“The lack of extinctions in Africa (Fig. 1) is especially notable given the long history of humans on this continent. The co-evolution of man with the African megafauna may have resulted in the evolution of effective anti-predator behaviours (Diamond, 1984; Martin, 1984).”[Italics added]
The animal capable of this seafaring--150 miles of open ocean with only the most primitive of boats--would have had to have been very familiar with and physically very well adapted to a marine environment. Not a likely accomplishment for a mammal specialized in exploiting the dry savannah environment.