

This theory is mentioned in the article as well, but it doesn't seem very plausible if we're talking about living into the 80s or 90s.

At that age, the effect should be reversed as people often need care themselves rather than caring for grandchildren (who may be adults by that time anyway)

I suppose the "grandmother theory" originally only explained why women live beyond menopause, not into their 70s, 80s or 90s.

Alzheimer's in particular does not reduce lifespan all that much; rather, it increases the resources that children and grandchildren need to use to care for the parent. So selection against Alzheimer's would presumably (we have very little evidence for any of this) be even stronger than selection against traits that, say, killed at 70.

Is actually caring for an Alzheimer's stricken relative a new phenomenon? Could it be that a hundred years ago, a chronically ill, continually deteriorating relative was just left in a room to waste away?

Professional Alzheimer's care is new;, without good professional work you're going to get sores and muscle atrophy and all kinds of other complications that can be fatal at old age. We've gotten very good at dealing with those, but not at actually curing the underlying disease. (Speaking from family experience.)

Maybe even more importantly, though, 100 years ago other, unrelated diseases of old age were likely to get you well before Alzheimer's really got bad.

Even today, people rarely die from Alzheimer's, because something else gets them first, as Alzheimer's is a very slowly progressing disease.

Increasing longevity to the point of incapacity may be a consequence of improved health care, rather than biological adaptation.

And while average life expectancy has been increasing for a long time (thanks to improved health care), the maximum life span has not. The oldest living people are still "only" about 115-122 years old.

Is this because "improved health care" only makes typical illnesses treatable/prentable whereas maximum lifespan (ie, the deteriation of cells/organs/etc with age) are not being focused on? Or is it because they are very challenging things to extend?

Primitive societies are known to abandon old people to die when they become a burden rather than an asset.

An ugly, indisputable fact. Saddening to see you downvoted.

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What's a nomadic tribe to do with someone who can no longer keep up? When there is insufficient food (commonplace in older societies) who is going to get the food?

When resources get scarce, there are a lot of ugly realities.

You might do better to provide citations over argument.

"Traditional nomadic tribes often end up abandoning their elderly during their unrelenting travels. The choice for the healthy and young is to do this or carry the old and infirm on their backs — along with children, weapons and necessities — through perilous territory. Also prone to sacrificing their elderly are societies that suffer periodic famines. Citing a dramatic example, Diamond said Paraguay’s Aché Indians assign certain young men the task of killing old people with an ax or spear, or burying them alive."

http://newsroom.ucla.edu/stories/jared-diamond-on-aging-1505...

Thanks, exactly.

But many animals reach sexual maturity much faster (e.g. cattle at ~40 months) so you would expect many more generations to be alive simultaneously.

I had the same reaction as you at first, but read carefully:

"Mostafavi and his colleagues tested more than 8 million common mutations, and found two that seemed to become less prevalent with age"

Old people are less likely to have this gene than young people. This can have two possible explanations: old people with this gene are more likely to are already dead (and thus not show up in the study), or the gene is becoming more prevalent in the younger generations. The statement "People without these mutations have a survival edge and are more likely to live longer" favors the first explanation.

This is not the effect of natural selection on gene prevalence. (And it would be weird for Alzheimer's, which usually occurs at an age where reproduction is often finished, and maybe even raising the kids is done already).

My apologies for editing my reply to add a third paragraph while you were replying to it, but I think that explains the relevance of the age dependence to this study: the decrease in the prevalence with age suggests that they actually do exacerbate the diseases they are associated with, and so the association is not merely a correlation.

Note that they are not suggesting these genes have been the subject of selection; it is the hypothesized number of such late-acting genes that were not found that are assumed to have been selected against.

> This puzzled me, as these Alzeheimer's and smoking results do not seem to be evidence of evolution; one would surely need to see a decrease over generations?

At least with smoking, we've been experiencing exactly that decrease for generations. Smoking has become vastly unpopular compared to how prevalent and socially accepted it used to be. Social factors had been one of the main driving forces, but that doesn't mean it has to be the only driving force.

The social issues complicate the problem of identifying genetic effects.

Could be that the genes have a deleterious effect in youth, not just old age.

Or it could be that aging is a complex process which a naive experiment like this is unlikely to shed much light on.

That is one of the hypotheses advanced in the article.