This is my theory as to how the flood story came about 1 day in the early days of Man - a hunter-gather came across some fossils of fish high in the hills. He took these to his local Shaman (or whatever the highest religious representitive (RR) was at the time). And said to them - 'I found these high in the hills, how can this be when we know that fish swim in the lakes'. And being a bright person, and not missing a chance to make sure everyone was in fear of the gods and in turn him, the local RR made up a wonderful story about god drowning the world to teach naughty followers a lesson and thus explain away how fossils ended up high in the mountains (because it was underwater for a while).