Finally a topic where my knowledge comes in useful.
> 1. What about rennin?
Only ruminant mammals (cows, deer, etc) produce rennet.
> 2. I used to think about 60% of the world population, including nearly all black African people and nearly all “Caucasoid” people (dark or white),could drink milk. Is it true that 70% can’t?
The spread is concentrated in west europe and west africa:
Lactase tolerance isn't super important to the digestion of lactose. Fermented dairy products such as cheese already have most of the lactose fermented away. It's also possible to have favorable gut bacteria who will do something similar.
> 3. Is that idea about the Mongls vs the Chinese commonly accepted, or controversial, or even crackpot?
It's half true. The mongols didn't consume fresh milk, they instead drank a fermented product called airag. This food holds the distinction of being the only alcoholic beverage that isn't made from a plant -- unlike cow milk, mare's milk is high enough in sugars to ferment into alcohol. In any case they wouldn't need lactase persistence because they were consuming fermented milk. They also ate a pretty substantial amount of horse meat.
You are right that having a renewable food supply would give you an *enormous* advantage in warfare. This is probably why they were able to conquer so much of the world. Another factor is the very high protein intake in their diet -- mongolian soldiers would have been significantly taller, stronger and otherwise more well-nourished than their chinese counterparts.
Posted March 6th