For historical perspective, 45 years before the release of ALIEN was 1934: Five years before the global conflict known as World War II.
Fun fact: Writers Dan O' Bannon and Ronald Shusett, creators of this movie, never had Science Officer Ash in their original script. The way the alien got onboard was arguably better in the original script, but it didn't make for potential sequels.
Then, a few years later, John Carpenter did the same thing for his movie, THE THING.
You can't call it a rip off when both men were originally friends, sharing a common love for old scifi movies and together created DARK STAR: a movie that openly mocked all of their inspirations and then some. Plus, most fans can watch both and never make the obvious connection.
The fact is, when the two creators of DARK STAR went their separate ways and made the two most powerful Science Fiction Horror movies in history, it makes sense that there would be so many similarities. There's a trove of more history when you fall down the rabbit hole of our review of ALIEN.