Recently increased prevalence of human forearm median artery: Microevolution | Hacker News Reader

Recently increased prevalence of human forearm median artery: Microevolution

onlinelibrary.wiley.com

78 pointsbookofjoe33 comments

>Analysis of the literature showed that the presence of the median artery has been significantly increasing (p = .001) over time, from approximately 10% in people born in the mid-1880s to approximately 30% by the end of the 20th century.

That is incredibly fast! That's three generations and change, which I find suspicious... if it indeed is progressing at that rate then I would be quite shocked to learn that the cause is purely genetic.

As another comment noted, "It’s enough that there is no decrease in the success of the offspring really. They don’t have to be more successful, just not less" - except for the prevalence to change that rapidly is very unusual. In the famous story of the peppered moths, the first black moth was collected 1811, they were about 50/50 in 1864, and they were at 98% in 1895. It took until 2003 for the melanic phenotype to fall to around 10%. A peppered moth generation is one year, versus a human's twenty-seven, and the reproductive pressure was massive (a white moth on black trees can't hide and is easily eaten by birds; I can't imagine anything similar for humans.)

Also quite interesting for the HN audience: >persistent median artery has been suggested to cause pain in carpal tunnel syndrome (Barfred et al., 1985; Lisanti et al., 1995)

> That is incredibly fast! That's three generations and change

A generation is between 20 and and 25 years - shorter towards the beginning of the period and longer towards the end

https://pubmed.ncbi.nlm.nih.gov/26630399

Generation length varies significantly across human history, geographical location, population groups etc. but the average across the last quarter million years is twenty seven years.

https://doi.org/10.1126/sciadv.abm7047

(2023-1880)/27 = 5.3 > 3

The quote was mid-1880s to the end of the 20th century, so you have accidentally added a generation.

Better-nourished mothers => chonk fetuses => thicker forearms => increased utilization of arterial supply in the immediate environment of the the fetal median artery => median artery is less likely to atrophy during a critical period of gestation.

No genetic mutations necessary.

I wonder how many other structures exist during gestation that don’t survive past infancy in the common case. This seems similar to baby teeth in that we need them early in life but they are replaced with more permanent structures or in the case; needed during fetal development but not needed at all after birth.

The circulatory system of a fetus undergoes many changes at the time of birth. For example, in a fetus, the left and right atria of the heart are connected (allowing blood to bypass the lungs, which are not yet in use). Upon birth, that "hole inside the heart" (the foramen ovale) normally closes, redirecting blood flow to the lungs for oxygenation.

For another example, look up the ductus arteriosus.

What is the environmental pressure? Just a mutation by itself doesn't spread, there should be some advantage? Shouldn't there?

> Similar[ly] the prevalence of other anatomical features such as spina bifida occulta and fabella has increased over the last 2–3 centuries. Evidence indicates that changes in the natural selection pressures acting on these specific anatomical features could have caused microevolutionary processes, leading to the observed increases in prevalence rates.

They seem to be suggesting that this is not necessarily an advantage. Modern medicine makes pregnancies with these genetic mutations more viable, so the dominance of the gene expression becomes a more significant factor than fitness.

It’s enough that there is no decrease in the success of the offspring really. They don’t have to be more successful, just not less.

Under Hardy-Weinberg equilibrium we wouldn't expect a trait to go from 0.1 to 0.3 in a large population in four generations. This suggests that the trait is subject to selective pressure.

There are a lot of potential causes. In my estimation the two most likely causes—

First, hitchhiking. The mutation could be near some other beneficial mutation. The closer genes together on a chromosome the less frequently they'll be separated by crossing over events.

Second, it could have some other, presently unobserved cardiac effect that reduces infant or maternal mortality.

My joke pet theory is that it helps with mouse control on PC's.

We are living our whole life at a desk, it is providing extra endurance for clicking.

We should just evolve ALPS key switches instead of fingertips already. Within a century, we'll look like the Prometheus engineer pilot.

IIRC we would see any trait spread into its equilibrium point unless selective pressure leaned on the scale. So, it could go from 0.1 to 0.3 simply because it's population equilibrium is there.

This should have a [2020] designation I believe.

I read the following:

> The focus of this study was not to analyse the prevalence of the occurrence of the median artery in relation to ethnicity, geographic origin or variations by sex, but to identify the global trends in its occurrence.

and thought "well that explains the astronomical precision of the p-value, they didn't take ethnicity into account!". You would expect to see variation prevalence vary by ethnicity, no big news here.

But no! Thinking about it more, that's exactly what is says on the tin: global ethnic populations changed in the last ~140 years, and with it, the prevalence of generic variations. Makes perfect sense ¯\_(ツ)_/¯

edit: I mean... assuming the causal link ofc. I'm assuming this makes more sense than some kind of evolutionary pressure that is selecting for forearm median arteries.

