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A testable hypothesis for your idea: do women age slower than men.

They certainly live longer

Good luck getting rid of all the confounders involved in testing that hypothesis. Men and women are much more different than in that one sex may lose more blood throughout their lifetime.

The way to test this is to follow all people that donate blood.

> A species that goes years without losing any blood, is effectively operating its body in an evolutionary out-of-spec configuration. It might work, for a while; but it wasn't heavily optimized for.

I think your/the hypothesis fails...take certain species of jellyfish that are biologically immortal, they don’t have blood at all.

Coincidentally they are considered top of the food chain, so another knock to the hypothesis.

I think she/he/it was referring more specifically to mammals and other animals with blood.

Why would you think they were referring specifically to mammals? Mammals isn’t mentioned once whereas species/animals is used very broadly In the context of very broad ideas of evolution.

In either case, the hypothesis suggests animals should live longer in the wild than in captivity. So let’s take the only mammal that isn’t more likely to die from age/don’t die from old age, the naked mole rat. We don’t understand why they die and any given day there is a 1 in 10,000 chance they die at any age. In captivity they should have the same lifespan as lab rats (6 years); however, in the wild the have an average lifespan of 17 years and in captivity that expands to 30 years (again in contrast to the hypothesis where wild animals of the same species should live longer than those in captivity).

> they don't have blood at all

Then that would be an evolutionary in-spec configuration, since Cnidaria-phylum species have evolved under selection pressure for hundreds of millions of years in that "never losing blood, because it doesn't have any" configuration.

For a similar reason, you wouldn't expect most plants to be troubled/benefited by the loss of solutes (= sap); a maple tree should probably live about the same lifetime whether it's tapped or not, because maple trees evolved mostly without predation. But for plants that have evolved over eons entirely under conditions of predation (e.g. where they're a primary ecological water source, like cacti in the desert—observable by the fact that they possess defenses to predation), you would expect a difference in longevity between those that lose transported solutes to predation, vs. those who don't.

I don't know but since I was young it always seemed intuitive that blood-letting ought to have some good effects. If xyz bad things buildup (say, heavy metals) exist in the blood stream, and you remove a few % of your blood stream to make new blood, haven't you removed some xyz bad buildup?

Yeah, but you've weakened your body's ability to manage whatever xyz bad buildup and are now asking it to produce more blood as well. IANAD, but I'm pretty sure the medical consensus is that it's a net negative stress on the body.

That sounds more like an argument in favor of having kidneys than anything else...

Yeah definitely. It's good to have an oil filter.

But what if you also change the oil?

Touché!

Make bloodletting trendy again!

Blood donors?

https://sciencenordic.com/body-death-denmark/frequent-blood-...

There is another than menstruating women group of people losing blood: blood donors.

If I remember correctly, it is beneficial for your health to donate blood if you have some predisposition for Fe accumulation (hemochromatosis in the severe form.

Here's a good-ish article on bloodletting in the modern age: https://www.artofmanliness.com/articles/the-potential-benefi...

TLDR: There are correlations that it reduces circulating iron, circulating cancer cells, arterial hardening, decrease insulin sensitivity, and it increased metabolism. Not a lot of super firm evidence though