If one may read the book, "Man of a Thousand Faces" By Joseph Campbell- they'll read it to notice a great book based on the same speculations.
The evidence is staggering, though it's still very debatable.
Personally, I think there was periods of time of which all religions (speaking as a whole) had.
Specifically, the idea of religion- it came from somewhere therefore in Neolithic times when religion first broke out they were all based on the same things.
But, then there were only few religions therefore the thought of them all being rooted together is quite plausible.
After-wards, of course the segregation of continental separation between certain peoples caused an eruption of purely original religions.
This is caused perhaps a few roots, that sprouted into many, various religions seen today; instead of having just one root.
What I'm saying is:
Roman Catholicism (600-100 B.C.E) + Hinduism (1200 B.C.E-c2000 C.E) = Indo-European Christianity evolving into what we all know today as typical Catholicism.
Today's Catholicism (Indo-European) + Duidry (European) + Shamanism (Native American) = something similar to new age Christian Paganism.
It's all quite interesting stuff really.