Razib said:
Why didn't they spread north as agriculture spread? And why didn't they spread east with agriculture?

Maybe they arose after the spread of agriculture North from the fertile crescent?

Many Arabs drink milk in the form of Kefir, and according to the site:

The beneficial yeast and friendly bacteria in the kefir culture consume most of the lactose (or milk sugar).

So Arabs evolved a cultural solution to the problem, while Europeans evolved a genetic solution.

cultural solution

Ba-dum bump