Why is it that almost every human culture has some notion of a soul? I don't necessarily mean the idea of an immortal soul that persists after death, but some kind of spirit or essence of self which is distinct from the body. There must be some reason for the near ubiquity of such a concept, and I am inclined to suspect an evolutionary survival benefit.
Years ago, when I was at graduate school, a confluence of three experiences got me to thinking about this. The most significant was the death of my grandfather. When my wife and I arrived at the airport and met my sisters and mother, they recounted having gone to view the body earlier that day, and I was struck by how each of them remarked that it wasn't really him. That is, it was clearly his body, but he wasn't there.
Within a few weeks of this, I was driving to pick up my wife after her night shift at work at the hospital, when I spotted three pigeons in the road in front of me, two of them dead. Fortunately, at 5 in the morning, there was virtually no traffic, so I slowed down to give the one live pigeon the opportunity to get out of the way. It seemed reluctant to leave its companions, however, and took a few moments to decide. At last it hopped up onto the curb, and I was able to proceed, my tires missing the fallen fowl by a healthy margin.
Almost the next evening, I recall watching a nature documentary on TV in which a gnu had just delivered a stillborn calf, attracting the attention of some hyenas. The mother gnu valiantly protected her calf through the whole night, while the hyenas waited with evident patience. At last, the mother gave up and left, and the hyenas ate.
The pigeon and the gnu I thought about for some time after that. It was kind of touching how the pigeon didn't want to abandon its friends, and the gnu's predicament was absolutely heartbreaking. Although I was not a parent yet myself, I could certainly identify with her, and could scarcely imagine a circumstance in which I would abandon my own child to hyenas.
And that's kind of when it struck me, reflecting on my grandfather's funeral and the viewing of the body. He wasn't there, in his body. There was no imaginable circumstance in which we would willingly cremate or bury my grandfather, but this was just a lifeless body, not him.
And so I suspect that we (and probably other species) have evolved a tendency to combine a number of sensory factors (things like temperature, movement, breathing, a pulse, and the like) into a kind of holistic sense of the presence or even identity of a person, distinct from the body itself. Something that would allow us to abandon our dead to the hyenas, without seriously compromising our powerful instinct to protect our living kin. We would not abandon our kin just because they had been injured or lost some simple tangible property, like a pulse; we'd need to be able to believe that they no longer inhabited the body, that they were no longer the person we cared about. Hence, something like a soul, a spirit, an intangible essence that we nonetheless feel we sense through all the tangible vital signs.
This doesn't mean we have a soul, of course. (Nor does it necessarily mean we don't.) It's simply an account of why all human cultures would have some kind of related concept, and incidentally a response to the common argument that we must have souls because every human culture believes in them independently.