The most important thing to understand is that extraterrestrials are aliens, organisms that share no ancestry with life on Earth, so they will be radically different. We have little reason to expect aliens to resemble reptiles, birds, insects, or any other familiar groups. The xenomorph, while vaguely humanoid, certainly doesn't look like it belongs to any known animal clades. It has metal teeth, it uses acid as a solvent instead of water, and is an "interesting combination of elements" according to Ash, suggesting an alien biochemistry. This isn't a humanoid dog or giant insect. A lack of shared ancestry doesn’t mean aliens won’t have some recognizable traits, or characters, as biologists call them. There are certain characters that have evolved independently many times: multicellularity, wings, limbs, eyes, and mandibles to name a few. We can refer to these characters as convergent. These are in contrast to characters that are a result of unique circumstances or concatenations; we can call such characters contingent. Convergent characters are possible because unlike other factors that drive evolution, such as genetic drift and mutation, natural selection is a nonrandom sorting process that can create the false appearance of design. Classic examples of this include cephalopod and vertebrate eyes: Similar camera "designs" evolved independently in both, though they differ in fine detail.
|left: eye of a vertebrate. right: that of an octopus. 1: Retina 2: Nerve fibers 3: Optic nerve 4: Blind spot. Illustration by Caerbannog, based on the work of Jerry Crimson.|
What we probably won’t see are aliens with contingent characters such as feathers or mammary glands; these are products of evolutionary concatenations that are unlikely to occur more than once. Consider, for example, that feathers and fur are both made of the protein keratin, and both are likely derived from scales. An analogous alien integument would be comprised of an entirely different set of amino acids and may be derived from integument that is itself quite different structurally from scales. The same applies to all-too-common "blue alien boobs". The closest analog to mammalian milk in a non-mammal lineage is the crop milk of some birds. We should expect alien "milk" to be at least as weird as esophageal and crop secretions.
|Sheepshead fish. Photo credit: Texas Parks and Wildlife Department.|
However, teeth like those those of fish and their land-living tetrapod descendants probably evolved from enamel-coated scales and armor within jaws derived from modified gill struts. This is likely another example of a contingent character: How likely is it that a completely different evolutionary lineage would evolve the same sort of jaws and evolve teeth derived from very hard scales? This is one area in which the xenomorph's frequent enemy, the Predator, has the it beat in terms of realism. The queen has what appear to be siliceous teeth (teeth composed of silica), and the “warriors” have metallic teeth. Both are plausible. Many organisms, such as diatoms and radiolarians, have silicon dioxide skeletons, and many sponges contain siliceous spicules for structural support and defense. The chiton, a mollusk, has a radula equipped with magnetite-reinforced “teeth” (Gordon & Joester, 2011). Some scaly-foot gastropods possesses shells and scales containing iron sulfide minerals (Pickrell). Analogously, the warrior aliens could be using iron minerals to harden their teeth and exoskeleton.
|Scaly-Foot Gastropod. Illustration by Rachel Koning|
|Image from Wikipedia. Illustration by Zina Deretsky.|
Surprisingly, one of the most implausible characteristics of the xenomorph’s anatomy is its chin. Aside from a vaguely similar structure in elephants and saber-toothed cats, the chin is found only in modern humans, and is probably the ontogenetic byproduct of reduced facial size (Holten, 2015). This, in combination with the suspiciously mammalian teeth, is probably yet another contingent character.
|Stop putting this on alien faces! (Illustration by Mikael Häggström)|
|This concept piece by Sekeris, which is apparently from AVP Requiem, |
is an excellent example of how the xenomorph's design
could be revised to have a more plausible body plan
while still retaining the Giger aesthetic.
|Radiolarians, real organisms that form skeletons composed of silica.|
One way for science fiction writers to address this impossibility might be to acknowledge that it doesn’t make sense. Evolution isn’t a perfect engineer; it produces all sorts of suboptimum designs—just look at the tetrapod retina or the laryngeal nerve! Humans could have the misfortune of being the right shape to trigger a facehugger attack. The parasitoid could latch on and implant an egg while being completely unaware that it’s injecting its offspring into something inedible and likely toxic; the nutrients the chestburster requires would need to be provided by a sort of yolk sack. Normally, after the larva matures, it might eat the internal organs of its prey as it grows inside its victim’s body cavity, but in a human, it rapidly begins to starve, forcing it to immediately chew its way out and escape. At best, its human host might provide a source of warmth. This could explain why we never see these creatures eat anyone. In Aliens, we see many bodies cocooned on the walls, but none of them seem to have been consumed. This could also explain why premature removal of the facehugger results in death: If the alien’s egg, embryonic sack, or whatever it is, ruptured, corrosive fluid would spill into the host’s thoracic cavity. This is still quite a stretch. Another thing to consider is that even though people can have large tumors in their body cavities without realizing something is wrong, a large, foreign mass would probably trigger an immune response; Kane would have become seriously ill, to say nothing of his perforated trachea. The implied horizontal gene transfer between an organism with sulfuric acid blood and a human would never work for reasons that should be obvious. Horizontal gene transfer does occur in nature—it’s not purely the work of Monsanto—but genes code for proteins which have a certain range of temperature and pH that they need to remain within to function. Moreover, genes aren’t Lego blocks that can be mix-n-matched to give an unrelated organism digitigrade legs and a tendency to walk on four limbs.
