This was on C4 the other night and was an absolute fascinating examination by Michael Johnson about the possible effects of the (un)natural genetic selection of black Americans by slavery to leave them genetically predisposed to speed. It's potentially highly controversial stuff and it remains difficult to prove as it's hard to separate the clear evidence of an effect on the gene pool of black Americans from the social factors at play in a culture that is highly focused on sporting success.
However, leaving that whole focus of the programme aside, it was still incredible simply as a review of the worst excesses of slavery and its impact on the people who were its victims. Johnson traced his own genealogy to reveal his own slave antecedents and it was occasionally moving stuff.
Check it out on 4seven or 4OD; it's well worth a watch.