Someone will probably know but I don't: Has there ever been a society or social system in history that did not contain a religious or spiritual element ... some god, some bigger-than-man something-or-other, some codified explanation for the various aspects of life that defy explanation?
Somehow I find it hard to believe there is or was such a gathering, but I just plain don't know.
Human beings have minds with which to think, and minds seek explanations and meanings. So, when the inexplicability of, say, dreams or death, asserts itself, some sort of explanation is in order. That explanation may be very 'scientific' or totally oooooeeeeeooo, but the desire for an explanation and understanding propels the scene.
Maybe human beings are just hard-wired for spiritual adventure.
Or maybe all of this speculation begs the question -- as I think it deserves to be begged -- of what, precisely is 'spiritual.'