Wednesday, August 24, 2016

Arrival and "Starfish Aliens"

Arrival, the upcoming adaptation of Ted Chiang's wonderful science fiction short story, Story of Your Life, will be hitting theaters in November. I'm hoping audiences will get to see plausible aliens in this film, because realistic aliens are about as rare as realistic spacecraft in science fiction. Though I wrote an article explaining the quasi-plausible nature of some aspects of the xenomorphs from the Alien series, I can't honestly say the xenomorphs -- exactly as presented in the films -- are possible as real organisms. However, I do think the Heptapods of Story of your Life and Arrival are entirely believable as products of evolution on another world.

First I should reiterate why we shouldn't expect to see real aliens that are giant, albino human bodybuilders or "blue space babes" (sorry Liara fans). Consider this simple thought experiment: imagine that the earliest chordates, the ancestors of the vertebrates, were wiped out in the Cambrian by a natural disaster. If this had happened, there would be no humans, no mammals, birds, fish... no vertebrates would exist. Instead, the terrestrial world might be dominated by arthropods (it kind of is anyway, but still) and terrestrial mollusks, perhaps even some strange forms of terrestrial cephalopods. Instead of schools of fish, we might have schools of fish-like slugs. Real "fish slugs" actually exist today, they're just not as dominant or as diverse as modern fish. Our hypothetical alternate Earth would be surprisingly alien, all because of one evolutionary lineage being snuffed out in its early years!

Phylliroe, a pelagic, fish-like nudibranch slug. Image from Deep Sea News; Photo (c) Fabien Michenet /

So if one small change in the evolutionary history of life on our planet could have produced a surprisingly alien world, why should we expect aliens to look anything like a Klingon, Asari, or Engineer? Even if microbial life were seeded by some humanoid precursor, as was the case in Star Trek and Prometheus, the argument against humanoid aliens still holds. There's no way to "engineer" out random mutations and extinctions from billions of years of evolution.  Aliens are going to be weird, and the aliens of Arrival are pretty weird... but are they believably weird?

The Heptapods. Painting by Rachel Koning.

The illustration above is based on the description provided in Ted Chiang's short story. At a glance, one can see that he took the term "starfish alien" almost literally. The aliens are called heptapods because of their seven-way (heptaradial) symmetry and seven limbs that serve as both arms and legs. Their bodies consist of a barrel-shaped axial pod equipped with seven eyes arranged in a ring near the dorsal surface and surrounding a respiratory spiracle that doubles as the vocal organ. They have a ventral orifice lined by bony plates -- jaws presumably -- and no obvious anus, though the they could have a U-shaped gut with the anus opening near the throat. It could be argued that radial symmetry isn't the best design for an active, motile, terrestrial creature, but that doesn't mean the design is unrealistic. Evolution isn't a clever engineer that produces optimum designs, it's a mix of random and nonrandom processes often leading to the modification of preexisting structures away from one function and towards another, a process known as exaptation. This process often leads to the evolution of body plans or traits that are less than ideal for the function they perform. So when critiquing the design of an alien from a movie, one should be careful when pointing out perceived "design flaws", because if the alien is a product of evolution, it wasn't actually designed in the literal sense.

Radial symmetry is generally an adaptation for a pelagic or sessile lifestyle. A body plan with limbs and sense organs positioned radially has obvious benefits for an organism that cannot move under its own power. Imagine being buried up to your waist with something you need to reach placed directly behind you! For motile creatures, bilateral symmetry is more common. A streamlined body is better for actively moving through water or crawling along the seabed. Gravity provides an up and down, leading to the evolution of dorsal, ventral and lateral surfaces. A mouth at one end and anus at the other allows for a longer, more efficient gut. Sense organs tend to cluster around the mouth, and since nerve conduction is not instantaneous, enlarged ganglia, or a brain, tend evolve near the sense organs and mouth. This trend in some animal lineages is called cephalization. So why would the heptapods, which are motile, highly intelligent, presumably terrestrial animal analogs, evolve radial symmetry? We could say they evolved from sessile ancestors that evolved to be motile later. To make things more complex, we could say they are like some echinoderms, such as brittle stars, in that they evolved from bilateral ancestors which in turn evolved into sessile suspension feeders, which then reverted back to a motile existence while retaining their radial symmetry!

