When Alien came out many people noted similarities between it and Mario Bava's 1965 Italian film Planet of the Vampires. Particularly familiar is a scene in Bava's movie where astronauts explore the ruins of a crashed spaceship and discover the bones of giant alien beings. At the time Scott denied having seen the movie. But now Prometheus seems to be totally nodding towards Bava - notice how the space suits are the same in both films?
If Alien was influenced by Planet of the Vampires, Prometheus at times feels like a remake. In Planet of the Vampires two human space ships follow a distress beacon and crash land on a planet where they start getting killed off and possessed by strange alien beings. The beings are trying to get off this planet and make it to another one... Earth!
Bava's film isn't great, but it's colorfully evocative and imaginative. It's certainly the real Alien prequel.