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.
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)
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-
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: http://learn.genetics.utah.edu)|
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
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)
Jiralhanae (Servus ferox)
Plausibility Grade: D
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