Another theory I've come across a few times in my studies is that while some people are truly "Earth souls" (meaning all previous incarnations - be they mineral, vegetable, animal or human - were on this planet), others originated elsewhere.   

Apparently homo sapiens were so disappointingly slow to develop during the first several millenia of existence that the "higher powers" finally petitioned certain of our "neighbors" to come help out. These "advanced" beings, for karmic reasons of their own, agreed to incarnate in human form here, to teach and assist us in "raising" and developing our consciousness.    One source says that the first two "waves" of these more evolved souls simply gave up and left, discouraged and disgusted by our "crude and vile forms" and behaviors. (hee hee -- sorry, that tickles my funny bone a bit)

But the third "wave" met with more success. And thus began civilization as we know it today.

Hmm. If the theory is correct, I wonder if we (and the rest of the planet) weren't much better off when we were still "crude, vile and unevolved".

;-)   daylia