Friday, January 27, 2017

The Aliens of Halo

The aliens of the Halo universe are, for the most part, a departure from the sort of humanoid, "rubber forehead" aliens we usually see in Hollywood SF. While most of them are tetrapod-like (bilaterally symmetrical and four-limbed) only two are very humanoid, with one being the unfortunate result of a low-budget, live-action series. The aliens of Halo feel a bit closer to what one would find in a science fiction novel and perhaps a bit closer to what might actually be out there. Since they're a step above the typical Hollywood movie alien, I've decided to review their plausibility and offer alternatives where their design falls short in that regard.

Plausibility Grades:

A:  The design is sufficiently alien to be considered a genuinely believable extraterrestrial organism. Such designs are impossible to confuse with any phyla found on Earth and may have a bizarre physiology or chemical composition. The Heptapods of Arrival are one of the few examples of this level of realism in a film.

B:  A plausible design but one that relies on a high degree of convergent evolution to justify its appearance. It must be a plausible example of convergent evolution, one that uses characters we can reasonably expect to evolve more than once under common selective pressures. I would give the Prawns from District 9 this grade.

C:  A somewhat plausible design, but one that suffers from misconceptions about biology, is lacking in scientific rigor, or takes convergent evolution a bit too far. If the mistakes were fixed, the design would be an A or a B. A "bad biology" example would be the xenomorphs from Alien. Many aspects of the creature are believably alien: the creature uses an acid instead of water as a solvent, its life cycle is unique, and it has a silicon-based exoskeleton. However, it also grows far too fast, the acid blood is absurdly strong for an organism, and its ability to "steal" genes despite its alien chemistry is simply impossible. The Predator is example of an otherwise good alien design that's a bit too humanoid to be convincing. If the Predator weren't obviously supported by a human skeleton, it would be an easy B.

D:  These may be plausible organisms, but they're not plausible aliens. They're most often aliens that are humanoid versions of Earth animals, e.g. humanoid cats. Such designs would require an extremely unlikely series of evolutionary coincidences. These aliens would make more sense as products of genetic engineering.

F:  These are either humans with some rubber glued to their head, humans with one small difference, or aliens that make no biological sense at all.

Huragok (Facticius indoles)

The Huragok, or Engineers, are one my favorite Halo alien designs. They're certainly one of the most plausible in that they couldn't be mistaken for any known animal phylum. They have a unique physiology and method of reproduction as well. They're described as biological supercomputers and as artificial constructs lacking true tissues and organs. I'm not quite sure what is meant by this. They must have connective tissues to hold their bodies together, something like a nervous system to process information, and muscles to move their bodies. Confusingly, it's also said they have structures that are virtually indistinguishable from tissues and organ systems. My interpretation is that they are a non-cellular form of life: They're composed entirely of organic nanomachines that are arranged into tissue and organ analogs. This is reinforced by the fact that they do not produce gametes that grow into adults. Instead, they replicate by working with one or more Huragoks to literally build a new individual with its parents' combined memories serving as heritable information.

At first glance, one would think their relatively few tentacles would be woefully inadequate for their purpose as dedicated engineers, but the Halo novels explain that each tentacle can split into thousands of cilia, allowing them to manipulate objects on a microscopic scale. The only potential problem I can see with their design is that they use lighter-than-air gasses to float about, which would make lifting heavy objects impossible—something that seems problematic for a creature that repairs machines. Their Forerunner creators had compact antigravity devices that the Engineers could have used to lift heavy objects, so this isn't really a problem within the context of the Halo universe.

Plausibility Grade: A

San'Shyuum (Perfidia vermis)

The San'Shyuum, or Prophets, were the leaders of the Covenant throughout the original Halo trilogy. They're a physically feeble species with a relatively small population and low fecundity. Their physical weakness is due to them originating on planet with relatively low gravity. This is further compounded by a sedentary lifestyle and a genetic bottleneck that occurred when they fled their homeworld. Judging by their hooked toes and vaguely primatoid body plan, an arboreal ancestry would be a plausible evolutionary background for this species.

There's nothing too particularly alien about the Prophets beyond the fact that their "ears" are located on the back of their heads. This is a bit strange, because they would have difficulty determining the direction of a particular sound. The ideal configuration would be laterally positioned and vertically offset ears. They have red respiratory pigment, implying something iron-based, like hemoglobin. They could be mistaken for a mammal if their weird ears were ignored. However, they do not have an overly humanoid body plan or a man-in-a-suit look, so they're somewhat plausible.

