I agree, intolerances makes you ill firstly with specific low-level symptoms, but can evolve in many different ways, including cancers, tumors, auto-immune diseases etc...
But especially in a survival contexte, because we dont know how all those ancient populations reacted to lactase or gluten for exemple, symptoms linked with gluten or dairy could be seen as " usual buisnesses ". And the transition between being sick and not and why, was not clear for them.
To be more explicit, i dont think symptoms of eating gluten or lactase by people who didn't tolerate them was as obvious as the symptoms you get from a cobra bait.
Therefore, i think we can easily be tricked by the fact that farmers or herders could have been huge crops ( gluten ) or dairy consumers without really being aware to how it harmed them because they didn't have lactase persistence. Wich also could have tricked us on the fact, that yamnaya ancestry clearly linked with herding culture, should have had those tolerances, but actually did not, or not much.
Especially in the contexte of Triticeae and Fabaceae, it's amazing how wide this culture spread, initially from the middle-east, then in all directions, maybe with already daily gastric symptoms.
Now, did the culture of dairy consumptions clearly peaked after a said population had the genes for lactase tolerance would be interesting to know.
I'll start with the bolded comment. There is absolutely no way you can know that.
There is, indeed, no way either of us can know PRECISELY how the people who lived before the first millennium BC who lacked even one derived allele for lactase persistence reacted to dairy products. That's why
I said that it's possible that their gut biomes might have been slightly different.
The
closest we can get is to test those people today who are having lactase intolerance symptoms. Now, I happen to be one of those people who actually was tested for it. In those days there was no testing for the LP gene. Lactase non-persistence or lactose intolerance was diagnosed through symptomology.
In my case, suddenly, in my late twenties, after having eaten tons of cheese, ice cream, puddings, butter in cooking etc. all my life I developed severe gastrointestinal symptoms. Not only did I present to the doctors as in pain and dehydrated, but I had lost about twenty pounds over a two and a half to three month period. Given I started out at about 120 pounds, it was serious. Given my history the doctors initially suspected some sort of cancer. After blood tests, upper GI series, lower GI series, Xrays up the wazoo and finding nothing, they finally decided to try what they called "food allergies". The actual pin ***** arm food allergy test showed nothing so the gastroenterologist decided to start me on what they call an "elimination diet" ironically enough. The first thing he told me to do was to cut out all dairy to "cleanse" my system. After a few weeks, when my symptoms did indeed abate, they administered the actual test, which is to give you straight lactose and then wait to see what happens. I wouldn't wish it on a dog. I might wish it on my enemies.
I've learned to manage it. No drinking milk, of course; no butter on toast, ice cream once in a blue moon; limit my cheese intake to mostly hard cheese and moderate amounts of it, discipline myself to have a slice of pizza maybe once a week or less, and take lactase pills.
If ancient people had a reaction even half as bad as mine it would indeed have killed them, and quickly.
Now, for the third time, maybe it wasn't as severe for them, but I wouldn't bet on it being totally benign either, or all those northern Italians of the first millennium BC would have started drinking the milk their cows produced as well, and we know that in the first millennium BC and during the Republic and Empire, they remarked on how much dairy the Celts and Germanics consumed and that they drank milk, but although the "Italics" certainly had dairy farms and made cheese, they didn't drink the stuff.
Oh, the ironic thing about all of this is that I carry the derived allele; both of them, so I should have no problem whatsoever with dairy.
I've discussed it with others who have the same history and genetics, some Poles among them, and all I can figure out is that in some people it just "turns off" with age, although not in infancy, as happens with people who carry the ancestral alleles.
Someone asked so where did it mutate. I don't think anyone really knows, although I remember a relatively old paper which speculated it happened somewhere in central Europe. We do know it was only highly selected for starting three thousand years ago.