Doing a deep dive into unsolved "story identification" questions on another forum, I came across two different people seeking this book about 10 years ago - neither got an answer. I have not been able to figure it out and thought I'd see if the brain trust here can give it a shot.
The novel or novella is from the 70s. The cover is an image of an old man's face merging with that of an ape-like alien. The story: A professor (or scientist) finds that he can send his mind/consciousness out (when asleep or meditating (?)) into the body of a juvenile member of an ape-like species on a distant world. He becomes a leader of these aliens and teaches them to use tools and develop a society. The ape-like aliens that make up his tribe are in competition with another, cro-magnon-like tribe. It is essentially an arms race between the two. The twist at the end: While awake on Earth, our protagonist discovers another human male is sending his mind into the body of the leader of the cro-magnon tribe.