I'm aware that homo-sapians could have offspring with homo-neanderthalensis, but could homo-sapians hypothetically have offspring with, say, homo-erectus or even homo-habilis? I know it could never have happened as only h. sapians and h. neanderthalensis ever occupied the same time and territory; But hypothetically, if we could transport any ancestor from the past into modern times, how far back could we go before we could nolonger have offspring with them?