That's a tough question, and the answer's not that simple! 

Chris was correct in his statement that all of these natural processes are interconnected and would have to cover a huge area for it to be viable.  Climate plays a large factor (e.g., how much water is available?  Is the altitude restricting oxygen access?), but the basic fact of the matter is that we were not evolved to be solitary OR sedentary.  Humans are unique to the rest of the primate family in our ability to walk upright and benefit from a highly varied diet, and we have historically been able to obtain our food only by collaborating within social groups and following food sources (sometime very large distances!).  We are also capable of storing energy in the form of fat and can survive for impressive amounts of time without food, but we will die within days without a source of water.

Apologies for the vague response!  The answer really depends on the climate, the region, and dietary needs to name a few.  I suppose it also depends on your definition of "sustain," because humans cannot survive in solitude forever.