Perhaps they are not. As I mention nearby, successful breeding can be measured on a continuous scale. We don't really know the success rate, other than it being

Perhaps they are not. As I mention nearby, successful breeding can be measured on a continuous scale. We don't really know the success rate, other than it being above zero because humans have some of their genes. Maybe 9999 cross-group cave-people boinked for every one cave-baby produced. (Bamm-Bamm?)