Originally Posted by pompousjohn

I studied these questions as part of my fundamentalist upbringing, the reason is that the birth defects resulting from inbreeding are caused by generations of cumulative genetic defects that require mating with a dissimilar DNA chain in order to suppress. When the human race was only a few generations old, and atmospheric radiation was low enough to allow lifespans close to a millennium, genetic mutation was minimal so this was not a problem.