Creation explained
At first there was light, i.e. a multitude of photon-analogous members of the boson crowd. And then came stuff, i.e. these fermion types. Shortly after Adam and Eve appeared. Subsequent to a few incidental troubles involving a snake and a fruit, Cain and Abel were born, a wee distance from Eden. Finally, from these four people, the whole of mankind was created. Nice, clean fun.
But hey, wait a minute -- how could that be? We have a slight problem here: Adam's and Eve's offspring were boys. Two boys can hardly beget babies. A further (but admittedly lesser) problem is that Cain got rid of Abel, sort of. Anyway, he did, according to the Scriptures. So the tough task of creating mankind fell on merely 3 people -- 2 males and 1 female. Or, to put it in another way, 2 parents and 1 son.
So, whichever way you cut this crap, it's obvious that you can't deal the cards without involving certain tricks that some might consider dirty. Consequently any true literalist believer of the Bible knows the answer to the most important demographic problem of mankind, the very first one. It is, evidently -- incest. And there are some people alive who seem to be ardent readers of Holy Books.
Reference:
The Holy Bible