Hypothetically God could have created anything or nothing at all, as a perfect being surely doesn't need anything. But Christians say that God chose to create a world with the capability of falling, which makes the world less than perfect in some respect. And if the world were not entirely perfect, then it cannot have been created by a perfect God. Second, there is the scientific or historical question. Fundamentalists believe this because it says so in the Bible, but there is no reason to think that the first few chapters of Genesis are anything but myth.
Science actually discounts the idea of there being two parents of all humanity.
A locus is the place on a gene that codes for a specific trait such as eye color. Alleles are the genetic code that says, for example, whether one's eyes are brown, blue, green, or whatever. Not only that, but the alleles are different from each other by over one hundred substitutions. Thus, the gene pool of the human population today is far too diverse to be the result of the inbreeding of a single pair of people a few thousand years ago. Since we are not descended from one man, Adam, as the Bible says, this undermines the whole basis of the Genesis creation story, original paradise, the fall of man, and original sin.