Earlier today, a discussion was started about what the oldest religion is. Interestingly, a few people said Buddhism was the oldest, but Buddhism came out of Hinduism. Hinduism was also a common answer, one some folks were very sure of, but Hinduism is only truly the oldest if you stop counting religions that fell out of practice. For example, the Egyptian pyramid texts are said to be the oldest religious texts in the world, so older than the Vedas, and yet we know that there were more religions that predated the pyramid texts.
Unfortunately, we'll likely never know what the actual oldest religion humans practiced was. All we can really do is speculate about what our most distant ancestors got up to, how their spirituality might have looked, perhaps it was shamanistic, animistic, but the truth is that information has been lost to time.