I just saw an interesting documentary on why the Kenyans are such good runners (https://www.youtube.com/watch?v=PTQVHZU4WzE). It is a bit long but I feel very informative.
If you do not have time to watch it, here is my summary: The main goal was to find out why the Kenyans are so much better that the rest of the world, and the researchers did this through comparing many aspects of the Kenyans against those of a similar group of Danish subjects. This included activity as children, diet, born/living at altitude as well as physiological attributes such as ratio of slow to fast twitch fibers, leg length, height, etc. They studied both the elite and untrained from the two countries. Nothing seemed to explain why the Kenyans had such an advantage, until...
SPOILER ALERT: The only attribute that had a strong correlation to running fast was volume, and therefore weight, of the lower limbs. If your calves and feet are smaller, you will be much more efficient, and therefore faster, than someone with exactly the same aerobic capacity but with heavier legs. As it turns out, the Kenyans and Ethiopians have a high concentration of this body type, but they do not have a monopoly. So, theoretically, anyone from any nationality could achieve the same results given the same "genetics". You cannot train your calves thinner, you have to be born with it otherwise the best you can be is a very fast donkey but never a racehorse (to paraphrase a quote from the end of the documentary).
Of course even if someone has the right physiology, he also needs the right desire and the right atmosphere to promote the quest for superiority. And the Kenyans have a lot of that.
One of the best images of the documentary was the Danish runner who moistened his heart rate strap by rubbing it in the snow and putting it on his chest.