Milk actually doesn’t have all that much vitamin D naturally.
It appears likely that there were multiple areas of origin.
no.
Exactly as I said: either didn’t occur, or didn’t spread in that population. (because it was not selected for as a survival trait - for whatever reason, or was selected against for whatever reason).
I’m not aware that this particular mutation has a downside in terms of metabolic cost, but if not, then I would expect the trait to be randomly present in some part of the population if it had occurred. It’s general absence either indicates that it never happened, or that something is actually selecting against it as a survival trait - which would imply more than simply a lack of opportunity to use it.
Thanks, Aspidistra and Tamerlane. So it appears that lactose tolerance appeared twice in human evolution, once in northern Europe about 9,000 years ago and once in northern Africa about 5,000 years ago. In both cases it was accompanied by the appearance of pastoralization (raising of dairy animals). There’s no way to tell what caused what.
There’s a small cost associated with just making the lactase enzyme – that’s resources that could be making other proteins (slightly larger muscles or a few more immune cells or whatever). I don’t know if there are any other disadvantages (it’s conceivable that for instance, the lactase enzyme interferes with some other protein, though I tend to doubt it).
And to the OP, the general answer, as said here, is that there weren’t any cows around in ancient Asian societies, so there was no advantage to mutants that could digest milk, so the mutant milk-digesting gene never spread in those populations. I wonder if the lack of cows could be traced to a lack of grasslands that would be suitable for feeding cows (as opposed to Africa, Mongolia and Europe).
The archaeological record is more complicated than the genetic record.
Animal domestication starts in the Middle East about 10-12,000 years ago. We know today that low-lactose foods like kefir and koumiss and yogurt and cheese have their origins around that area and that the current populations have mixed numbers of lactose tolerance: not as high as northern Europeans, not as low as East Asians. This implies that some selection of tolerance occurred.
How it spread is highly controversial. The direct route appears to be from the steppes north of the Arabian peninsula but that is contested. A mapping of tolerance levels shows a gradual rise as one goes to the north and west. It’s fairly certain that both herding and tolerance reached Scandinavia by about 3000 BCE, but nobody agrees on the exact route. Some people make a connection to the spread of Indo-European languages and some people think this is hooey.
Why it spread appears to have several components. The usual story is that dairy products gave an additional source of food and had high levels of important nutrients like calcium that weren’t available easily from northern plants (no soybeans, e.g.). Possibly the extra calcium helped women survive childbirth. Tolerance may also help with calcium and vitamin D absorption. Tolerance is a dominant mutation. A mere 5% advantage in surviving offspring would spread the gene as much as it has in the time period allowed.
I’m not aware of any evidence at all that any mechanism selects against tolerance. There may be a resource difference in making the lactase for life but it appears to be so slight that it makes no measurable distinction.
Fair enough - it was the ‘no point inventing a food you can’t eat’ bit that threw me - because the process of selecting the mutation in western populations must have happened in a context where there were people without the mutation, who couldn’t eat the food (unless it spread through a whole population before anyone tried eating dairy, which seems unlikely, because that’s not selection)
Except for milk, we’re not talking about a food that (pre-mutation) nobody can eat; we’re talking about a food that no adults can eat, but children can (All babies can digest lactose. In intolerant adults, that ability goes away at 2 to 10 years of age, whereas in the mutant lactose-tolerant ones the ability never goes away)
So even in a population of lactose-intolerant adults, a source of milk is still valuable for feeding babies and children. And of course, once you’re feeding babies from cows, a mutation so an adult can take advantage of that milk is clearly advantageous.
That’s rather extreme.
IANAA but many of my friends and colleagues have been Asian-descended, and many have been among the most ravenous cheesehounds and ice cream lovers I know. Asian folk can correct me but my perception is that (1) some feel no ill effects at all from even a significant amount of dairy; (2) others get a mildly to moderate upset stomach or bloat, etc. if consuming a substantial amount of dairy; (3) relatively few have such a serious reaction that they’d just be like “get that away from me or I’ll have explosive GI problems, hives, etc.”
Yes, they can eat yogurt with live cultures as well as various cheeses that have undergone microbial fermentation (which is a fancy way of saying fermentation in which microbes have broken down carbohydrates - i.e. lactose).
Generally the harder the cheese the lower the lactose, and fermented cheeses, like feta, are a-okay.
I seem to recall a quote about China saying something like : “The Chinese eat everything under the sun”. I remember backpacking around China /Tibet around 1986 or so (yes, I’m old!) & seeing stuff like pig embryos & other bizarre (to me) stuff for sale in the markets ('Course, show 'em the ingredients of a typical Swanson TV dinner & they’d prolly 'a blown chunks!).
As someone before mentioned, I saw water buffalo in Tibet/ Nepal, but it was a much more grassy kind of landscape than alot of China. (And of course, the Yak butter lamps are a big part of Tibetan buddhist cermemonies).
Gotta think if dairy was available and practical it would be a big part of the Chinese diet. I think a big part of the question is the evolution of rice in China. I don’t know, can cows eat rice? Wouldn’t they mess up the paddy’s? Isn’t meat more of a condiment in most asian cooking?, maybe just because asians have historically had to eat “lower on the foodchain”, because of lager populations?
From what little I know, rice is an integral part of the landscape (terracing, land ownership, rice paddys, etc), and culture of China, so maybe that would be one place to begin an exploration of the question.
Well, thanks all for bringing back memories of Freak Street and Durbar Square in Khatmandu circa 1987.
Yesterday when I was young!! : )
I spent a week on Freak Street ca 1987! Say, you aren’t a math teacher named Jenny, are you?
Weren’t we all! 
Thanks for the information, Exapno Mapcase! I just want to add that cheese was a very valuable food because it was compact and kept for a long time without refrigeration. (Is it certain that the earliest cheeses were fully digestible by the lactose-intolerant?)
(Genetics is helping solve prehistoric mysteries. As just one example, the cows of Tuscany seem related to cows of Asia Minor, perhaps shedding light on Etruscan origin, and even perhaps suggesting a factual basis for the Etruscan origin myth in Virgil’s Aeneid !)
Naw, I couldna been her, wrong gender. Unless the trip was even longer and stranger than I remembered and included a cross dressing interval, the memory of which I’m now repressing. Heh Heh :eek: