This is a legitimate question I do not think anyone has ever answered fully. General answers like altitude, hills, plyometrics, etc. are part of the equation but Mexicans, Americans, Kenyans, Somalians, New Zealanders, Irish, Canadiens, JApanese, etc, are ALL putting in hard work.
If you have an idiotic response to volunteer such as drug use contributing to these performances, please, refrain.
But my sincere question; what do people think enables the Ethiopians like Bekele, Gebreselassie, Sihine, Geneti, Abiyote, Mezgebu, Tola, Abera, Jifar, Admassu, etc. to end up so far ahead of EVERY other country?
These people are human beings with beating hearts, 2 legs, etc. What are they doing to perform like this?