And DC Toronto, while one can debate the contributions of nature (genetics) versus nurture (surroundings of an individual during life), you cannot compellingly make the "nature" part go away from the peer-reviewed research literature.

"International athletes differed in their mtDNA haplogroup distribution relative to the general Kenyan population. They displayed an excess of L0 haplogroups and a dearth of L3* haplogroups." (from Mitochondrial haplogroups associated with elite Kenyan athlete status).

"...the success of these elite athletes... must be the result of a wide combination of advantageous genotypes... evidence for a genetic component in physical performance is mounting, and more than 200 identified genetic variants may contribute to the observed variations in physical fitness" (from Genetic aspects of athletic performance: the African runners phenomenon).