Alien was produced by Gordon Carroll, David Giler, and Walter Hill through their company Brandywine Productions. Before Alien, Brandywine only had two films to its name, Women in Love (1969) and The Student Body (1976). As you can probably guess from the titles, about the only thing these films have in common with Alien is that they're also films.
The company found its calling with Alien, and every movie they have produced since has been a part of the franchise in some way. It even produced the spin-off films such as AVP: Alien vs. Predator and the kinda-sorta-not-really prequel Prometheus.
Gordon Carroll, David Giler, and Walter Hill were instrumental in making Alien the film it would become. Let's look at a couple of their big moves:
(1) Perhaps most importantly, they got 20th Century Fox to distribute the film, saving it from becoming a low-budget Corman film. Not that there's anything wrong with a so-bad-its-good science fiction B movie—but would we be Shmooping it if were Mystery Science Theater 3000 fodder? Let's just say we're glad the money came through.
It makes sense that 20th Century Fox was in on it, as the company has a rich history of science fiction: the Star Wars films, the Planet of the Apes films, the Alien franchise, Predator, Avatar, and The Day the Earth Stood Still. Among others...
(2) Giler and Hill also took the script through several drafts. In "The Beast Within: The Making of Alien" documentary, Alan Ladd Jr. states that the Giler and Hill "rewrote the script from top to bottom." Or did they? As Dan O'Bannon says in the very same documentary, "The only thing I could see [they were] doing was just stirring around the elements." And then there's Carroll, who noted that Giler and Hill changed the "whole character of the film" but not its "spine."
We're probably never going to get to the bottom of this (and really, does it matter?), but it seems fair to say that Giler and Hill left their mark on the script.
(3) Of Giler and Hill's contributions, Ronald Shusett said they provided one of the best aspects of the story (spoiler alert): Ash being a robot and the way he's revealed to be.
(4) During the pre-production phase, Giler and Hill also decided to make the film's lead a woman. That's rare today and it was even rarer then, but you can thank Ripley—and Giler and Hill—for characters like Sarah Conner, Dana Scully, Katniss Everdeen, and Tris Prior.