I can only recall the broad theme and a few details on this one. The protagonist, who was a scientist of some sort, has fled his own time to return to man's early period. I'm not sure of the exact time period, but I would guess it is very early on in the era of Homo Sapiens.
I cannot remember the exact reason the scientist has come to this era, but it had something do to with the protagonist's own era. I can't remember if there was a nuclear war, or other man-made disaster. I do recall that the protagonist hopes to impede scientific progress, and preserve the simple harmonies of where and when he is now. To that end, I think he has established himself as some sort of wise man in the village (not sure).
The conflict in the story comes when another in the tribe, a man whose name I cannot recall, shows an interest in experimentation and scientific instincts of his own. The protagonist, recognizing the path the other man is on, tries to thwart him. I believe he misleads the man, tries to ridicule him, and even briefly ponders killing him in order to stop the process of discovery.
I remember the resolution to the story fairly clearly. Another tribesman comes running up to the protagonist and excitedly tells him that the other man has discovered something. The protagonist, realizing what has happened, tells the tribesman to tell the other man to grind up the yellow stone and add it to his mixture (something like that). The protagonist realizes that man's nature is to experiment and discover, and that his efforts to stop it are a fool's errand. Even if he were to have stopped the other man's efforts, there would eventually be another and another, etc. like him.
This one is an older story, for certain. I think I read it as a little kid in the early 1980s, and it wasn't a new story then. I read it in an old anthology that was my dad's. If I had to guess, I'd say 1960s at the latest.