Thirty-three years ago Ridley Scott's Alien unleashed on an unsuspecting public one of the most virulent, relentless monsters in movie history.

Audiences in 1979 were shocked when, in a now-classic scene, the creature burst onto the scene after tearing, ripping, and biting its way out of John Hurt's chest.

Featuring Sigourney Weaver, Tom Skerritt, and Ian Holm, Alien is a stunning science-fiction-horror parable that brings to life a creature so utterly different from us, so hostile, so unimaginably other that it is simply called the alien. (Its faux-scientific name, xenomorph, is derived from the Latin xeno, meaning strange or different, and morph, or form.)

Scott, 74, returns to the scene of that most murderous crime in his latest big-budget opus, Prometheus, which opens Friday.

Starring Noomi Rapace, Michael Fassbender, and Charlize Theron, Prometheus, a sort of prequel to Alien that reportedly cost more than 10 times the original's $11 million, promises to answer a question fans have posed for decades: Where did those vicious, slimy dragons — who are equipped with not one but two sets of snapping jaws — come from? What demon gave birth to them?

Alien stands with Scott's next film, the 1982 sci-fi masterpiece Blade Runner, as one of the most influential genre films in cinema.

It spawned three sequels, each made by an up-and-coming film master: Aliens (1986) from Avatar auteur James Cameron; 1992's Alien 3 by David Fincher (Fight Club, The Social Network); and Alien: Resurrection (1997), which introduced French director Jean-Pierre Jeunet (Amélie, The City of Lost Children) to an American audience. And it has inspired countless academic studies. (The eminent British film critic David Thomson dedicated an entire book to the Alien quartet.)

What makes the Alien pictures and the alien critter so compelling?

Scott's creation shocked audiences more than any other films, save for 1973's Exorcist, first because it upended genre conventions, chewing up and spitting out the sacred cows established by earlier sci-fi pictures.

A haunted-house story of a kind, Alien is set in the confined space of a battered, stained spaceship that has been invaded by a single alien.

The ship looks more like an 18-wheeler than the magnificent, white spaceships the public was used to from Star Trek and 2001: A Space Odyssey. The comparison is apt: The Nostromo is an industrial hauler transporting a mineral refinery.

Unlike the bright, shiny college grads running the Starship Enterprise who are attired in shiny, color-coded uniforms, the crew of the Nostromo (the name comes from Joseph Conrad's 1904 novel set in South America) is a ragtag collection of misery-warts, misanthropes, and misers who eke out a living hauling rocks. The ship's engineers aren't charming ladies' men like Scottie, but discontented working-class stiffs (Yaphet Kotto and Harry Dean Stanton) dressed in dirty overalls. The captain (Skerritt) isn't a gung-ho world-savior but a nonchalant slacker more inclined to listen to music than lead.

Scott added to the film's suspense by casting unknown character actors — no one knew who would be killed and who might survive. Skerritt, the likeliest hero, dies an unremarkable death. In another inversion, Skerritt's subordinate, an inexperienced woman named Ripley (Weaver) rises to the occasion and slays the beast.

Weaver's hero, who appeared in all four Alien films, became a prototype for a new generation of movie heroines who outfight, outshine, and outlast the men.

Alien destabilizes gender roles in a more profound way: The xenomorph who attacks Ripley's ship gestated inside Hurt's character, a man, after he had an egg implanted in his chest. In this universe men are violated and impregnated and give birth, no less than women.

The biological horror unleashed in Alien owes a great deal to Swiss surrealist painter H.R. Giger, who helped Scott's team design the alien.

Giger has said he tried to envisage a world where the ultimate horror would befall humankind: our extinction at the hands of a superior creature.

Alien and its sequels are evolutionary parables that drive home some ugly truths we've been hearing for a century from the likes of Charles Darwin, Friedrich Nietzsche, and evolutionary biologist Richard Dawkins.

The world Scott and his heirs created is a cold, pitiless, meaningless place where the only purpose of life — including human life — is survival and self-perpetuation. The xenomorph is the ultimate predator and the ultimate survivor. It represents the rapacious movement of life itself without sentiment, morality, love.

The horror we feel when faced with this monster is the horrible realization that life itself, the "river of DNA," as Dawkins calls evolution, continues without concern for human values. It has no other purpose than to move forward, and it'll consume anything in its way.

The ultimate irony of the Alien quartet is betrayed by the subplot that unites them. Each film features characters who are determined to capture, tame, and harness the alien as a biological weapon on behalf of a government or private corporation. In Alien, the android Ash (Holm) is programmed to bring back the creature regardless of the cost in human life; Aliens, about a group of marines sent to battle an alien outbreak, features a corporate stooge (Paul Reiser) who has the same orders. But Ripley thwarts his plans. We may be horrified by the alien, but we also envy it. What Wall Street firm wouldn't love to have brokers with the alien's survival instinct? What corporate head doesn't dream of dispatching his or her duties with such cold efficiency?

In the fourth film, Alien: Resurrection, we arrive at a world where moral values are erased. The only thing that matters for the characters in Jeunet's film is acquiring power over others. Set on a military research station, it's about a group of scientists who undo Ripley's death by cloning her so that they can extract the alien inside her. Their experiments include impregnating human test subjects with the creature, a singularly unhealthy procedure for the hapless civilians conscripted for that purpose. The scientists and their military taskmasters care about only one thing: having the alien's power. They speak about its beauty. Its purity.

Ironically, the only character who has a sense of human decency and compassion is an android (Winona Ryder).

Alien was marketed in 1979 with a clever tagline, "In space, no one can hear you scream."

It's an apt phrase. In this film quartet, the world is a pitiless place where no one would be moved by your screams.