where did the 'mates' come from

So this is a question that a mate of mine's grandfather used to ask any all door knockers. I don't know the answer though I was given a hypothetical once by a couple of mormons so would be interested to hear your views. As a precursor I am not a religious person nor am I trying to shit stir. Just wondering what your opinions are (for all I know the good book may have the answer)

Here goes...


When Adam and Eve had kids who did the kids then mate with in order to populate the earth??? Given that there was only ever Adam and Eve to start with?