I remember in one of our class discussions, lactose intolerance came up and I thought that a change in the lactase (the enzyme that breaks down lactose or milk sugar) gene made it possible for expression after weaning. This meant that individuals could digest milk and other dairy products and supplement their diet with other sources of calcium and vitamin D. To me, that seemed a completely sufficient explanation for why the new allele for lactase was selected as advantageous. After all, we’re all told to drink milk to build strong bones, so why wouldn’t the same apply to early shepherds. This explanation is known as the “Calcium Assimilation Hypothesis.”
We need vitamin D to absorb calcium. That vitamin D can come from one’s diet or come from sunlight. So in geographic regions of less sunlight, there is a greater need for a diet containing vitamin D than say in equatorial regions. In other words, the Calcium Assimilation Hypothesis really only holds for northern European populations.
From here on out, I think it’s important to consider lactose intolerance as more than the digestion of milk, or more specifically lactose, but as an evolutionary culmination of culture, geography, and sunlight.
To note on your last paragraph, I think that point of view should be expanded to all traits , not just lactose intolerance. I think it is incorrect to say that the answer to why a certain trait is present in our gene pool has one answer. Rather, there are many correct hypothesis where no one hypothesis is more true than another and all of the predictions can be integrated together to find the real and complex answer. Surely one hypothesis may hold more weight in why the trait is present than another, but that does not falsify another valid explanation.
Lactose digestion is a good example of how the human diet can provide a selective pressure for gene expression. In the discussion of the first article you cited, the authors suggest that dairy-based farming was likely more protected against food supply fluctuations than cereal-based farming since crop failure was not a threat. In my contemporary nutrition class we briefly discussed lactose digestion and its relationship to the development of agriculture was highly emphasized. Humans did not have a source of dairy until animal domestication and the introduction of animal husbandry. I think that this strongly relates to culture and how different populations use dairy differently and developed various types of farming at independent times.