One guy lost in the wilderness would have a lot of problmes and probably not make it, no matter what others say, imho. And die of disease, of hunger, etc...
If everything went upside down, for everybody, humans would start again, joining in tribes, eventually, but i really think the first generation would fare very badly.
And let us not forget the inherent badness in man. The worse kind of people would allways fund a way to band together and barbarism would be on the menu....
It starts in the "how are you doing" threads. While everyone there thinks ecolis food is gross, I'd happily eat that! Especially more than fast food. I have a stomach like a horse.