In pre-history there were a few sub-species of humans that aren’t around any more. Over the years there have been several theories as to why this should be…
Here’s a rather garbled version of the latest theory. Basically all the human sub-species were keen on inter-breeding, but the non-modern-humans had antigens which seemed to provoke immune responses far stronger that those cause by modern humans, and HDFN did for all the other sub-species.
I suppose it is as good a theory as any…