Well, it wasn't easy, but I found it. I had actually given up hope, but then I was thumbing through my recently-purchased guide to the ABC network's movies of the week back in the '70s, and there it was. Your movie is
The People, a TV movie from 1972. All along I had been thinking that it was based on a very minor science fiction novel that I used to see in the bookstore WAY back in the day, and it turns out I was right (although I had actually been half-convinced that the book in question was called something along the lines of
The Kin of Atu.)