A lot of these runners come from ethnic groups which live in highlands and mountains: extensive aerobic training in lower-O2 environments, then competing at sta

A lot of these runners come from ethnic groups which live in highlands and mountains: extensive aerobic training in lower-O2 environments, then competing at standard elevations, seems to be the most important advantage: https://www.researchgate.net/publication/225064362_Kenyan_and_Ethiopian_Distance_Runners_What_Makes_Them_So_Good https://www.researchgate.net/publication/225064362_Kenyan_an... There is probably a minor genetic component, microevolution promoting higher hemoglobin/etc than average, similarly to many Tibetans. But childhood conditioning seems to be a more powerful effect.