Well...I'm assuming we're pretty much in conjecture land here anyways. But it seems that the earliest evidence of writing and art suggest some sort of 'more than just what's visible' or 'more than meets the eye' thinking.
"Around 30-35,000 years ago there was an explosion of symbolism in Paleolithic human culture around the world, primarily represented by cave art. This cave art is usually located in hard to access underground spaces that must have had significant meaning for the artists and those who would have experienced these strange images by torchlight; And strange they are. Whilst many of the images are naturalistic images of humans, mammals and birds, there is also extensive representation of therianthropic beings, that is part human, part animal shape-shifters. There are also many beings that seem to be distorted humans, perhaps better described as
humanoid. These images suggest that the Paleolithic artists were attempting to tell stories and incorporate messages and meaning within the stories, which they deemed important. The fact that many of the beings represented in the cave art are of a
supernatural quality is symptomatic of what we might call folklore."
Shamanic Explorations of Supernatural Realms: Cave Art - The Earliest Folklore
So...from that I might conclude that AS humans started to capture/write/draw their thoughts on cave walls ie started to share complex information via language or 'be-human,' they had thoughts that today we would think of as other worldly....as the quote says, 'of a supernatural quality.' IOW, I'd say theists existed at those earliest times even though it may have been simple beliefs that some things in the natural world had agency and they may have tried to figure ways to 'deal with' that agency...whether to appease, negotiate, laud or fear/love it. And...I just
imagine that among those were some who said...or thought to themselves, "yeah, I don't think that volcano is making decisions whether to destroy us or not."