1. since the 1970s, much archeological study have suggested, and many scientists have come to agree, that Homo Erectus had the same upper cranial capacity and indeed the same brain as Homo-Sapien - a view very different from that we had come to accept up to that point (see Mr. Tea's comments) -- they had more hair, yes, but was just as "intellegent" as us -- extending the history of man on earth to 4,000,000 years.
2. it is commonly accepted that vegetation was much more abundant prior to the last ice-age (10,000 years ago) - thus it is likely that food was everywhere for the taking - thousands of kinds of fruits within reach or an easy climb, leaves, etc., etc. -- I'm sure many thousands of species of these plants did not survive the ice-age.
3. the same brain as us but zero advancement of technology - no tools, no advanced language system, etc., why? only conceivable reason is there was no need.
4. what we do know for certain is that civilization began about 10,000 years ago - agriculture, division of labor, advanced language systems, centralized power, hierarchy, domination, exploitation, slavery, and technology -- our of necessity, because of dwindling resources.
5. also it is commonly accepted that major diseases such as cancer did not exist prior to agriculture, when our diet changed from consisting of thousands of kinds of plants to a dozen or 2, and the advent of large quantities of cooked animal protein.
6. studies of tribal and other societies of today which have more or less retained the characteristics of the way of life of ancients seem to at the very least partially agree with this view -- the tribes of Indonesia, Africa, etc. again, there are volumes to be said about this, but i don't have time here to even cover my own very, very limited research and knowledge.
7. lastly, and fully realizing that this is the easiest one for people to ridicule, but should be mentioned none the less -- the myths and stories of all peoples and civilizations on earth: China, Egypt, Maya, Aztec, Aboriginal Australia, Persia, Greece, etc., etc, etc, all contain a form of paradise-lost theme. much like the garden in the christian bible.
of course myths of paradise can be attributed to the nostalgic nature of humans and a number of other explanations, but the possibility that these alarmingly similar stories coming from remote parts of the earth, from disparate cultures having no connection to eachother for thousands of years, were actually derived from some kind of memory, or even direct oral tradition originating from our ancesters, should not be dismissed.