Flagella and Cilia

Flagella and cilia are by definition structures of unicellular organisms; therefore they are only likely to appear in the case of simple microbes, and perhaps in macroscopic variants with hypothetical supercells. Flagella are most useful in a fluid medium, occurring with waterborne organisms.

The structure of the flagellum

Cilia may function effectively in fluid and on relatively dry surfaces.

The structure of cilia

The exhibited structure is an important indication of the environment to which the organism is adapted. Aliens with these limbs immediately indicate biological simplicity, likely having consciousness no more advanced than stimulus response.


The mere presence of legs does not in itself indicate a wealth of information about the creature’s ancestral history. Rather we must attend to their shape, muscularity, quantity, and type of digits, all of which hold these cues.

Terrestrial insects have segmented limbs with multiple joints, a structure which would be burdened by excess body mass lacking special musculoskeletal reinforcement. Thus these limbs are most likely to emerge on small or particularly light creatures. Limb orientation causes the body to rest lower to the surface, empowering vibrational sensory organs to better detect tremors caused by the motion of other organisms. Therefore, segmented limbs are likely to manifest alongside antennae or analogous sensory organs.

The variety and simplicity of insectoid limbs is better suited to less massive creatures

Most terrestrial creatures have limbs with two primary joints, the first and uppermost allowing rotation, and the second and lowermost allowing hinged, ninety-degree motion. These limbs are better suited to supporting organisms with higher body mass which dwell on the planetary surface.

The range of motion in human legs is similar to most other mammals

However, these limbs also prove functional in aquatic environments; amphibians and some waterborne mammals such as otters maintain this musculoskeletal structure. Webbed digits are perhaps the best indicator that terrestrial limbs have adapted for extensive use in aquatic environments.

Despite their terrestrial ancestry, the otter’s limbs adapted to an aquatic environment

On earth, bipedal stature enables fine motor control and manipulation with the forelimbs. However, there is no reason to believe that two legs are normative for species with grasping forelimbs, nor is there reason to believe that two legs necessarily imply the use of grasping forelimbs; this is merely an ancestral relic of quadrupedal chordates which happened to be the progenitors of most extant vertebrates. Grasping limbs may exist alongside tripodal, quadrupedal, or even centipedal structure. Similarly, bipedal posture may occur in organisms which possess no additional limbs or limbs which are not capable of digital manipulation, as in the case of dinosaurs and avians.

Avians are necessarily bipedal, as their forelimbs are specialized for flight

However, having an excessive number of limbs becomes cost-prohibitive as the creature becomes more massive; a pair or two of muscular legs may function as effectively as and far more efficiently than four pairs of spider-like appendages. Therefore there must be evolutionary significance for the given number of limbs if it comes at high metabolic cost—multiple pairs of legs, for instance, may enable the creature to sprint at a rapid pace for an extended period of time.

The horse’s powerful legs enable it reach speeds of up to 55 mph (88 kph)

Multiple legs may even coemerge with prohibitively large body mass on planets with fragile crust, as the creature’s weight could be distributed across many feet to reduce the force exerted on one any location and consequently preserve the structural integrity of the surface. 


Fins do not necessarily suggest aquatic habitation or ancestry; they also serve the function of thermal regulation, as evidenced by the dimetrodon. Their orientation on the body is an appropriate cue: the conjunction of pectoral fins and dorsal fins indicate adaptation to aquatic environments.

The pectoral fin of an aquatic species is a purely locomotive appendage

If only the dorsal fin is present, it is likely used purely for thermoregulation, and we may safely assume that the creature lives on the planetary surface.

The dimetrodon’s dorsal fin played a crucial rule in thermal regulation


Similar to fins, tails do not necessarily imply an aquatic nature. Many terrestrial creatures have or once had tails which assisted in balancing the body mass, protecting the hindquarters from biting insects, and in some reptiles, defense from predation.

If the tail has developed specialized fins, and if it supplants locomotive legs, it is likely aquatic in nature.

The shark’s tail fin is its primary means of locomotion

If the animal exhibits precise control over its tail, it is probably for self-balancing.

The cat’s tail improves balance on narrow surfaces such as tree limbs

If the tail simply swings as a pendulum, it is likely a repellant to insects.

The horse’s tail primarily acts as a deterrent to biting insects

If the tail sports a heavy growth or keratinized spines, it is almost certainly purposed for defense.

Ankylosaurian dinosaurs used their heavy, club-like tails to deter predators

Alternatively, if the tail easily detaches without medical complication, as with some lizards, it may prevent capture.

Some lizards practice autotomy, sacrificing their tails to escape predation

If the function can be gleaned from appearance, the ancestry of the organism may be accurately deduced, with particularly interesting implications for defensive adaptations in prey species; see “Psychological Implications of Ecology”. This structure is theoretically viable and functional within most clades of organisms; the question as to its basic necessity and efficacy relative to other analogous structures remains to be answered.


Although tentacles are typically characteristic of aquatic species, we cannot by their mere presence assume aquatic habitation or ancestry. Although it is rare, some terrestrial species on earth have tactile tentacles—the star-nosed mole is a notable such example. Perhaps the best indication we have as to their function and origin is the degree and precision of motor control.

Sensory tentacles, like those of the star-nosed mole, perform only simple movements typical of the finger’s range of motion, probing the environment to relay highly detailed sensory information to the brain. This can reflect aquatic or terrestrial origin.

