Earlier quoted context omitted.
If it's so powerful, why did it not take over the rest of the world, or at least the rest of Eurasia? You'd think just a few people having kids with foreign populations would make the gene conquer the whole planet? They had cows in other places as well, right? One guess might be that winters in Europe were harsher than other population centres (China, India, Middle East), and periodic famines meant that some people w…
> If it's so powerful, why did it not take over the rest of the world. Time. For going from 1/10 000 000 to 25% of the population in 300 generations (7500 years), you need to have a gene that give you 5% more offsprings at each generation.
And climate. Milk was an advantage against other food sources available at the time in northern Europe, but this wasn't necessarily true elsewhere.