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The deep ancestral condition of all hominins was pale skin. Beneath their black hair, chimpanzees (Pan troglodytes) have pale skin at birth and in youth.> The hominin line of descent that branched into H. sapiens had to develop skin that began dark and persisted as dark as humans lost body hair and began living in savannah environments with less shade than forests, according to the currently accepted hypothesis. As population bottlenecks in Africa formed the species H. sapiens, presumptively the typical skin color of all modern humans was dark (within a pattern of continued variation among individuals and a continued capacity for individual skin color to vary over the course of seasons).
These two comments, separated in your text by references, seem opposites (ancestral condition was pale...began dark), and the linked texts do not support both of these opposites. But I think you meant humans began pale, became universally dark, then some became pale again.
In any case, all the references you provide support the original post's hypothesis: skin color was affected by both diet and interaction with the sun, each round of research adding a new mechanism to the previously suspected mechanisms (not refuting the previous).
The original post here adds to the collective info you supplied, and neatly explains an otherwise startling variance between their hypothetical charts and the actual charts, that would seem at first blush to invalidate the prior theories but in fact confirms them.
You wrote a lot in your reply, and HN appreciates and votes up for effort. With the mix of personal anecdote (lived everywhere, I'm nordic and my wife's Asian), the gist is that you're adding insight.
However, with your two major paragraphs proposing at least one view that's opposite to your linked research (skin that began dark vs chimps pale skin), and with none of your comments or links addressing or negating the original article's added insight about grain and the loss of vitamin D, I can't tell what you're intending to convey.
It seems your exec summary might have been, "Without addressing the original article which I call 'just a hypothesis', I'd like to say I've seen skin shade variations everywhere including seasonality in my own kids; I am linking research that the linked article builds on; and populations' genes are getting more mixed though even people with pale skin have it for different genetic reasons."
Ok. But not one word of that refutes (or ever directly comments on) the thesis of the original article linked here.
To summarize your reply while tying in the original article, we could say (in a sort of TL;DR):
"The original article explains historically sharply defined regional skin color bands in terms of UV exposure and diet, adding grains and the gulf stream to prior theories involving UV and fish and meat. This explains regions that were problematic for earlier theories. But in today's highly mobile world, people like my own family are becoming more mixed than ever and these regional bands are becoming less sharply distinct. We should be cautious of the increasingly arbitrary politics of race."