Plausibility Grade: C-

Sangheili (Macto cognatus)

I think the Sangheili are one of the best alien designs in the Halo universe and one of the better video game or movie aliens in general. While their body plan is definitely tetrapod-like, it differs in detail enough to be justified as an example of convergent evolution. Other tetrapod characters, such as jaws that evolved from gill struts, are absent. Jaws have evolved multiple times within Metazoa (animals), and never exactly the same way. For example, though cephalopods (octopuses, squids etc.) have rather parrot-like beaks, they overlap in the reverse orientation, have a different muscle arrangement, are made of chitin rather than keratin, and house a radula instead of a tongue. Sangheili jaws aren't exactly like those of any animal on Earth, and that goes a long way in keeping this species from being "lizard men". Their hands are rather human-like, but this is mitigated to a small degree by the inclusion of a second thumb. 

Their physiology is also believably alien, with purple blood and two hearts. In the animal kingdom, respiratory pigments span the entire rainbow. For example, the purple blood of the Sangheili could be due to having a respiratory pigment similar to hemerythrin, which is iron-based and turns violet-pink when oxygenated. Some animal lineages also have multiple "hearts" (cephalopods have three; oligochaetes have five). Sci-fi writers shouldn't use this as a justification for greater durability however. Animal's such as cephalopods may have more than one heart, but they cannot survive the loss of one any more than a human can survive the loss of a major artery.

Sangheili society appears heavily based on the Samurai, complete with a Bushido-like code of honor. One gets the impression that their entire race is composed of warriors, which raises the question as to what a Sangheili math professor is like, or a plumber. Halo sidesteps this problem to a degree, and even directly addresses it later in the series, by showing that non-warrior roles are largely filled by other species, resulting in a sort of species-based caste system. This problem is explored later in the series following the collapse of the Covenant when the lack of diversity in Sangheili society proves nearly disastrous.

It's implied that the Sangheili are sexually dimorphic and that the females remain on their homeworld to fulfill non-military roles. Most males are said to be monogamous, while the most proficient warriors are polygynous and have an "alpha male" status involving preferential access to mates. Based on this, and following the principles of anisogamy, one could expect the males of the Sangheili to be considerably larger, competitive, and risk-prone, as the stakes for mating are so high. This leaves only half of their population to fill non-military roles; consequently, they would probably need a creche system to raise their young, which seems to be the case based on the source material. Overall, their society isn't too particularly alien, but it does have enough nonhuman elements to feel plausible.

Plausibility Grade: B

Unggoy (Monachus frigus)  

The fan wikis, and perhaps the source material, describe the Unggoy in a somewhat biologically unsound way. Firstly, there seems to be some uncertainty as to whether they are arthropods or vertebrates. They would be neither—these are aliens we're talking about. A vertebrate is a member of the subphylum Vertebrata, which in modern phylogenetics comprises a clade defined by evolutionary relationships. Thus, while the Unggoy may have an endoskeleton, this doesn't make them vertebrates. It's possible for an organism to have an endoskeleton yet lack vertebrae, as is the case with echinoderms such as sea urchins. They wouldn't be arthropods either for the same basic reason. The best way to describe them is to say they are bilaterally symmetrical, tetrapodal, metameric, animal-grade organisms, with both an endoskeleton and an exoskeleton. That's a mouthful, so we could say they belong to the clade Monachata and call them monachates, not vertebrates.

Secondly, it's said in the source material that they "breathe" methane, which is plausible, but it would not be analogous to breathing oxygen. The correct term is "methanotroph". The methane would be a source of carbon and energy that would need to be oxidized into something metabolically useful, such as formaldehyde. This means the Unggoy would breathe methane and oxygen, and methane would be a sort of food for them. Since methane is lost in the atmosphere as a result of photochemical reactions, which is why 90 percent of atmospheric methane on Earth is biotic in origin, the methane of the Unggoy homeworld would probably have a biotic origin as well. This is implied in the source material by their swampy homeworld.

