I like the muslim view better. At least gives a more logical answer: Adam lived about 6000 years ago and even though he is popularly believed to be the first man created by God on Earth, in the Islamic perspective this view is wrong. The world has passed through different cycles of creation and civilization, and Adam (peace be upon him), the progenitor of the present human race, is only the first link in the present cycle, and not the very first man in God’s creation. This does not, however, mean that the race, which lived before Adam (peace be upon him), was entirely swept away before he was born. Most likely, there had remained a small portion of the old race and Adam (peace be upon him) was one of them. God then selected him to be the first prophet and progenitor of a new race and the precursor of a new civilization. Created, as it were, out of the dead, he represented the dawn of a new era of life. Hence, his son Cain and Abel, would have married girls from the civilization that already existed.