Originally Posted by
Xrayman
I was actually under the impression that not every "spirit" goes into a human being-Tibetan Buddhists believe that in fact rebirth depends on what you did in a previous "life" whether you earn a place outside of samsara. Humans are but one of many lifeforms that our essence could be rebirthed into.
but i could be wrong
Agreed, although I don't think that's specific to Tibetan Buddhism, as far as I know both Mahayana and Theravada Buddhism, and whatever other variations on Buddhism there are out there, have the same concept.
Based on a "ladder" model, some Buddhists believe in different planes of existence, including, but not limited to, the ones which we observe in our world (Humans, animals, insects... and some may say they observe ghosts, who knows?). Where one might be reincarnated depends on his karma which is obtained through his or her intentions and actions in all previous lives, not just the previous one.
But as Xrayman said, he could be wrong, for that matter, anyone could be wrong.