the stipulations of a soul are too arbitrary. What creatures get a soul? what creatures don't? Some people say certain animals don't have souls. Does a soul then boil down to the creature's capacity for knowledge and intellectual ability? That is what it sounds like these people would be saying, which would mean that infants wouldn't have souls until a certain point. But then, when does a soul arrive? If it is always there, then it must reside in every creature, even in the most mindless insects, which clearly operate on a pre-programmed script to survive. Humans do the same thing at a different level. And that I think, is the only raisin the concept of a soul or spirit exists. Nothing justifies their existences, except the fuzzy idea that we could live on after our bodies die because we've already conjured up all these gods and cute divine prophecies so why not a ticket into an afterlife?
There is a possibility, but there's also a possibility that some alien race has conceived something like our concept of a soul on a much higher intellectual level that we could never comprehend. Something like Cthulhu-Intellect. Where the gap between us and them like the gap between an insect and us now. entertaining ideas, useless to both the insect and us, though.