As far back as Enki filling the first mans lungs with with air (latin = spiritus) we can see some concept of the hidden 'vital spark' of being alive.
People used to call mercury 'quicksilver' because of the way it would move like it was 'vital'. We attribute a certain special 'something' to being alive.
I contend that we have to have a word that describes the expression of being alive with a seat of consciousness in words that we all share.
Put a halo of spirituality with it's nebulous parameters form and 'purpose' around it and you have the xian 'soul'.
Nothing special, just a shared concept arising from a need to explain and describe the phenomena of consciousness in language.