Two things I've picked up from reading:
Altitude Training and Athletic Performance (Wilber, pg. 171):
"... the Kenyans' relatively good running biomechanics and physiological economy may have been affected by a unique viscoelastic component in the Achilles tendon and triceps surae muscle. This is a novel hypothesis that remains to be evaluated objectively."
Better Training for Distance Runners (Martin and Coe, pg. 26):
"Ankle flexibility seems best displayed at major international track meets by African runners, particularly those who ran barefoot as children. Their style shows the knee of the supporting leg well in front of the ankle, giving the foot an greater range of motion throughout takeoff."
Whether they truely have genetically greater "viscoelasticity" in the ankle/lower leg, or if it's due to the lifetime barefoot activity, who knows.