I’ve read a claim that chimpanzees and early hominids did in fact interbreed during a 9 Million to 6 Million years ago time frame, though of course they were much closer genetically then. Here are two cites (though neither appears to be the longish article I read a few years ago 
).
On another matter:
I’ve a question about the relative contributions of crossover and mutation to genetic diversity. I’d have assumed that crossover would usually be more important over a short period (few thousand years) especially when “irrelevant” mutations are ignored.
If we assume that crossover is more important, the homogeneity of cheetahs is due strictly to the (roughly 10,000 years ago) bottleneck, estimated at six individuals or less, which leads to all cheetahs still being related much closer than 1st cousins.