Frankenstein & The Bible (Allegory)
Although the crew is not responsible for the existence of the alien, they help to facilitate its birth by letting Kane back onto the Nostromo without a routine quarantine. The drive for scientific advancement is actually programmed into Science Officer Ash, who is motivated to return the alien monster to Earth. Thus, Ash (and by extension the crew) becomes a kind of Frankenstein, bringing to life a monster over which he has no control, and which is driven by violent motivations. This narrative has many parallels with Mary Shelley's classic novel Frankenstein. In Frankenstein, an ambitious scientist (Frankenstein) creates a grotesque creature, which eventually turns against him. Thus, we can see the story of Alien as allegorical; it is precedented by other works that explore themes of monstrous creatures turning against their keepers. Of course, any direct comparison with Shelley's novel eventually breaks down, but the parallel is a helpful way to understand the monsterous aspect of the alien as a consequence of human curiosity.
When Ash calls the alien "Kane's son," he is issuing a double entendre, alluding to the Biblical story of Cain and Abel. Cain kills Abel out of jealousy and is punished. By calling the alien the son of Kane, Ash is implicating man in the birth of monster. Indeed, the monster is born from within a human body, Kane's. This is an allusion to Judeo-Christian theology, that while evil is foreign to man as he was originally designed, through some fault of man, evil is born from within him. "Sin" or hubris leads to death both in Christian theology and in Alien. Both "sins" are initiated by a search for knowledge.
Ash the Android (Symbol)
When Ash is revealed to have been an android all along, not a human as previously believed, he becomes objectified in the eyes of the viewer, and becomes a kind of symbol. When Ripley realizes that he has been programmed—like "Mother"—to retrieve the alien and bring it back to Earth, she sees Ash as an extension of and symbol for corporate interest and objective logics that value abstract "progress" over human interest. In the moment that Ash is revealed to be a robot, he becomes a stand-in for the corporation employing the crew, which has no qualms with letting them die in order to bring the alien back to Earth. An android is itself a symbol and stand-in for a human, but with all of the humane qualities missing. Ash is able to walk and talk like a human, but he has emotions or desires of his own, and no investment in any kind of ethical code. Indeed, he looks down on his human counterparts for what he calls their "delusions of morality." Therefore, Ash becomes a symbol for progress and science, insofar as progress and science come at the expense of humanity and human interest. While the rest of the crew desperately wants to survive and to protect one another, Ash does not have to worry about these things, because he does not have a conscience, and is instead a constructed robot. Thus he becomes a symbol of objective thought without conscience, and its attendant amorality.
MU-TH-UR/Mother (Symbol)
"Mother" is a profound and multi-layered symbol. It represents human ingenuity, technology, and familial care all at once. Mother is the main computer system on the ship, and initially wakes the crew from hypersleep at the start of the film, initiating the central conflict. Like Ash—an android programmed by the corporation—Mother can be seen as a symbol for the corporation's influence on technology and its favoring of progress over humanity. Mother leads the crew astray in their journey and is programmed not to help them conquer the alien. Mother's main function is information exchange, and as such is a symbol of the limitations of technology, and (similar to the symbol of the android) its inability to prioritize ethical or human interest.
Additionally, with a nickname like "Mother," it is impossible not to interpret the central technological system as a symbol for motherhood and maternal energy. When they hear the signal in the beginning, someone makes the joke that "Mother is calling" as though someone's literal mom is calling them on the ship. The system is nurturing and authoritative, offering answers and instructions in a calm feminine voice. The crew members put their trust in Mother (however misguidedly) in a familial way. Thus, Mother can be seen as a symbol both for technology and maternity.
The "Unknown" (Motif)
An integral motivic feature of any thriller/horror story is the unknown. When we do not know something, it challenges our sense of comfort, and makes the story more suspenseful. Alien uses the terror of the unknown throughout in its depiction of the alien and the havoc that it initiates. Not only is the nature and origin of the alien unknown—so much of what is terrifying about the alien is that the crew members do not understand what it actually is—but also, once it escapes into the ship, the crew members have no idea where it is lurking. Victims of the alien are almost always approached silently, and have no awareness of its presence until it is too late.
This motif of the unknown also brings to mind the theme of mortality in general. Death is the ultimate unknown, the ultimate darkness, and in any horror film, it is the greatest threat. The unknown is terrifying precisely because it holds the promise of violent death.
Sex & Reproduction (Symbol)
Sexual symbolism and the signifiers of reproduction are present throughout the film, perhaps most obviously when Kane is attacked by the facehugger and later gives birth to its offspring, the alien. The facehugger penetrates and inseminates Kane, which becomes both symbolic of human reproduction (penetration and impregnation) and literally, a reproductive process all its own. Ridley Scott was explicitly interested in using sexual imagery in the production of Alien, which he claimed was meant to represent the idea of fearing one's offspring, and the horrific side of reproduction. Thus, the reproductive imagery is meant to symbolize its sometimes horrifying effects. In interviews, Scott described the alien's lifespan as accelerated, lasting only a few days. At the time of the alien's death in the film, it is essentially an adolescent or a young adult, the "nightmare offspring," a literalized representation of the fear that one's child will grow into something horrible.
Additionally, the entrance to the alien spaceship on the mysterious planetoid can be interpreted as a vaginal opening, and its womb-like interior is filled with thousands of eggs. The alien spaceship is, therefore, a reproductive symbol of a womb, the basis of life. Furthermore, the main computer unit on the Nostromo is called "Mother," which, as we said, can be seen as a symbol for maternal care. Mother is programmed to take care of the alien, symbolizing the archetypal mother protecting her offspring.
Sexual images are present elsewhere, as when Ash tries to kill Ripley by shoving a magazine (what seems to be a pornographic magazine) in her throat to suffocate her. Rolled up in a cylinder, the magazine becomes an explicitly phallic object, and Ash's forcing it down her throat comes across as an act of sexual violence, a forced and fatal fellatio.
