That's why scientists are confused that genetic testing seems to show only genes from male neanderthals entered humans DNA. If human women and Neanderthal males were making babies, why weren't human males and neanderthal females?Note, however, that Lions and Tigers seem to have diverged ~3.9MYa and Humans and Neandert(h)als only ~500kYa, so mating barriers are surely more social/behavioural than genetic.
The most likely reason is some biological problem. Maybe their children were mules, maybe they died in the womb, maybe they had severe genetic problems. No one really knows.
So I'm going along the lines that they just can't get a live birth due to differences in hormone levels, which really screw with human embryo's in a Neanderthal's womb.
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