not necessarily uncommon just the fertility of such mating and their decedents might have been lower than replacement rate. if a such mating pair were only able

not necessarily uncommon just the fertility of such mating and their decedents might have been lower than replacement rate. if a such mating pair were only able to produce 2 or less fertile children on average then their line would eventually die out. Its also possible that it was like mules where the offspring are starile 99.9% of the time but every once in a while one is able to reproduce successfully for some reason.