Tonus -I don't know. Curiosity? A desire for some degree of certainty? These are traits we still carry with us today, and they likely would have been much more useful in our distant past.
The lengthy and rather scientific process you theorized these primitives may have carried out to arrive at a conclusion of God seems much too laborious to just appease curiosity. Imagine a tribe of people, mostly concerned with finding food and staying alive, sitting down and determining 'we MUST invent an explanation for X". It goes against the notion that these people, the males in particular, were all about practicality and efficiency.
Based on what we understand of these primitives, it seems it would have been more logical to attribute anything mysterious to themselves, and thereby gain an upper hand amongst themselves. To have attributed these powers to someone outside of themselves undermines the logical process you described and further, undermined man all the way to now.
The desire for certainty in these people was supposedly seen and carried out in their actions. In their supposed daily fight for survival. We are led to believe today that these primitives MADE their future possible by virtue of their physical strength and force of will.
But if they indeed had faith in their god or gods that they could aid them in their survival, or even utterly dictate their fortune or misfortune, then it wasn't simply an invention to appease curiosity or create certainty.