The generations of man had devolved into corruption, sin and violence. And there were giants.
For context, the characters would have been stone age and early iron age. I think the “fall” of man was what we now call the Neolithic Revolution: one of the most important period’s in early human history. This was the relatively short period in which small clans of hunter-gathering cultivated wheat, tying them to one location. The first stone tools and pottery are found, and the storage and surplus of food allowed the clan to expand into the first permanent human settlements at the end of the last major ice age, approximately 12,500 years ago.
There are many fantastic theories of antediluvian races and civilizations, the most famous being Atlantis.
What we do know: Gobekli Tepe was a major complex in southern Turkey that dates from the pre-pottery Neolithic era, between 9500 and and abandoned in 8000 BCE. It was literally filled in with rock and rubble where it lay until its discovery in 1963. This predates Stonehenge and the first Egyptians by thousands of years. It is assumed that the megalithic structure was the first communal building project, and could have been created when man was still nomadic. The importance of a ritual site led to planting crops and tending herds to support both the builders and pilgrims to the location. In other words, the site was the impetus for settlement, if only for certain times of year.
And there were still megafauna during this period: mastodons, giant sloths. They would exist until the younger Dryas Ice Age.