The mystery behind the East African runners

BBC Sport - East African runners: what makes them so dominant?: "Put 'East African running' into a search engine and you'll get thousands of results exploring the question of what makes these long distance runners so good.
Everyone is searching for the secret explanation so what does the research evidence point to? There is not one simple answer but here are my thoughts.
In athletics terms many people associate East Africa with Kenyan and Ethiopian distance running. Intriguingly, the majority of these runners come from three mountainous districts beside the Rift Valley: Nandi in Kenya, the most successful district, and Arsi and Shewa in Ethiopia.
Some people say that since these runners and their forebears live and train at altitude, they're bound to be good. 'Altitude natives', through long-term exposure combined with endurance training, have increased red blood cells which is one neat explanation of their excellence in endurance events..."