if this guy is who he claims......SPOILERS
Then a good portion of the planet's human population are his grandchildren! Perhaps most, since he is the only primordial human that's immortal!
shareNot necessarily. This is a bible story. So the world is only 6000 years old. Cain was married to his sister Awân and they had one son together. And five generation later they where all wipe out when God sent his flood upon the earth.
How the world became populated with 7,5 billion people in so short time, the bible it is not so clear on. Maybe that was the work of Jack/Cain. But according to the bible it is the three sons of Noah that is suppose to populate the earth again.
Going by this movie, he should be able to survive a flood.
That said, he mentioned not knowing whether or not he had parents, so I'm assuming the Bible wasn't an exact backstory for him as Adam and Eve were clearly named as Cain's parents.