In other words, you prefer not to answer the question. I understand.
You didnt ask one.. you didnt even imply one..
If the third human on this planet didnt come from the so-called (lets call them)Adam and Eve.. where did "IT" come from?..
There must have been a third human at some time.. thats the way numbers work.. and procreation..
ELSE.. a very large even more simplistic Yarn must be built... to explain it all..
Science fiction requires logic or whats the point..
Reality need not be logical at all.. (nothing to prove)..