What arguments and writings have been used to defend the Genesis account of the fall as a historical event? Is there evidence apart from the writings in Genesis that man was at one time not fallen?
I'm assuming, you're looking for evidence of a transition from unfallen-ness to fallen-ness, not merely evidence of fallen-ness.
If a satisfactory answer to your question requires, in a Young Earth view, something like an archeological artifact (say, a book that Adam wrote in the few hours/days before he Fell), I think you'll be lacking.
In a philosophical sense, C.S. Lewis in Mere Christianity tried to make a "logical" case for the necessity of evil being a corrupted good, rather than an eternally evil bad. But again, this may be more of a view of the "fall" of the devil moreso than mankind.
If one holds to a literalist-old-earth Creation view, there could be some evidence in extinction rates. Reasons to Believe, an old-earth think tank, says: