Contrary to the case of blue eyes, we can demonstrate multiple origins of the ability to drink milk into adulthood. Adding coolness, some origins appear to track to camel-raising; others to domestication of cattle.
Taken from my Spring 2014 MCB184 class; references appear in the Powerpoint/Keynote in the next image.These ideas build on preceding Blue eyes material
Lecture #1
Lecture #2
References embedded