Hi Curt!
According to the work of Julian Jaynes, pretty much everybody prior to the time of Christ (He draws the line at roughly 2-3000 years ago, though not for religious reasons) hallucinated out of the right side of their brains. I’m guessing your hypothetical caveman baby would be subject to these and other, worse, indignities.
The OP is the basis of “Charley the Australopithecine,” a recurring character in **Tom the Dancing Bug. **
Well, Sherlock Holmes didn’t know that the earth orbits the sun. Hedidnt want that kind of information cluttering up his brain.
Only in Cafe Society. 
Even when you’re being frank and earnest, too!
Doesn’t Jaynes describe this as a cultural phenemona, rather than a genetic/biological one? Quoting one section of a second Wikipedia article, it mentions “[The breakdown of bicameralism] was also evidenced in children who could communicate with the gods, but as their neurology was set by language and society they gradually lost that ability.”
So it’s more a matter of nurture than of nature.
If I may…
You know those caveman characters in the Geico insurance ads? The Cavebaby would grow up just like that.