Unggoy blood is a bioluminescent blue. Since their mouths and eyes are not constantly glowing, it can be inferred that this only occurs when they are injured. Glowing blood would probably attract the attention of predators, which is actually what some dinoflagellates (a type of single-celled protist) do: They draw attention to animals that threaten them. I doubt this would translate well at higher trophic levels however, since whatever predator might eat a smaller predator would probably eat the Unggoy too. Instead, the Unggoy's blood could be a way to alert other members of their social group to danger visually rather than vocally. Their blood appears to be blue even while not glowing as well; this could be the result of a copper-based respiratory pigment, like hemocyanin, which reinforces the idea that they actually breathe oxygen along with methane.

The available material suggests the Unggoy reproductive strategy is r-selection, i.e. they produce large clutches of low-cost offspring. This makes them useful cannon fodder but also makes it difficult for their masters to control their population. The r-selection strategy is somewhat at odds with the strong bonds they feel for their offspring. While this may help make their exploitation by the dominant Covenant species more tragic, it doesn't make much sense if their young are abundant and easily replaced. This could be reconciled by having the bond Unggoy parents feel somewhat proportional to amount of energy invested into a particular child. For example, Unggoy parents may not feel too upset at the death of newborns, but may be devastated by the loss of a child who had nearly reached adulthood.

Plausibility Grade: B-

Yanme'e (Turpis rex)

Image from
I love eusocial insects, so I'm always happy when a space opera setting has a species based on them, provided they're designed well of course. The Yanme'e aren't bad; however, their biology could use a little more fleshing out. It's conceivable that a vermiform alien lineage could have undergone arthopodization, the evolution of the suite of characters that comprise the arthropod body plan. That being said, we should not expect an arthropodoid alien to have exactly the same type of body segmentation as arthropods on Earth.

An example of the variety of ways arthropods body segments have combined into tagma (Source:
The Yanme'e soldier shown in the image above this article is the most plausible version I've found. Unlike insects, which have three tagma (combined segments with a shared function), the Yanme'e have five: a head, neck, thorax, pelvis, and abdomen. The thorax, pelvis, and abdomen each have a set of legs. Yanme'e workers and soldiers also have six wings mounted on the thorax, with the anteriormost pair hardened into beetle-like elytra. Some of the designs have three pairs of compound eyes rather than one. The non-insectoid segmentation, the presence of six wings instead of four, and the six compound eyes help communicate that these are aliens, not insects. 343 Industries should take this further and add that they have lungs rather than a tracheal system (they're too big for that) and a closed circulatory system similar to those of annelids (worms). Their green blood could be a result of a respiratory pigment similar to chlorocruorin, which is found in some annelids.

Being eusocial, the Yanme'e have a queen, reproductive males, and various classes of workers. I'm not sure if it's clear that the workers consist of non-reproductive females like ants or males and females like termites. Eusociality occurring in an alien species is fairly plausible, given that it has evolved in wide variety of animals on Earth including hymenopterans (ants, bees, wasps), termites, thrips, aphids, beetles, shrimp, two species of mammal, and even a species of flatworm. Their social organization isn't a simple cut-and-paste version of a bee colony either: The queen is immobile and bloated with eggs much like a termite queen, and the males are wingless and actually care for and transport her. Wingless males that remain near the queen suggest that they might exhibit a high degree of inbreeding like thrips and unlike ants and bees. Another possibility is that the males shed their wings after a nuptial flight.

I'm glad to see that while the Yanme'e soldiers are fearless, more coordinated than human soldiers, and obey orders without question, they're not mindless and appear to function well without a queen. They even appear to have ranks, perhaps based on maturity, that imply some sort of chain of command. Too often the queen of a fictional eusocial species is presented as the "brain" of the colony, when in reality, she's closer to being its gonads. There's even some room for emotion among the soldiers, such as jealously, and there are a few "unmutuals" who refuse to cooperate with the rest of the colony. The workers and soldiers are described as being highly intelligent with an unusually strong grasp of engineering, mathematics, and science. In terms of society and behavior, they're one of the better portrayals of a eusocial alien species I've seen.

The person Bungie hired to write the background material on the Yanme'e homeworld, Palamok, screwed up: For some reason they decided that the likely homeworld of a giant, flying insect analog should be a planet twice the size of Earth with 2.2 times the surface gravity... why? Arthropods are generally small animals for a reason. To make things worse, the Yanme'e fly... and do so with antigravity devices attached to their exoskeleton. If they need antigravity devices to fly in 1g of gravity, how could they have evolved flight on a world with 2.2 times Earth's surface gravity? These two things do not make sense together. 343, if someone from your staff is reading this, there is an easy way to fix it: Make Palamok the name of a double planetary system, one with a larger body that is twice the size of Earth and another closer in size to Mars. Say that the Yanme'e's ancestors evolved on the larger world as small, insect-like organisms, but were somehow transplanted onto the smaller moon where they evolved to be much larger.

