The idea of “body-snatchers” is an old one in science fiction. How old, I don’t know, but when Heinlein wrote his novel The Puppet Masters, he apologized for using such an old and hackneyed plot. And he wrote well before Jack Finney wrote his story.
(Heinlein’s book reads like “James Bond meets the Body Snatchers”, and it’s interesting to note that it predates both “The Body Snatchers” and “Casino Royale”, the first Bond book)
Maybe one of these days they’ll do a decent movie version of it.
I agree that the original stories weren’t reactions to McCarthyism – the theme of soul-stealing and dehumanization is powerful enough without reference to the current political situation. But I’ll bet it helped get the first movie version made.
By the way, a lot of 1950s SF was about such alien soul-stealing, but most of that never got associated with political situations. Look at The Trollenberg Terror (aka The Crawling Eye), or any of Nigel Kneale’s BBC serials (all later filmed) about Bernard Quatermass. Every single one of them is about aliens taking over human beings.
But the Jack Arnold film Invasion of the Body Snatchers is the one that I’m sure is at the root of what everyone has in mind. It even inspired humor – a scene in Airplane II (where the hero leaps out of a truck full of pods, in a great throwaway joke) and on Saturday Night Live (the skit “Invasion of the Brain Snatchers”, where Republicans leave pods labeled “Reagan” under people’s beds. “But…you always used to be against holding onto the Panama Canal!” “WE Were Wrong!”)