I wonder ... if there was one first human couple, would not everybody be a direct descendant of them?
Depends what you mean by "human"! And also what we decide about the recessive/dominance of souls, and whether one can have a "half-soul"....
Okay, OEC hat on again.....
Adam and Eve boh had full souls. Their children had souls. When a soul mates with a non-soul, I'm going to say.....their children would either have souls, or no souls - there's no such thing as half a soul.
Half a bee, philosophically, Must, ipso facto, half not
But half the bee has got to be, vis a vis, its entity. D'you see?
But can a bee be said to be, or not
to be an entire
When half the bee is not a bee - due to some ancient injury?
So do offspring have, or have not, souls, with one parent of each? The flood story would suggest they do not, with chidren only having souls if both parents did (apart, perhaps, from certain special individuals).....Adam's lineage perhaps, due to inbreeding, were "all-souled"....
But post flood, assuming Noah and kin were all "full souls", then everyone AFTER that would be full-souled, as there were no non-souls around to mate with. So pre-flood there were lotsa NON humans (who got drownded), and post-Flood everyone was a proper human, with a shiny soul and everything.