I'll ignore the homeworld issue, since it seems to be in contradiction with how they're described elsewhere in the canon, such as how they rely on antigravity devices to fly. It's also something that could be fixed fairly easily. Overall they're pretty good "bug men", but they could stand to be a little more alien in design. A very non-humanoid appearance could have made them creepier, more unnerving, as well as more realistic. 

Plausibility Grade: C

Lekgolo (Ophis congregatio)

The Lekgolo are an excellent example of how to make an alien genuinely alien. Despite the humanoid appearance seen above, the Lekgolo are actually colonial, lithotrophic, vermiform (worm-like) organisms that form highly motile, strong, sapient colonies. Individually, they're unintelligent, but they can link together to form a progressively larger and intelligent neural network, essentially an enormous composite "brain". As a colonial mass of muscular worms, the Lekgolo can take a variety of shapes, ranging from humanoid soldiers to massive spider-like tanks, by attaching themselves to a makeshift skeleton. 

Based on the available background material, the Lekgolo seem to be at least partly lithotrophic, meaning they use an inorganic mineral substrate, such as iron or sulfur, as a source of electrons and energy. This isn't a very efficient means of energy production, and on Earth, this occurs only in Bacteria and Archaea, so the Lekgolo could be mixotrophic, meaning they use a variety of energy sources, not just inorganic substrate. Their mineral diet was substantial enough, however, to allow them to eat technological artifacts left by the Forerunners, which triggered a war with the Covenant early in its history. Their blood is yellowish or orange. This suggests their blood contains high amounts vanabins, which are vanadium-binding metalloproteins. Vanadium is a rare element found in several alloys, so this fits with their artifact-eating habits. We don't really know why some real-world species concentrate large amounts of vanadium in their blood. Oxygen-carrying doesn't seem to be the reason, though it could be in an alien species.

Plausibility Grade: A+

Kig-yar (Perosus latrunculus)

I love these guys, but they're not realistic aliens as presented; there may be ways to fix that however. A few things for the wiki writers first: If they're aliens, they're not birds or reptiles. Furthermore, birds are dinosaurs, so it's probably more accurate to say they're dinosauroid (meaning dinosaur-like) in morphology, which they very much are. Many non-avian dinosaurs possessed feathers and beaks, so discussing the Kig-Yar in terms of being more avian versus reptilian isn't really accurate. The Kig-Yar are feathered, scaled, bipedal, have tridactyl hands, some have beaks, they're oviparous, they brood their young, and they have hollow bones - all characteristics of dinosaurs. Their young even have downy feathers! The only thing that suggests an alien origin is their purple blood. 

There are three Kig-Yar subspecies shown above going left to right: Ruutian, T'vaoan, and Ibie'shan. The Ibie'shan should be considered members of a separate family at a minimum, because their head morphology, which is more crocodilian in appearance, is considerably different from the other two. All three are so dinosaurian that I just cannot view them as aliens. One solution that would be quite interesting in my opinion, and "lore friendly", would be to posit that they are not aliens at all, but are rather a lineage of theropod dinosaurs that were translocated, or uplifted, over 66 million years ago by the Precursors, who were active in the Galaxy for billions of years, seeding planets with life. This is reinforced by the fact their homeworld, despite being a moon orbiting an ice giant, even resembles a Mesozoic Earth and is only 40 light years from our solar system. The only problem is their purple blood, but this could be explained away as being a result of genetic engineering, perhaps to adapt them to the atmosphere of Covenant ships.  

Plausibility Grade: D (if they're true aliens)

Jiralhanae (Servus ferox)

The Jiralhanae, or Brutes, are clearly very mammalian in appearance. I think it's unfortunate that in many works of science fiction, the furry, brutish species is so often modeled after cats, bears, or apes, while saurian species are always scaly. In reality, many dinosaurs were fuzzy, even some of the big carnivores, and some mammal ancestors may have been scaly. In some of the Halo games, shaved portions of the Brutes appear scaly, and I think it would be a good idea for 343 to incorporate this in future designs.