The star-nosed mole’s tentacles are highly sensitive tactile sensors

Locomotive tentacles undulate, thrusting away the fluid medium in which the creature is suspended and withdrawing for another thrust with minimal water resistance. It is theoretically possible for locomotive tentacles to emerge in terrestrial environments, but the dragging motion they would enable is less efficient and practical than the sprinting motion enabled by legs. If the creature evolves to secrete a viscous fluid through which it transits, similar to the snail’s mucus, this may improve the viability of terrestrial tentacles albeit at increased metabolic cost. Terrestrial tentacles are more likely to emerge in creatures with low body mass.

The tentacles of the jellyfish are highly specialized for locomotion and predation

Grasping tentacles must exercise the finest motor control, having a muscular structure which allows for fluid, coordinated, and constrictive motion. The octopus is perhaps the best known example of this structure, sufficiently coordinated that it can grasp, lift, and twist objects in its environment. Tentacles such as these may emerge in aquatic or terrestrial environments, but we would expect terrestrial grasping tentacles to exhibit a greater degree of control and coordination, as the medium of air presents less resistance than water.

The octopus is smart and dexterous enough to open jars with twist-off lids

The flexibility required of tentacles makes them unlikely to occur with chitinous exoskeletons or calcium carbonate shells, although it is possible for the body of the organism to be protected by such a structure while the appendages are covered only with the sensitive dermis.


Wings may perform the function of thermoregulation in some organisms, but this structure is sufficiently complex and expensive that it is unlikely to emerge to fulfill only this role. Insectoid wings are simpler and more efficient, but there are many extant examples of insects shedding or reducing their wings and flight muscles to focus their metabolic energy into reproduction. Thus we may assume that creatures with wings are capable of flight, or else their ancestors were once capable of flight and the wings remain as vestigial organs.

To distinguish between the two, we may study their size and motion. Wingspans of at least the height of the organism, and often many times longer, indicate an aerial species. Smaller wings are likely vestigial or thermoregulatory structures.

Most flighted birds have wingspans several times longer than their height

Further, if the rapid motion of the wings creates thrust, we can be certain that the creature is an aerial species. If, however, the wings simply vibrate, they are likely used to cool the body.

Some flightless beetles retain vestigial wings for thermal regulation

Theoretically wings may occur with mammalian, scaly reptilian, avian, and insectoid skin. Typically, the lighter structure of avian and insectoid skin is preferred. Wings are not necessary for a species to become airborne, however; see “Life in Exotic Environments” and "Jet Propulsion".

Muscular Foot

The muscular foot is a particularly useful locomotive tool when the organism is the subject of heavy predation and must be able to conceal its vulnerable body in a tough, armored hide.

The scallop’s muscular foot grants mobility without compromising its protective shell

However, its relative inefficiency for locomotion in aquatic and terrestrial environments makes it unlikely to occur in complex, civilized lifeforms. It suffers from the dragging motion of tentacles but lacks the power afforded by the structure and number of tentacles in other organisms. Due to its relative simplicity, however, it may be useful in high gravity environments, particularly if it is self-lubricating. If it is indeed self-lubricating, this is a clue to the creature’s terrestrial ancestry; otherwise we may safely assume that the organism inhabits an aquatic environment. The muscular foot is likely to occur alongside leathery mammalian hide, reptilian scutes, or a calcium carbonate shell.

Jet Propulsion

Jet propulsion is principally useful when the creature exists in a fluid medium dense enough that the concentration and expulsion of said fluid produces thrust. In the overwhelming majority of cases, this occurs in aquatic species which can create their own jet stream from the surrounding water.

Salps propel themselves forward by channeling a jet stream through a hollow body chamber

However, this may not always be the case; air with sufficient density may enable a creature to remain airborne via the same mechanism. The appropriate conditions for this to occur are most likely on super-Earths with particularly thick atmospheres, environments where we may expect carbon-based life to exist outside of the traditional habitable zone. See “Planetary Conditions” and “Life in Exotic Environments” for more detail. Aquatic creatures making use of jet propulsion may have coela as their sole locomotive organs, although this mechanism may also exist alongside tentacles as in the nautilus.

The nautilus may steer its jet propulsion with primitive tentacles

Terrestrial creatures making use of jet propulsion may have specialized fins to assist in turning as well as to maintain buoyancy and balance in the air.

View the full text here: An Analysis of Alien Prospects.pdf

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Comment by Zaikan on November 11, 2017 at 12:58pm

True. Flagella and cilia are covered as a sort of due diligence in this broad-based examination. Usually when we entertain the idea of alien life, we are less interested in microbes than we are in multicellular creatures or civilized peoples. So it may also be that there are analogues of flagella and cilia which operate in multicellular organisms. Flagella, for instance, are functionally similar to tails. Cilia, alternatively, usually cease to perform a locomotive role, instead functioning in filtration in the respiratory tract or as ostial structures in filter feeding organisms such as bivalves.

However, sustained flight depends on several variables such as gravity and air density which in extraterrestrial environments will not necessarily correspond to the values present on earth. Lower values of surface gravity reduce weight and make it easier to achieve lift. Higher values of air density enable greater buoyancy with less work. The issue of the efficiency of scale remains, but variation in these environmental conditions grants structural viability in ranges of mass which would not be physically possible here.

Comment by Pretty Angelic Pleidian Babe on November 11, 2017 at 11:35am
Many of these evolutional adaptations are dependant on the scale or size of the living creature.

Flagella,cilia and wings do not scale up in size efficiently.


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