Thursday, January 7, 2016

How Alien is the Alien?


The titular creature of the Alien series is one of the greatest movie aliens of all time, but how realistic is it? Obviously the filmmakers and H.R. Giger were not setting out to make a hard science fiction film, but just for fun, let's imagine that they had. What changes would need to be made to make the creature plausible? Discussion of how real aliens may appear is obviously speculative, but we can make educated guesses informed by an understanding of evolution and the history of life on Earth. The laws of physics apply everywhere in the universe, and the fundamental principles of evolution are likely to apply as well. 

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. 


The xenomorph isn’t like Chewbacca or the Gorn in that it’s not a "space mammal" or a "space reptile". Sure, it does have some recognizable features, such as vertebrate-like teeth and tetrapod-like limbs. Convergent evolution might be able explain the teeth, as they are similar in general shape and function but different in compositional detail. Moreover, the sort of tooth morphology the xenomorphs possess have evolved more than once:

Archosargus probatocephalus
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

Of course, I need to discuss the creature’s iconic inner jaws and phallic head. H.R. Giger explained that the elongated cranium houses and powers the piston-like eversible pharynx. This strange design may seem fantastically alien, yet the inner jaws are a fairly fish-like characteristic. Thousands of fish have pharyngeal jaws, many with teeth, but the moray eel is by far the most xenomorph-like. Morays cannot produce negative pressure with their mouths to suck food into their throats, so they have evolved mobile pharyngeal jaws. Unfortunately, the combination of pharyngeal jaws with already exceedingly fish-like mandibles and teeth make for a highly contingent set of traits. Considering the jaw diversity among the animals, a more believable design would probably resemble a combination of the Predator's jaws with the rather hideous eversible pharynx of polychaete worms.

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) 

Judging from the structure of the hands and feet, the xenomorph’s legs appear to be supported by an endoskeleton, while the rest of the body appears largely exoskeletal. One could argue that the creature’s design is far too humanoid to be realistic, but this seems to have been mitigated slightly with later incarnations. The queen, which I assume represents the most mature, or developed, form of the species has six limbs, and the more recent designs of the warrior alien have digitigrade limbs and very long tails. I don't think these changes make the body plan alien enough though, especially for a creature called a xenomorph, a name that literally means "strange form". Tetrapods have an unusually small number of limbs compared to other complex animal phyla. If the fish we evolved from had developed, for example, paired anal fins, terrestrial vertebrates might have been hexapedal! Moreover, the bones of our hands and feet are derived from a particular type of lobed fin from a particular lineage of fish. There is little reason to expect primate-like hands on any alien species.

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.
Chemical Composition and Physiology

The most unusual aspect of the creature is, of course, its acidic blood. It may be more correct to say that it uses acid as a solvent the same way Earth life uses water. There has been some speculation in the exobiological community that "life not as we know it" might use non-water solvents such as sulfuric acid, ammonia, or methane. Some of these have limited observational and experimental support (Stevenson et al. 2015). For example, acidophilic microbes such as Ferroplasma thrive in extremely acidic environments with a pH of 3 or below (Oren, 2010). Ash states in first film that it’s an “interesting combination of elements”, so the xenomorph might use something like silicone polymers as its macromolecules while concentrated sulfuric acid serves as a solvent (Asimov, 1981). Such a creature could survive in temperatures that would boil even the toughest organisms here on Earth. Considering the boiling point of sulfuric acid is greater than that of lead, such a creature might even survive a brief bath in molten lead as the Alien did in the third film. This would also make the xenomorph something of an extremophile compared to its relatives back on its home planet. Its native world would have a much higher mean temperature than Earth, too hot for liquid water, but ideal for seas of sulfuric acid. A silicone-carbon-sulfuric-acid-based xenomorph nicely explains the glassy exoskeleton of the creature, siliceous teeth of the queen, and its extremely acidic bodily fluids. 

Radiolarians, real organisms that form skeletons composed of silica.
If the xenomorph is filled with concentrated acid, how would it parasitize a human host, swap genes, and get something to eat on a human-made spacecraft? The xenomorph's reproductive cycle is closest to some parasitoid wasps in that it lays an egg or embryo inside a victim that later hatches and kills the host, often by eating it from the inside out. However, the sort of universal alien parasites and infectious pathogens that pop up in science fiction are largely absurd. Real parasites adapt to their host’s immune system and physiology over millions of years; this is why so many parasites depend on specific hosts to complete their life cycles.

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 designsjust 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.  


What do these things eat, anyway? The xenomorphs are never clearly shown eating human flesh, and the alien in the first film was able to reach maturity without an obvious food source. Since the Nostromo was a towing vessel transporting a mining station with 20 million tons of mineral ore, the least handwavy explanation would be that the xenomorph is a lithotroph, an organism that uses the oxidation of iron or sulfur for biosynthesis. There are many bacteria and archaea that produce energy this way, and some of them, such as the aforementioned Ferroplasma, produce sulfuric acid as a waste product (Oren, 2010). The xenomorph could be a macrofaunal analog to such microbes. Sulfuric acid is often the byproduct of mining activity as well, so the mining station could have been a nutrient-rich haven for such a creature. However, the problem with this idea is that such a metabolism would be very inefficient. It certainly would not be efficient enough to allow for the insanely fast growth rates seen in the film! More realistically, the “resin” that comprises the xenomorph's hive could be explained as an autotrophic symbiont that the they use as a food source such that a producer-consumer arrangement is in place. The deep sea tube worms that live near hydrothermal vents have loosely analogous arrangement. 