I like the fact that they come from a high-gravity homeworld. There is growing evidence that "super Earths" may be more common than worlds the size of our planet, so at least some alien species should originate on such worlds in works that involve many aliens. Unlike the Yanme'e, the Brutes actually look like they could support their weight on such a planet. 

Their social structure is a fairly unoriginal pack hunter culture. The detail about pheromones communicating emotions is something that could be expanded upon to make them a little more interesting however. Another issue is that I can't find any information about Brute reproduction or sexes, which I think presents another opportunity for the writers. Let's consider what we do know: They possess a pack hunter culture in which "alphas" dominate the pack, and a subordinate can only displace the alpha with a duel to the death. Mature Brutes eventually stop broadcasting their emotional state to their pack members, suggesting some physiological change as they age. Female Brutes seem to be absent. With all this in mind, I would make them protogynous sequential hermaphrodites, meaning they are born female and become males shortly before or after they achieve alpha status. The alpha's pheromones, along with those of the other pack members, could have regulatory function in controlling the sex of pack members. By combining reproductive behavior more commonly observed in fish with a creature that looks mammalian, it would help communicate that these are aliens and not simply beast men. One potential problem with this is the fact that, if I remember correctly, they refer to each other as "brothers" and generally use male pronouns. Since their speech is being translated, this could be because their language is mostly gender neutral while English is not, so a term like "pack sibling" becomes "brother" when translated. 

Plausibility Grade: D

Sharquoi (Drinol)

Virtually nothing about this species has been revealed, so I can only comment on their appearance. While they have a rather humanoid body plan, their head is convincingly alien, with a single, large compound eye reminiscent of those of copepods, and a unique jaw morphology.

Plausibility Grade: B


The Forerunners are the creators of the Halo rings and many other megastructures, including Dyson spheres and shell worlds. They were wiped out by the Flood 100 thousand years ago, and passed down the "Mantle", a role of galactic stewardship, to Humanity. While the Forerunners come in a variety of higher "mutant" forms, in their most basal state, they appear very human, with the only real difference being found in their ear and nose morphology. If they're intended to be aliens, I would give them an F; they're little different from Star Trek rubber forehead aliens. However, within the canon, it's suggested that the Forerunners are genetically related to humans in some way. This strongly implies a common ancestry. It's also said that they were seeded on their homeworld by a much more advanced species, the imaginatively named Precursors, 15 million years before humans evolved. If they're related to humans, the simplest explanation is that they're descended from early hominids, making them more distantly related to humans than orangutans but more closely related than gibbons. This is a semi-plausible way to have hominid "aliens" in a science fiction setting. It makes far more sense to have the humanoid aliens who evolved from Earth hominids, but were translocated somehow, than to say humans descended from aliens. Basically, the writers of the Halo canon avoided the biggest biological error in the film Prometheus. Humans are demonstrably related to other primates here on Earth. We did not come from space. 

Plausibility Grade: N/A (not true aliens)

The Flood (Inferi redivivus)

The Flood is a difficult one for me to grade. On one hand, the species is a convincingly alien superorganism with a complex life cycle reminiscent of some real-world parasites, the life cycles of some parasites such as malaria and liver flukes being examples. Some parasitoid wasps even employ viruses to alter the behavior of their host, effectively turning them into "zombies". On the other hand, the Flood is a sci-fi "omniparasite", one that can infect a plethora of unrelated species. Real parasites co-evolve with their hosts over millions of years and are often limited to one or two them. The fact that it's difficult for real parasites to infect new species that, compared to an alien, are virtually identical in terms of chemical composition, makes it hard to believe any such super parasites could exist somewhere in the Galaxy. The Halo canon includes something of a handwave to explain this: Flood super cells. Supposedly, the Flood have cells that can break down virtually any cell they contact, analyze them, and then mimic them, thus consuming their contents as food while simultaneously acquiring the information stored in the host's nervous system. I guess this would be the biological equivalent of "uploading" a mind by incrementally replacing and replicating the function of each neuron. The Flood would be fairly plausible if it were limited to one or two related species, but the "omniparasite" aspect bumps it down from an A to a B. 

Plausibility Grade: B


They're just humans in makeup - 343 Industries, why would you do this?

Plausibility Grade: F

Primary Halo references for information and images

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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