|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 characters such as feathers or mammary glands; these things are products of unique evolutionary histories that are unlikely to occur in sequence more than once (Cohen & Stewart, 2002). The insulatory integument of an endothermic alien will be considerably different from feathers, more so than feathers from mammalian fur, which are both composed of keratin. And the closest non-mammalian character to milk glands are the “milk” producing crops of some birds!
|Sheepshead fish. Photo credit: Texas Parks and Wildlife Department.|
The queen has what appear to be siliceous teeth (teeth composed of silica), and the “warriors” have metallic teeth. Both are plausible. Many organisms here on Earth, 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.|
Perhaps surprisingly, one of the most implausible characteristics of the xenomorph’s anatomy is its chin! Aside from a vaguely similar structure in elephants, the chin is found only in modern humans, and is probably the ontogenetic byproduct of reduced facial size (Holten, 2015). It is what Jack Cohen would call a "parochial" character, a unique trait unlikely to be found in another evolutionary lineage.
|Stop putting this on alien faces! (Illustration by Mikael Häggström)|
"The chin thing (remember when I mentioned that... ) could be relatively easily explained, I think...
Many animals have a well-developed mandibular symphysis. The extreme examples that immediately came to me when you hinted at that in the earlier Fb post are the machairodontine cats.
The reason for their "chin" (minus the mental protuberance - which is the part that I think you are referring to) is two-fold:
one, developing a stabilizing flange for the increased maxillary canines;
two (most important for our discussion), the increase in size and homogeneity of the mandibular incisors (and the concomitant reduction in size of the mandibular canine to be almost identical to the incisors). They essentially take the lower canine, reduce it, and replicate into the symphysis.
So, they have relatively large, samey-shaped teeth in the anterior of the mandible.
Now that we have a large, vertically-oriented, flat mandibular symphysis, we need a protuberance. The easiest way to get this would be to make either that caniniform tooth or the entire anterior mandibular set of dentition double-rooted instead of single-rooted. Tooth roots
Aliens fans who are reading this are probably yelling at the screen, saying that they look human because of the horizontal gene transfer that occurs during the creature’s parasitic stage—I address this in the next section.
Chemical Composition and Physiology
|Radiolarians, real organisms that form skeletons composed of silica.|
One way for science fiction writers to address this impossibility might be to simply 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 general shape to trigger an attack by a facehugger acting purely out of instinct. It 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 be eaten; they only have holes in their chests. 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-and-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. 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 might 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|
Having sulfuric acid blood and at least partially silicone-based biochemistry, an extreme growth rate of less than a month could be mildly 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 Prometheus, a film that dismisses everything 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 what it looked like: 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 "phylum", 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, in my opinion, 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 the 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 hypothetically 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!