It's not difficult to imagine the ancestral heptapods filling niches analogous to those of some cephalopds, octopodes in particular, living as intelligent, opportunistic, benthic predators. Chiang suggests that the heptapods have an endoskeleton of some kind, with structures similar to vertebrae in their limbs, so it's not hard to imagine a terrestrial clade evolving from marine or aquatic heptapods. If they were like octopodes, their intelligence and tool use could, under the right conditions, evolve into something comparable to human intelligence and technology.

I'll refrain from discussing their language for the sake of avoiding excessive spoilers.

A heptapod hand from Arrival

Much the way the Interstellar gave us our first realistic wormholes and black holes, I'm hoping Arrival will show us aliens that feel as though they really evolved on distant planet. Perhaps it will inspire future film and video game writers to be a bit more imaginative with their alien designs.

We'll see on November 11th!

Thursday, January 7, 2016

How Alien is the Alien?


The titular creature of the Alien series is one of the greatest, if not the greatest, movie aliens, but how realistic is it? To put it another way, what changes would a hard science fiction writer have to make if they were to “reboot” the Aliens universe? The design at least follows a useful rule of thumb when it comes to creating plausible aliens: If a biologically literate person can’t place it within a known phylum, it’s probably sufficiently weird to be considered alien. The reason this is a useful rule is because an alien would be more distantly related to a vertebrate than a vertebrate is to a bacterium! Ignoring some galactic panspermia scenario, a real alien wouldn’t share any biological ancestry with life on Earth.

A lack of shared ancestry doesn’t mean aliens won’t have some recognizable features, or characters, as biologists call them. There are certain characters that have evolved independently many times here on Earth: multicellularity, wings, limbs, eyes, and mandibles to name a few. These are characters that Jack Cohen, zoologist, alien designer and author of Evolving the Alien, refers to as "universals". The laws of physics are the same everywhere in the universe, and while mutations are random, natural selection is not. This is the principle that underlies convergent and parallel evolution. A classic example of this are cephalopod and vertebrate eyes: The the same basic camera design evolved independently in both, but they differ in 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 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!


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 composition. Moreover, the sort of incisors the xenomorphs possess have evolved more than once:

Archosargus probatocephalus
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

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.

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) 

Regarding the chin issue, Douglass Rovinsky, a man who understands mammalian carnivore anatomy very well, offered a solution:

"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 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."

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.  

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 remedied somewhat 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. Tetrapods have an unusually small number of limbs compared to other complex animal phyla. This is probably an "accident" of evolution: If the fish we evolved from had developed, for example, paired anal fins, terrestrial vertebrates might have been hexapedal! Bipedal locomotion and digitigrade feet are fairly plausible, as both have evolved independently in various tetrapod lineages.   

Chemical Composition and Physiology

The most unusual aspect of the creature is, of course, its “acid blood”. It would be more accurate to say that it uses acid as a solvent the same way Earth life uses water. There has been some speculation in the astrobiological community that life not as we know it might use non-water solvents such as sulfuric acid, ammonia, and 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 silicone polymers with carbon side groups 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.
We have a creature that burns holes in floors (and flesh) when injured, but this makes a near-impossibility even worse: how would this thing parasitize a human host, swap genes, and where would it get something to eat? The xenomorph is closest to some parsitoid 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 nonsense for the most part. Real parasites adapt to their host’s immune system and physiology over millions of years; this is why so many depend on specific hosts to complete their life cycles.

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 designsjust 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.  


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 normal food. 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. Moreover, the “resin” that comprises their hive could be explained as an autotrophic symbiont that the xenomorphs use as a food source. 


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 might 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”. 

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… 


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 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.

Hard SF Reboot

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.

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, the first thing I would do is 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 beg 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 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.

Final Thoughts

Hopefully you’ve found this thoughtful analysis of a man in a rubber suit coated in KY jelly entertaining! My ultimate goal was to show that sometimes creature designs like the xenomorph aren't all that alien when compared to real-world organisms. Many of the strangest aspects of its design and anatomy could be produced by evolution. Ironically, its more familiar features, such as its humanoid teeth, chin, and body are difficult to explain using evolutionary theory, not acid blood, metal teeth, and parasitic larvae. We should expect alien life to be very strange!

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 

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