The Last Woman on Earth (1960)

There's a movie called The World, the Flesh and the Devil, in which Harry Belafonte is trapped in mine cave-in, and when he finally gets out, he discovers that the world has ended in his absence. He eventually meets other survivors; first a woman, then a man; and much of the thrust of the movie's script is about how the end of the world changes things (the woman is clearly interested in Belafonte, but since she is white he struggles with the fact that 'people wouldn't approve' of a multi-racial romance, even though there's basically no other people around) and the rivalry the other man feels toward Belafonte over the woman's affections. There's also a 1985 film from New Zealand called The Quiet Earth which has a similar setup: two men and a woman in an empty world, with rivalry between the men over the woman, though many of the other details are different.

I mention these two films because they are both significantly better than The Last Woman on Earth, another cheapie from Roger Corman, which shares the same romantic/sexual triangle. I suspect Belafonte's film may even have been a direct inspiration for this one, as it came out the year after that movie.

In Last Woman on Earth we meet an unethical businessman, his wife and his lawyer. The three of them are combining business with pleasure in Puerto Rico, by way of a scuba dive. While they are under the waves, however, all the oxygen temporarily vanishes from the air, causing everyone but them to suffocate to death. If you're thinking 'I find it hard to believe they are the only people on Puerto Rico who happened to be scuba diving at the time' then you are not alone, but you'll have to accept it as a conceit of the film, I guess.

The film addresses some similar themes to that of Belafonte's (though without the racial angle), but it doesn't do much to make any of the characters likeable, so the squabbles between them really didn't carry much import to me.

Given the existence of similarly-themed, but much better made films, I really can't recommend this one.