Of course, it's plausible that the culture came first drinking milk gave some advantage to all, because milk has nutrition other than lactose in it, but it especially benefited adults with mutations that allowed lactose digestion.

WSJ: Matt Ridley on Human Culture's Effect on Genes | Mind & Matter

Clinical studies do suggest the importance of bacterial flora in other conditions that include lactose intolerance, atopic dermatitis and some other immune-mediated, rheumatologic and allergic diseases.

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