Prometheus (2012), the long-awaited first installment of prequels to the iconic Alien horror film series, was not originally well-received by some fans of the franchise. However, its Deacon Xenomorph creature turned out to be the key to Alien biology. The Deacon was briefly introduced to the world during the film's intense ending, but despite its lack of screen time, it had a huge impact and confirmed a concept that had already been introduced in the original quadrilogy—the beloved parasitic organism is directly influenced by the genetics of its host.

The original Xenomorph in Ridley Scott's Alien (1979) and those seen in James Cameron's Aliens (1986) were more or less the same, only with slight differences, which have been attributed to either directorial decisions or to the species's physiological development. Such differences have shown up throughout the original series; for example, the Xenomorph's head is alternately depicted as being either smooth or rough; they are also shown to have anywhere between three and six fingers.

Related: Alien’s Original Xenomorph Origin (Before Prometheus)

The concept of the Xenomorph's appearance being influenced by its host was actually first introduced in Alien 3 (1992), in which the creature's host was a dog. Consequently, the Xenomorph, referred to as "the dragon" in the film, displayed some attributes and behavioral characteristics of canines. This concept is what Prometheus builds on, not only confirming it, but also further developing it. The introduction of the Deacon Xenomorph is the first step in this development, which is continued in Alien: Covenant (2017).

Prometheus’ Deacon Is A Predecessor To Alien's Xenomorph

Alien Xenomorph

Prometheus' Deacon has several traits in common with the classic Xenomorph of the Alien series; however, it also has some major differences. Both creatures are endoparasitoids, meaning that their life cycle involves growing within a host organism, which ultimately dies; both have an elongated head, an inner set of pharyngeal jaws, and a lack of visible eyes. The major differences between the Deacon and the original Xenomorph have to do with their embryonic, gestational and maturational stages of development.

Xenomorph hosts are impregnated with an embryo by a facehugger, which is hatched from an egg laid by a queen. The embryo then develops within its host and ultimately emerges as a chestburster after gestation, violently killing the host. The newborn chestburster then gradually matures into an adult Xenomorph. The Deacon's life cycle seems to be quite different: Elizabeth Shaw (Noomi Rapace) is impregnated when she has sex with Charlie Holloway (Logan Marshall-Green), who had been infected with the Engineers' pathogen; some type of gestation takes place inside of Shaw, and she gives birth to the trilobite, a facehugger-like creature; the trilobite impregnates the Engineer, who is later killed when the Deacon bursts fully formed from its abdomen.

The Deacon doesn't share an identical form of life with its successor, but it does provide the foundation for the Xenomorph's evolution, which, as implied by the ending of Alien: Covenant, is genetically manipulated by the android David (Michael Fassbender) to create the so-called perfect organism. In this way Prometheus' Deacon truly is the key to Xenomorph biology.

Next: What Alien’s Space Jockeys Were Like Before Prometheus’ Engineers