Interesting. As a Chinese I have always wondered why lactose intolerance is so common in China but not in the west. Turns out I asked the wrong question :)
It seems even more unlikely, given that China has had so much interaction with the nomad peoples from Mongolia, Manchuria, and central Asia, who lived mostly off of their herds and milk-products.
Dairy products never really made it into Chinese diet.
The next question to ask is why diary products didn't make it into Chinese diet despite the interactions with the nomad peoples :)
I still find it interesting, since many of the more successful Chinese ruling dynasties have central/north asian roots (Tang, Yuan, Qing). Considering that Genghis Khan is probably an ancestor to most of the eastern half of eurasia, once might expect those lactose-tolerant genes to be more prevalent.
This is simply wrong.
Maybe some 16 million. But that has also been more of a hypothesis. See http://clubtroppo.com.au/2012/10/15/why-genghis-khan-wont-ha... for one of the refutations for instance.
I do this not to participate in the argument but to point uninvolved readers to objective sources.
Chinese and Altaic people are not related. See http://en.wikipedia.org/wiki/Altaic_languages
By Altaic I mean Central Asians like Mongolians and Turks/Turkic.
When equating "people" with the language they speak, something that was very popular in the 18th & 19th centuries[0], it is important to remember that the ancestors of many people you know today probably spoke a different language 200 years ago.
[0] For a good historical summary, see http://www.ucpress.edu/book.php?isbn=9780520244559
Nature's solution was to make sure that anyone old enough to eat real food would suffer greatly if they refused to wean themselves.
But once people had animal milk available, it was a gigantic natural advantage to be able to drink it as adults -- not just for nutrients and calories, but much more significantly, to avoid dehydration, which was a major killer back then.
You may not know much about lactation, but nutrition is a big part of it. If there is a famine/drought, there is no milk to be had, doesn't matter how strong you may be.
But don't take my word for it; here's what Slate has to say: "With Cain weaned, Abel could claim more of his mother's attention and all of her milk. This kept a lid on sibling rivalry [...] while allowing women to bear more young."
http://www.slate.com/articles/health_and_science/human_evolu...
"There is a common misconception that malnutrition greatly reduces the amount of milk a mother produces. Although malnutrition may affect the quality of milk, studies show that the amount of breastmilk produced depends mainly on how often and how effectively the baby sucks on the breast. If a mother temporarily produces less milk than the infant needs, the infant responds by suckling more vigorously, more frequently, or longer at each feeding. This stimulates greater milk production."
http://rehydrate.org/breastfeed/faq-maternal-nutrition.htm
Do you have a citation saying otherwise?
N=1 anecdote, my daughters mother was very easily able to tell how her diet affected breastfeeding. A lot of it had to do with water intake.
http://www.npr.org/blogs/thesalt/2012/12/27/168144785/an-evo...
"Milk is uncontaminated by parasites, unlike stream water, making it a safer drink."
http://darwinstudents.blogspot.com/2009/02/evolution-of-lact...
Humans convert grass and water into starvation.
> Nature's solution was …
What made sense for an individual human is for everyone to be lactose-intolerant except them. This is what gives the best results for the individual in the near-starvation scenario.
Your suggestion sounds very much like “group selection”, an idea with many high-profile detractors (http://en.wikipedia.org/wiki/Group_selection ). The only argument I read against it was in “The Selfish Gene”, but according to the Wikipedia page, I could have picked from other high-profile scientists. Something looking like group selection may work in some cases (“The Selfish Gene” itself describes John Maynard Smith's counter-proposal of Evolutionary Stable Strategies), but offering such an explanation as obvious amounts to ignoring a lot of debunking by many bright people.