It is indeed a very intriguing subject. As you've already said, there are many similarities between religions all around the globe. I don't know whether you're familiar with Mircea Eliade, a Religion Historian, but he's made a point that one of the first manifestations of the sacred is within Shamanism.

Also, some of the theories concerning how the population got to spread around the globe is centered around the ice bands that formed over the oceans during the glaciary.