From Scientific American
When the starting gun fires at the Olympic track in Rio de Janeiro, there is little doubt who will be in the lead. In the Men’s 1,500 Meters Asbel Kiprop will be up front. In the women’s 5,000 meters Almaz Ayana will run away, and she may also take the 10,000 Meters. In the marathon Helah Kiprop will push the women whereas Eliud Kipchoge will be the one to watch among the men. In the Men’s 800 Meters, David Rudisha will likely hold his title and maybe break his own world record.
In other words most of these races will be dominated by runners from, or with roots in, east Africa—namely Kenya and Ethiopia, with a few Eritreans and maybe a Ugandan also standing out. Mo Farah, currently at the top of the ranking for 10,000 meters, was born in Somalia and raised in the U.K., and now trains in the U.S. Bernard Lagat, who just won the U.S. 5,000-meter Olympic qualifier (at age 41) is Kenyan-American.
East African runners have dominated for the two decades since Kenyans started winning in the mid-1990s, followed by Ethiopians shortly thereafter. This has lead to great soul searching on the part of former distance powers like the U.S. and U.K. Yet reasons for that dominance remain hotly debated, and science has had little definitive to say about it.
The reigning theory in the West is that runners from east Africa have some evolutionary advantage over runners from other backgrounds.