The conclusions probably depend on our confidence in their sampling and in the sampling from earlier eras, and I can’t judge that. For purposes of discussion, I will assume that they are right that there has been an increase in retention of the artery.
Well, “just basic stature” is as morphological as it gets, but anyway, Google tells me there are some new claims about wisdom teeth.
Who knows. IMO the most reasonable hypothesis is that the artery retention is not the key selected trait (assuming selection is happening at all) but that something else is changing and the artery is a consequence. I’m not sure drift is a good hypothesis biologically speaking but I think it’s a reasonable possibility. And I suppose it’s at least rational to speculate about selection on arm strength or dexterity or something but I think that’s unlikely and it’s probably too simplistic.
The paper is paywalled, but I do have questions based on the abstract. Are the various studies of the same population? If not, how would it be known that this isn’t a case of geographic variation? How can the results be extended worldwide? And as with increased height, can this be due to non-genetic factors?
But if this is a real and heritable trend, it must be selection, since drift isn’t nearly that fast. It must in fact be quite strong selection. But selection on what? Do modern humans have some strong need for increased blood flow to the arms, or is there some other trait under selection for which this is just a side effect?
That doesn’t seem all that reasonable. It would have to be very strong selection to account for the claimed change in frequency. Is there any evidence of such selection? Is there even evidence that this trait has a genetic basis?
If not genetic, that would leave some physiological response to environmental, dietary, or behavioral factors, maybe with epigenetic mediation. Anything else you would suggest?
Other than making an inference from the trend of the trait itself, it would seem there is no further evidence. It is really difficult to envision how any such direct selection would work, which I took as the reason that @sfmatheson offered that it may be incidentally associated with another change.