It's also worth mentioning that if we assume that interbreeding didn't occur somewhat frequently then that means that non-African humans evolved from the same a

It's also worth mentioning that if we assume that interbreeding didn't occur somewhat frequently then that means that non-African humans evolved from the same ancestors that the Neanderthals' and Denisovans evolved from, while most people in Africa evolved from another group or groups. The earliest modern human remains found outside Africa date to about 200,000 years ago, yet almost all people born outside Africa are descendants of people migrating from Africa about 70,000-50,000 years ago. We don't really know what happened to the first waves of emigrants but we do know they didn't leave many traces. The traces we have found don't indicate hostility so the easiest explanation is that they simply lived alongside the Neanderthals and Denisovans and that the Neanderthals and Denisovans were more successful. Since we know there were interbreeding I find it very likely that those were isolated incicents, considering the human sexual drive. :)