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.


Life Cycle

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 facehugger doesn’t appear to be a juvenile so much as an asexual life phase devoted entirely to parasitizing another organism. In some respects this is like the epitoke of some polychaete worms, a pelagic reproductive worm that buds off from a benthic parent. Polychaete epitokes swim to the surface where they release their eggs and sperm before dying soon after. However, the facehugger deposits an egg inside the body cavity of a living host, a character shared with parasitoid wasps. 

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

The growth rate of the xenomorph is absurd of course. The alien appears to grow from less than a meter to over two meters tall in less than a day! I’ve always been annoyed by how no intermediate growth stage is shown between the chestburster and adult in the first film. The only movie to come close to that was Alien 3, which showed an intermediate form molting during the fan scene. 

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. 


Ancestral xenomorphs on their hot, acidic homeworld. Illustration by Rachel Koning

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.

Hard SF Reboot

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.

So what would a hard SF reboot of the Alien universe be like? What changes would need to be made? Since Alien does not require handwavium technologies such as magical faster-than-light drives to work as a basic story, I would set most of the events in the series within our solar system. The gas giant that LV-426 orbits could be replaced with Jupiter, and Acheron could be swapped out for Ganymede or Callisto. This would make the Nostromo, a towing vessel transporting a mining station, considerably more believable. While asteroid mining within our solar system is something that may happen within the next 100 years, there aren’t many conceivable reasons for why a starship would transport mineral ore 39 light years between two planetary systems! Moreover, a more realistic slower-than-light setting would fit other aspects of the fictional universe. Alien takes place in 2122 and Aliens in 2179, which is a believable time frame for an age of solar system colonization. The timescales described in the movie fit an interplanetary setting as well: The Nostromo would have taken 10 months to return to Earth according to Lambert’s line in Alien, which is a realistic travel time for an advanced rocket traveling from Jupiter back to Earth, but not Zeta2 Reticuli to Sol. If the Narcissus, the shuttle that Ripley used to escape the Nostromo, had drifted “right through the core systems” (replace “core systems” with “inner planets”), and ended up drifting to the edge of the solar system, then having her lost for 57 years works too.

That might as well be Jupiter in the distance.
Changing the setting to our solar system would increase tension by having the aliens practically on our door step. It would also raise the question as to why the derelict ship crashed on one of the Jovian moons: Was the ship on its way to Earth? If so, why? Were they going to invade? 

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! 


Asimov, I. (1981). Life Not as We Know it: The Chemistry of Life. Cosmic Search, 3(9), 5. Retrieved from

Cohen, J., & Stewart, I. (2002). Evolving the Alien. London: Ebury Press.

Gordon, L., & Joester, D. (2011). Nanoscale chemical tomography of buried organic–inorganic interfaces in the chiton tooth. Nature, (469), 194–197. doi:10.1038/nature09686

Holton, H., Bonner, L., Scott, J., Marshall, S. D., Franciscus, & R. G., Southard T. E. The ontogeny      of the chin: An analysis of allometric and biomechanical scaling. Journal of Anatomy, 226(6), 549-59. doi:10.1111/joa.12307

Oren, A. (2010). Acidophiles. Wiley Online Library. doi 10.1002/9780470015902.a0000336.pub2

Pickrell, J. (2003). Armor-Plated Snail Discovered in Deep Sea. National Geographic. Retrieved from 

All rights reserved to 20th Century Fox Film Corporation, Sega of Electronic Arts, and SyFy of NBCUniversal.


  1. The chin anatomy issue 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 form along the mesial-distal* axis of the jaw, however, depending upon the size and shape of the root itself (i.e. are the roots conical or are they flat and splayed?) there could easily be mental tubercles that would cause the mandible inferior to the teeth to protrude beyond the anterior aspect of the teeth.

    *symphysis-mandibular condyle

  2. Great discussion! I always thought Alien would have been much more believable set in our solar system as per 1981's "Outland," with the "derelict ship" on a moon of Jupiter or Saturn. Certainly this would feel more consistent with the otherwise at least superficially "hard" sci-fi aspects of the Alien universe.

    The only good reason I can imagine going outside the solar system for raw materials (given an economical FTL drive) would be if the solar bodies were already "owned" by national or competing corporate interests and/or were set aside as a kind of "nature preserve." (This latter seems unlikely given the generally exploitative sensibility of the Alien universe.)

    Shameless plug for my own take on the prospective Alien life cycle and the nature of the Space Jockey's "biomechanoid" superciv:


    Further discussion in the AVP galaxy forum.

  4. More discussion on Reddit

  5. I love this! Please do more!
    And clearly, the generation ship that crashed on Ganymede wasn't headed for Earth, it was headed for Venus :)

    1. Good point! It's a great place for some sulfuric acid rain : )

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