One of my teachers and friends, Dr. Joel Hoffman has a blog post about whether or not the apostle Paul believed in a historical Adam.
It's interesting - we debate whether or not we think that there really was an Adam (or Abraham, or Moses, or...), but we always assume that our ancestors (who were, depending on your bias, more faithful, more gullible, more ignorant...) had no doubt at all. Joel points out that we might be way off base:
I think that the whole notion of “historical” is a modern one, created by modern science, and that it’s this entirely modern approach that pits history against myth. Paul didn’t believe in an historical Adam or a non-historical Adam. He just believed in Adam. It’s only as modern readers that we divide things — for ourselves — into historical and non-historical.
The entire way that we engage in thinking about truth, facts, history, science, etc. is an inherently modern framework. That doesn't mean it's bad (it is, in fact, the framework in which I most comfortably think!), but assuming that it's the only way to think, and that, therefore, it's the way in which our ancestors thought, is problematic, to say the least.
Assuming that it's the way that God thinks would be, I'll just throw out there, even more so.
If you want to believe that Adam was a historical figure, then go right ahead. But, don't assume that others thought the same way, just because you do.