This movie is definitive in putting the viewers in a situation where we find that we have made it extremely difficult to see things in its simple state and rather complicate it with all our cynicism and rationality. Being modern humans we like to believe that as we have gained more cranial volumes, have become more intelligent but fail miserably in explaining and bridging the differences of a few thousand years. The sense that Shepherd starts to develop in spite of all his efforts that Charlie the caveman would never actually be able to adapt to this world is the realization that the film portrays.
We find that with all our modern methods and abilities Shepherd does starts communicating to Charlie though in a vague way but still the actual situation could never be properly conveyed to Charlie. It's a classic example of generation gap where the newer generation wants to learn but fails to explain itself to older generation and the older generation is just afraid of the change. Moreover the surgical team is so mechanical that they just want to cut him up for analysis. They don't even consider Charlie a human but a relic, a living fossil.
This film has taken much care in addressing the topic as it is quite believable movie for such an impossible plot. It could have turned out as a jerky and plastic movie if it was not treated well on scientific and performance aspects. I don't know how many people have watched this movie but I must say it is pretty engrossing. The portrayal of a confused, depressed and angry caveman in modern world by John lone is very convincing and touching as well. This can rank among some of the best sci-fi movies of its era.