Genetic lesson for Molly because she asked
Inbreeding doesn't cause genetic mutations however the effects of genetic mutations appear more often in situations of incest because the offspring are more apt to carry two sets of DNA (alleles-DNA that code for same traits) with the same mutation. Everyone is a carrier of some genetic mutation however you always carry a second allele which is unlikely to carry the same mutation as the first.
As for Adam and Eve (haha) they most certainly could have populated the entire earth because God would have made them perfect without any genetic mutations. Their kids may have developed ones but likely different ones and could therefore inbreed without worry. -- I'm just kidding with you. I'm not one too take religion too seriously but we do all come from one woman and man in Africa which scientists call the genetic "Adam" and "Eve". I believe that the female common ancestor is known through mitochondrial DNA which is only passed on through the mother. Not sure how they figured out the common male ancestor. Also I don't think these two people were alive at the same time. Not sure on that though.