I found myself wondering the other day if ancestor worship could have turned into polytheism, as noble or aristocratic houses worshiped their ancestors (in the style of the Roman ancestor veneration and display in the atria) and elevated them to the status of divinities for the consumption of the common people. Is there any evidence of this having happened?
It is impossible to know for certain, because religion predates writing by thousands and thousands of years. Interpreting religious beliefs in pre-literate societies and cultures is difficult, as there are no religious texts or commentaries to aid our understanding.
I am personally inclined to think that the idea of spirits and gods evolved from the concept of deceased human beings. It makes sense to me that the first "gods" were ancestor spirits. But, the true origin of religion will sadly never be known :-)