We need to retire the idea that evolution has a goal, or that evolutionary pressure explains everything. There are tons of things in the human body that are very suboptimal or are purely accidental, from the tons of "junk" DNA, our difficult birthing, the appendicitis, wisdom teeth, the way our knees fold, differences in earwax, etc, etc.

https://en.m.wikipedia.org/wiki/Human_vestigiality

https://www.scientificamerican.com/article/no-humans-have-no...

"Evolution" is just a (fairly tautological when you think about it) observation that "traits that manage to get passed on, subsist". It weeds out very bad traits, but the rest? It's just a giant lottery, not a great design with a teleological goal of "improving".

(And the human species has been short-lived so far, and may wipe out themselves without the "help" of a meteorite. In the same vein, cockroaches could be argued to be the pinnacle of evolution.)

While I agree with you, and also find the evolution = improvement views kind of annoying, I think you're also stating something that's accurate but perhaps misleading. Because when you say "good" or "bad" people are going to mistake that for what they personally think would be good or bad. When in reality good = makes you more likely to successfully reproduce, and bad = makes you less likely to successfully reproduce.

The best example of this is intelligence. In modern times, intelligence has become substantially inversely correlated with fertility. And as intelligence has high heritability, we are actively selecting against intelligence. And in fact we are already seeing generational declines (which had been previously been increasing for as long as IQ has been measured) in intelligence. [1] And this trend will continue until the point that intelligence and fertility once again become positively correlated. Nobody would ever say intelligence is a "bad" trait, but in evolutionary terms it has become one.

[1] - https://www.sciencedirect.com/science/article/pii/S016028962...

The study you linked to doesn't find heredity to be the driver of decline of IQ (which they pointedly reminded is not a measure of intelligence):

Regardless, we believe education, test-taking, and media exposure emerge as potential moderators for explaining the observed gains in three-dimensional rotation scores and declines or stagnation in matrix reasoning, letter and number series, verbal reasoning, and composite ability scores.

> There are tons of things in the human body that are very suboptimal or are purely accidental, from the tons of "junk" DNA, our difficult birthing, the appendicitis, wisdom teeth, the way our knees fold, differences in earwax, etc, etc.

Right, but evolution also can explain those; many of these are tradeoffs for things that do deliver benefits. Difficult, premature births are the cost of big brains, for instance.

> "Evolution" is just a (fairly tautological when you think about it) observation that "traits that manage to get passed on, subsist". It weeds out very bad traits, but the rest? It's just a giant lottery, not a great design with a teleological goal of "improving".

The tautological explanation is extremely powerful, because it means even a small variation that delivers a marginal but consistent advantage is going to be selected for. For instance, a person with dark skin can survive in Northern Europe, and a person with light skin can survive in sub-Saharan Africa, but the ideal skin tones for those regions that optimally trade off vitamin D production with protection against skin damage from the sun are probably pretty close to the ones typical to their indigenous populations.

The actual catch is that there’s a lot of path dependence and all changes are stepwise. You can’t just install a new trait or body part that’s been designed from scratch; it has to evolve from an earlier, similar thing. If you look at the bones of your hand, the bones of a bat’s wing, and the bones of the front paw of a dog, the bone structure is basically the same; it’s just by changing the proportions that you can get the structure of a flapping wing, a front foot, or a hand with an opposable thumb.

Evolution does have a goal - spreading genes. Of course when we say "it has a goal" that doesn't mean it intelligently plans how to get there. Nobody* thinks that.

I think what you probably meant to say is that evolution isn't a perfect optimiser.

It's also not optimizing for "maintain health in your 70s/80s/90s". A faster decline is probably evolutionarily advantageous to the small group of hunter-gatherers.

A bigger group of hunter-gatherers is more evolutionarily advantageous when it comes in conflict with a smaller group.

Not in a lean winter, and not if you’re worrying about a bunch of frail revered elders during battle.

Not if half of them are 80 >.> Seriously not every trait is revelations beneficial nor had to be

From section 4:

> The present study used an Australian sample of European origin. These results were comparable to those reported in black South Africans, their white counterparts and Malaysians of similar birth years; all these groups had a prevalence of approximately 30%

Simpson's paradox...

Are we seeing

1 A random mutation that is “testing the waters” if this brings some evolutionary benefits?

2 A mutation that already provides a benefit? (Which one would that be?)

3 A mutation that is neutral and is neither beneficial nor detrimental?

It exists in all fetuses but disappears after a point in development (or, as appears to be increasingly common, does not disappear).

Rather than look for genetic mutations as the cause (highly improbable given the time range) it is almost certainly an environmental trigger: mothers getting better nutrition during pregnancy.

I think that's an excessively intentionalist view of evolution. Evolution doesn't test the waters, in the sense that it inhibits a harmful mutation from happening again, nor are mutations likely to be purely good or bad; mutations are random changes, and are (definitionally) more likely to be passed on if they increase reproductive fitness, and that's about it.

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