Why have African’s evolved to be better runner’s whereas Europeans tend to be better swimmers?

Question: Why have African's evolved to be better runner's whereas Europeans tend to be better swimmers?
- Keywords:
  africa,
  athlete,
  athletics,
  european,
  run,
  swim,
  trait
Asked by fisherjwh to Helen, Jenni, Mark, Martin, Stu on 24 Jun 2011.
- Jenni Tilley answered on 20 Jun 2011:
  
  Bit of a misconception there…
  
  West African runners tend to be very fast and it’s believed to be because they’re all related to one tribe and have one super speedy ancestor.
  
  I’d be careful about making generalisations in sport though – take a look at this article and see what you think:
  http://www.timesonline.co.uk/tol/comment/columnists/guest_contributors/article2189194.ece
- Helen O'Connor answered on 24 Jun 2011:
  
  remember too that thousands and thousands of Africans live no where near water – its such a huge continent. Whereas most europeans are near a body of water. This means we learn to swim when we are young, and then we develop an interest in swimming and more of us might become olympic swimmers!

Question: Why have African's evolved to be better runner's whereas Europeans tend to be better swimmers?