I learned something new today. While watching the video, which describes a little about the union
We know that humans have a common female ancestor, "Eve," from a couple of hundred thousand years ago. We know this because everyone comes from a common mitochondrial ancestor, and women only transfer this gene. Everyone has the mitochondria DNA of their mother, and no one has it from their father. This transfer of genetic material suggests that those with Neanderthal DNA were probably the combination of a Male Neanderthal and a Female human.
The thing that I learned is that mules are not necessarily sterile. A mule is a cross between a male donkey and a female horse.
"Most documented cases of mules/hinnies being fertile have been in the female mule."
There is a belief that humans have progressed faster than other apes because women tend to practice hypergamy. They are attracted to superior males or at least their equals. They are the gatekeepers for reproduction, so Neanderthals were not as primitive as many think. Ask any woman if they might consider mating with any other type of ape. We need not ask men because there is enough evidence that some men have unusual tastes.
It is interesting to think that the Neanderthals may have been the superior of the two species or were so similar in appearance that sexual exchange was typical when they met. Still, due to genetics, only the female progeny of female humans were able to carry on.