Hacker News Reader

Comment by tptacek | Hacker News Reader

This article uses the example of the Tsimané people of Bolivia as a model for pre-industrial society, and notes that a very low percentage of older Tsimané experience measured dementia (something like 1% compared to 11% in North America). But it's also the case that the at-birth life expectancy of a Tsimané person is quite low compared to the rest of North America; a smaller percentage of Tsimané survive to the age at which Alzheimers becomes a major epidemiological issue in modern societies. How does a researcher account for that? It seems like it could be a confounder.

View on HN

I often wonder what the effects of reduced infant mortality will be on the population. For all of human history before about 100 years ago, we were aggressively selected for survival at birth or shortly after. Now, we're not. All those people who would have died might not only suffer other health problems in their life but propagate the "die as a baby" genes to their offspring. Optimistically, it might mean that humans both become dependent on those medical technologies that save babies and mothers, and maintain the capability to provide them so we're fine. It could be a positive direction for evolution, like how we're already adapted to depend on ancient technologies like housing, clothing, and cooking. But maybe it's just bad and future generations will be generally get less and less healthy.

The head sizes of babies are actually getting larger over the past decades [1]. This might be because of better nutrition, but maybe it is by a large part because we can do caesarean sections in every hospital now and head size is not as limited as it was before anymore.

[1]: not the best source, but one I could find now: https://www.tandfonline.com/doi/full/10.3109/07853890.2011.5...

The corollary to that is that narrower (maternal) hips are also not being weeded out of the gene pool.

I doubt that infant mortality was mostly because of genetics, since natural selection would make it hard for that gene to spread unless there was some other benefit (e.g. how being a carrier for sickle cell anemia is good, even though having sickle cell anemia is bad). The main reason for high infant mortality was probably because of illnesses and famine, not genetic.

That's irrelevant to some extent. It still means that those who could better resist illness and famine had a better chance to live, which is likely to have some genetic correlation. Even when the famine is extreme, it still selects for the families cold hearted enough to be able to consciously feed only those who have a chance to survive instead of trying to spread their resources to all their children, which itself may have some genetic source that can be selected by this pressure.

Not that either of these are good or necessary for a healthy species, mind you. We will always adapt to our environment and the selective pressures that match it, even when it's one of our own making. If that means fewer children die young, that is an excellent outcome - evolution is never goal oriented anyway.

It could be partly due to normal harmful random mutations reducing the mother/baby's fitness, which are now being allowed into the gene pool more than before.

Yes, but there would still be a very high selective pressure to have more resistance to illnesses and famine.

I'm more worried about pollution and all the "lifestyle disease" that fuck up our gene pool personally (diabetes, obesity, hormone disruptors, &c.)

> a smaller percentage of Tsimané survive to the age at which Alzheimers becomes a major epidemiological issue in modern societies. How does a researcher account for that? It seems like it could be a confounder.

This would be an issue if Alzheimers is part of a phenotype that raises all-cause mortality in the young. I'm not aware of any research on this. Alzheimers is known to be correlated with certain lifestyle and immune factors, and these factors might be expressions of a common phenotype that also increases early mortality and is moderated by pre-industrial environments---e.g. increased risk-taking or decreased immunity.

A common phenotype hypothesis might explain some of the difference in Alzheimers incidence between Tsimane and Westerners, but it would be surprising if it were the main reason. Assuming the hypothesis only, an Alzheimers delta of 11x for those over 65 implies a mortality rate delta of 11x between the under-65 phenotype groups. That is a massive effect, and if it includes child mortality, we would expect to see genetic drift over time and a lower genetic predisposition to Alzheimers among Tsimane (has that been studied?).

Do you suspect a higher percentage of those of the Tsimane who died young would have developed dementia after 65 (which seems to be the cutoff they're using)? That would be an interesting result, but unless there's some reason to think it, it's just one of many potential hypotheses.

I don't know! I think the subtext of the article is that the Tsimané have, ceteris paribus, a lower incidence of dementia --- that if you could keep the lifestyle and environment constant and raise the at-birth life expectance of these people to modern standards, they'd still have low rates of dementia. I'm wondering if that's true, and how you'd go about investigating that.

But wasn't that also the case in antiquity/middle ages? People not experiencing Alzheimer's because they did not live long enough?

Right, so the premise of the article is an inquiry into whether there is something neuroprotective about the lifestyle or environment of premodern civilization.

I'm guessing that Cicero, Galen, et al. would have primarily been looking at upper-crust Romans. Tribal groups are necessarily more equal.

It is estimated that about 12% of Romans that survived first 10 years of life (half of them died before that, but cest la vie if you don't have vaccines and antibiotics) lived to be 70 years or older.

Currently, this figure is 43% worldwide from birth. But fertility is also way lower as a response to low infant and child mortality.

So 3.5x increase in over-70 population seems a decent guesstimate. In developed countries even more.

  In a study of all men of renown, living in the 5th and 4th century in Greece, we identified 83 whose date of birth and death have been recorded with certainty. Their mean +/- SD and median lengths of life were found to be 71.3+/-13.4 and 70 years, respectively.

https://pubmed.ncbi.nlm.nih.gov/18359748/

Dementia starts to be noticeable in modern society after about 75. I haven't found a source showing the distribution of age at diagnosis.

Also the title is highly misleading: Alzheimers is only one form of dementia.

From another source:

  A growing number of studies indicate that the prevalence 223.267.315-322 and incidence270.320-329 of Alzheimer's and other dementias in the United States and other higher income Western countries may have declined in the past 25 years,270,277.320-328.330-333 though results are mixed. 60.261.334-335 One recent systematic review found that incidence of dementia has decreased over the last four decades while incidence of Alzheimer's dementia, specifically, has held steady

Reply on news.ycombinator.com