All the movie versions, including the 1951 rendition, are based on the great 1938 sci-fi story "Who Goes There?" Written by John W. Campbell at age 28, it was his last major piece of fiction. After that, he concentrated solely upon editing Astounding Science Fiction magazine, the key vehicle in launching the Golden Age of Hard Science Fiction. In the summer of 1939 alone, Campbell published the first stories of (among others) Isaac Asimov and Robert Heinlein.
"Who Goes There?" is the story of American scientists holed up for the winter in a research station in Antarctica. They find an ancient spaceship buried under the ice and dig up a frozen body, which (foolishly) they allow to thaw. The alien wakes up and begins to eat people.
Ho-hum, right? But the thing has a peculiar talent: after he eats somebody, he can split into two and change himself into that person, physically and even mentally. Paranoia, carnage, and more paranoia ensue. If the alien(s) eat everybody at the station, they'll then eat the first supply plane pilot in the spring and take over the human race. How can you tell who is man and who is monster?
This helped inspire Heinlein's Puppet Masters and the film "Invasion of the Body Snatchers."
Campbell had written the same idea at least once before, in a light-hearted story called "The Brain Eaters of Mars," but "Who Goes There?" was clearly a climactic effort for Campbell.
The Wikipedia page on Campbell offers a wild biographical theory about the origin of this concept in Campbell's youth:
"His mother, Dorothy (née Strahern) was warm but changeable of character and had an identical twin who visited them often and who disliked young John. John was unable to tell them apart and was frequently coldly rebuffed by the person he took to be his mother. ... As Sam Moskowitz has written about Campbell in his early critical study of science-fiction writers, "From the memories of his childhood he drew the most fearsome agony of the past: the doubts, the fears, the shock, and the frustration of repeatedly discovering that the woman who looked so much like his mother was not who she seemed. Who goes there? Friend or foe?""