Rejuvenating the blood cell population
science.org
science.org
“…the rebalancing towards the innate immune system with aging is very likely an evolutionary adaptation to keep up acute responses with age while relying more on B-cell memory for the adaptive immune system to handle variations of the usual pathogens.”
Selective pressure does not operate after the reproductive phase of life. There is little or no “evolutionary adaptation” to cope with degradation of function with aging.
(The so-called grandmother effect is still a weak hypothesis.)
If so, then it's really hard to explain how female humans manage to live 40 years past their reproductive age.
Animals also live significantly longer in zoos.
Average age of death has gone up, due to asprin, antubiitics, vaccines, improved hygene, and so on.
I don't buy the wear and tear side of thibgs, and zoo animals, pets have vaccines, health care, medicine, and checkups too.
In warm regions, perhaps HVAC / having at least rudimentary shade is equally important.
And houses for shelter just as long.
Unless you think people lived without houses in the 1800s, and heat, it's modern medicine that's changed, dramatically, the outcome here.
Ans there are plenty of moderate climates that had no better life expectancy than colder ones.
We had a lot of shaping by selection of existing traits but not that much long term evolution. We're essentially "running on the same hardware" as these pre-house Homo Sapiens.
What allowed prehistorical Homo Sapiens to live for 50 years now lets us do so for 80.
You need to look at life expectancy rates for the last 200 years. The big change is in the 1900s.
Not when we first built warm, heated houses, thousands of years ago.
You want either modern medicine and sanitation, or low population density. The latter is very hard to combine with low wear and tear. So it's really both medicine and comfort.
Humans and many apes probably had similar lifespans in the wild, but humans have this huge post-reproductive lifespan in safe environments that other apes don't.
Human males don't, though. They're often reproductively capable at 60.
> The evolution of the vertebrate immune system occurred in geographically limited populations. Before machine-mediated transportation— trains, planes, boats and cars—individuals were likely to be exposed to the majority of pathogens in their local geography by the time of reproductive age. As T and B memory/stem cells can survive an individual’s lifetime[60,61], they should be sufficient to provide adaptive immune memory to all local pathogens. Thus, the generation of new T and B lymphocytes in later life was probably no longer advantageous, whereas the production of short-lived myeloid cells would remain important for acute innate responses, even in later life.
Also, I thought the adaptive immune system came about in marine vertebrates? Doesn't water mix things up pretty well?
Re-evaluation of the immunological Big Bang
https://pubmed.ncbi.nlm.nih.gov/25517375/
A cold-blooded view of adaptive immunity
Individuals in different biogeographical niches can have distinct “passages of time” in their immune systems, depending on their exposure to endemic pathogens.
> Doesn't water mix things up pretty well?
So does air, but you don’t see a homogeneous distribution of airborne pathogens around the world :)
By the way, thanks for those papers, they reminded me of single-domain antibodies (or “nanobodies”), which appear to have evolved independently in cartilaginous fish and camelids: https://journals.plos.org/plosbiology/article?id=10.1371/jou...
This hypothesis would be testable by looking at migrating animals, such as gray whales or arctic terns.
Even if we don't know the exact mechanism, it seems like we have evolved a longer lifespan in a safe environment than any other ape, while our lifespan in the wild was probably comparable to many apes.
Whether this long lifespan after reproductive age was selected for directly (e.g. grandmother hypothesis or patriarchal effect), or whether it happened because of some other evolutionary accident, it seems to be real.
Lowe points out here that there's a rebalancing of blood composition that happens later in life that would likely have been beneficial in the past, which is (very weak) evidence for a selective pressure that increased post-reproductive lifespans.
...isn't it just better healthcare and nutrition?
Mean lifespan has been dragged down mainly because of high rates of infant mortality and, as you allude to, various untreatable diseases. But as we make progress on those, the mean has been shifting up.
We have done quite a lot in terms of infant mortality, but relatively little to extend the maximum age people reach. Take 1,000 random people born 100 BC vs 1,000 people born in 1900 and the oldest person from 1900 will very likely have lived longer but probably less than 10 years longer.
Being able to consult the elders when the group faces perplexing problems (who may have lived through something similar, or at least heard stories from the recently deceased generations who have) might mean the difference between between 20% of the population starving and no one starving.
From this point of view, mice live roughly 2.5X their reproductive lifespan; very similar to human post-reproductive lifespan.
There is nothing unusual or special about human lifespan, or even maximal longevity. Some normal mice (not growth hormone mutants) make it out to 1400 days.
The heritability of life expectancy increases with age in both mice and humans. Or crudely put—bad luck kills you in youth; bad alleles kill you in old age.
The lymphoid strain can specialize into t-cells, which is the speciality of the thymus.
https://m.youtube.com/watch?v=LmpuerlbJu0
The myeloid does provide macrophages, but they are of little value compared to an activated b-cell on a mission.
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