I think it was the humanoid appearance of the original Alien that inspired the "gene-stealing" ability, but thanks to advances in special effects, the xenomorphs don’t have to look like a guy in a suit. For example, the alien queen, which was designed by James Cameron, has the most convincingly alien form with its six limbs, weirdly jointed legs, and a mouth full of sharp, almost saurian teeth. Since the species is eusocial, filmmakers and game developers can introduce new forms as morphologically distinct castes, not hybrids. After all, do we really want things like ostrich aliens? This has been the approach taken by some video game developers. Aliens: Colonial Marines had several castes: Some spit acid and others served as walking bombs; both have real-world analogs among the termites and ants.
The creature’s senses are never really explained in the film series. In Alien 3, the creature’s POV is shown with a fish-eye lens effect. Since the image is obviously visual and in normal color, the only explanation that makes sense is that the creature’s faceplate, that shiny part above the teeth, is a large compound eye. There are some real-world arthropods (copepods to be specific) that have a single, central eye located on their heads. With the creature being so large, individual ommatidia probably wouldn’t be visible, so such an explanation wouldn’t require the design to be altered, that is unless we’re talking about the original, which had a skull face underneath a clear carapace. Subsequent designs lack this feature however. One of the tubular structures along the side of the head could be explained as some sort of hearing organ. Not all animals that can hear have outer ears or tympanic membranes, tuataras being an example. Chemosensory (smell and taste) organs could be located inside, or on, the xenomorph’s pharyngeal jaws; this was already hinted at in the third film when it first detected an embryonic queen inside Ripley.
The life cycle of the alien has no obvious analog in the animal kingdom, but it does share some features with animals from various phyla. The full life cycle as shown in the films consists of an egg, facehugger, chestburster, and an adult.
The egg appears to be a multicellular organism in its own right, with radial symmetry and muscular lobes that open during hatching. The egg may be more correctly described as a polyp, not unlike those of cnidarians (jellyfish, hydras etc.).
|A cnidarian polyp|
The next stage would have to be a true egg with its contents protected from what would be the toxic environment of a human body. It would need a nutrient rich yolk to sustain the embryo as well. Most parasitoid wasp larvae devour the organs of their hosts and pupate within the victim’s body. The xenomorph larvae, on the other hand, seem to violently erupt and escape from their hosts. A science fiction writer looking to add some realism could explain this behavior by saying that the larvae would remain inside their natural host species, and that they burst from their human host’s body and abandon it due to it being inedible, if not toxic.
Based on the “dogburster” shown in the third film, the next stage seems to be nymph-like, basically a miniature of the adult. Finally, the adult stage is reached, which is split into a reproductive caste, the queen, and one or more worker castes. Life cycle stage names with a more scientific quality could be as follows: polyp, epitoke, egg, larva, nymph, and adult.
|"Dogburster" from Alien 3|
Given that the creature has sulfuric acid blood and at least partially silicone-based biochemistry, an extreme growth rate of less than a month could be handwaved as being a result of an extremely fast, high-temperature metabolism. This could be suggested by showing droplets of water boiling away as they drip onto the growing creature’s body. At least it would be more believable than a creature that magically grows to full size in about eight hours.
The xenomorph’s backstory is explained by intelligent design. This is unfortunate in my opinion. The idea, apparently, is that the creature is just too strange and deadly to be the product of evolution. I am, of course, referring to the premise of Prometheus and Alien: Covenant, films that dismiss much of what we know about the origin of humans and other species on Earth in favor of an "ancient aliens" scenario.
I had hoped that the "Space Jockey", the name given to the dead alien pilot shown in Alien, would turn out to be the corpse of a strange alien life form related to the xenomorph. If the xenomorphs had evolved on the same planet as the Space Jockey, and were part of the same alien clade, I would expect them to have similar anatomical characteristics and body plans due to shared ancestry. Prior to Prometheus, I imaged the derelict ship in Alien to be a colony ship transporting the flora and fauna of the Space Jockey's home planet, with the xenomorphs being a particularly dangerous predator that had escaped captivity. I also imagined that the ship's similar appearance was because it was a bioship, a living organism engineered to serve as a starship, and I had hoped this was what the name "Engineer" was referring to.
|There are few things we can be confident about when it comes to the appearance of alien organisms, but we can be fairly certain that they won't look this human.|
With all of the aforementioned points considered, the xenomorph would be a fairly believable alien if modified to address some of its impossible characteristics. Even unmodified, it’s considerably more believable than aliens like the Klingons, which are just violent Californians with armored heads, or UFO aliens, which resemble overgrown fetuses. I think this modified xenomorph would work rather well in a hard SF setting, like that of SyFy’s The Expanse, which scores about a 4 on the Mohs scale of science fiction hardness.
|That might as well be Jupiter in the distance.|
A more realistic portrayal of space flight could be incorporated. Artificial gravity would be provided by linear acceleration ("up" would be the ship's direction of travel) and by the use of rotating crew modules. The events of Aliens could take place on a terraforming installation on Mars, which again, would bring the aliens progressively closer to Earth, thus raising the stakes with each movie. The final film in this hypothetical reboot series could take place in Zeta2 Reticuli, which now serves as the location of the xenomorph homeworld, named Acheron in reference to the original films. A group of heroes, perhaps an adult Newt and a group of scientists and military personnel, might make the tough decision to travel for 40+ years to reach Zeta2 Reticuli to discover the ultimate source of the derelict ship and its cargo.
Please share your thoughts in the comment section and let me know which alien species you want me to discuss next!