Roll of the Dice

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When we encounter an alien, what will it look like? Will it appear as one of the cute baby-like aliens with large dark eyes that often are described when UFO abductions are claimed? Will they be merely humans with warts and other facial accoutrements as so often is seen in science fiction movies? More than likely they will appear as nothing we have seen before. With entirely different environmental conditions, it is highly likely that an entirely different range of species will have evolved. There truly is no better guide to get ideas on what an alien may look like than to look at the organisms that populate the Earth for what possibilities may be out there. Earth’s varied environments invite a variety of organisms that we see even today.


One of the first things that we see when we look at life on Earth is that it often has some form of symmetry. Symmetry refers to how parts of an animal or plant are situated in relation to other parts. Most animals that we are familiar with are bilaterally symmetrical; that is, the one side of the body is essentially a mirror image of the other.

Many animals display radial symmetry such as jellyfish, starfish and sea anemones. Many plants are radially symmetrical. Radial symmetry means that the life form is shaped like a wheel with every side being essentially the same as the other side. Then again there are some creatures on Earth such as the sponges that show no symmetry at all.

Clifford Simak used radially symmetrical aliens in his book They Walked Like Men. Hal Clement, master of alien designs as a creation of their environment, created a jellyfish-like alien with radial symmetry in his science fiction crime drama novel Needle. Iain Banks, in his Culture series, also describes an alien race with radial symmetry, the Morthanveld, which look like spheres covered with hundreds of tiny spines.

The options are there and often you don’t have to look beyond your own backyard. Then again, science fiction authors are not always happy with the status quo and have come up with forms of their own.

Damon Knight, famous more for his editorship, wrote all too few science fiction tales. In his novel Rule Golden, the aliens are 
trilaterally symmetrical. Robert Sawyer’s more recent novel Illegal Alien bested Knight’s symmetrical organization by creating aliens that have quadrilateral symmetry.


What else do organisms on Earth possess that might be found on aliens? We seem to have an innate bias and assume that all sensory organs are found on the head. Just look at most of the aliens depicted in science fiction films. Though most animals on Earth have sensory organs on the head region, it is far from universal. In actual fact sensory organs will appear wherever there is a need.
In the insect world, sensory organ placement takes on some very unexpected turns. The antennae of the male mosquito have thousands of tiny hairs which are sensitive to the sound made by the wing beats of a female mosquito. Katydids and their relatives have tympanic organs which are on their front tibia. This organ, similarly to the mosquito antennae, allows the katydid to detect sound waves.

Some insects have organs of smell in their antennae. Closely related to smell is the sense of taste. Taste organs can be found on the feet of some species of flies and butterflies, allowing them to taste anything that they land on. Imagine tasting your food with your feet. Placement of taste receptors, though, goes even further in catfish. Catfish often live in murky waters, usually at the bottom of lakes and rivers. Their taste organs cover their entire bodies.

We also tend to assume that all living things have the same five senses as we do, sight, hearing, taste, touch and smell. Again we look no further than Earth’s animal kingdom for differences.

Bats and cetaceans can “see” through the interpretation of reflected sound, very much like a radar system called echolocation. This allows them to move through complete darkness without crashing into objects and to also track food sources. The Kdatlyno found in 
Larry Niven’s Known Space series are large, eyeless and noseless, with but a gash for a mouth. Above the gash mouth are goggle-shaped tympanums which allow them to “see” via sonar. Science fiction author Hal Clement in his short story “Uncommon Sense” has come up with an alien that “sees” by its sense of smell.

Magnetoreception is another sense that humans do not possess. It is the ability of an animal to orient itself based on the direction it is facing with relation to the Earth’s magnetic field. It is what allows some birds to navigate the enormous distances that they do every year. For birds, the need for the sense is essential. However, some bacteria appear to orient themselves according to the magnetic field, as do cattle which appear to align themselves along a north-south direction. What advantage it may have for bacteria and cattle is the subject of further research.

Fish have sensory systems that are also very alien to us. Ever wonder how a large school of fish appears to swim as a unit, almost as if they were one organism? It is the lateral line that runs down the centre of their bodies that allows them to detect differences in pressure and movement, thus allowing a school of fish to respond quickly as a unit.

Sharks and rays also possess sensory organs called ampullae of Lorenzini; some primitive bony fish such as sturgeons and reedfish also possess them. The ampullae of Lorenzini allow fish to detect electrical fields in the water. The ampullae are jelly-filled canals that are open to the outside world through a pore in the skin, ending in jelly-filled sacs. Distribution on the body depends on the animal species. By detecting electrical fields, sharks, for example, can pick up the electrical signals of thrashing prey and rays detect prey that are invisible beneath the sand.

In humans, there is a vestigial organ called the vomeronasal organ, found in the nasal region, which is actually an auxiliary olfactory sensory organ. In other mammals it is used to detect the pheromones secreted by others of their species, marking territories and sexual state. In reptiles such as snakes and monitor lizards, it is known as the Jacobson’s organ. The Jacobson’s organ lies on the tips of their forked tongues; in effect they are smelling the air with their tongues. In turtles, the organ is used to smell underwater.

Even plants have sensory organs. For a plant growing in the ground, it is imperative to know the direction of gravity and have the ability to detect it. In mutations where the genes for this ability are changed, a plant cannot grow upright.

There are also some plants that are insectivorous; that is, they feed on insects and other small animals. In order to do so, they can use a passive approach such as we see with the pitcher plants that have funnel-shaped leaves where water can pool and in which an unsuspecting insect can fall and drown. Others such as the Venus fly trap have modified leaves that snap shut when an insect wanders aboard. To do so, the plant requires receptors that are sensitive to vibration in order to trigger the trapping mechanism.

Even where animals have the same senses as we do, sight, hearing, smell, touch and taste, there are differences. For example, most mammals can only see black and white, a but few see colour as we do. Bees and dragonflies on the other hand see the ultraviolet part of the electromagnetic spectrum which is invisible to us. Some birds, too, have been found to be able to see in the ultraviolet range. Others, such as pit vipers, which include rattlesnakes, and the boa group of snakes have an ability to detect infrared, essential in tracking down their warm-blooded prey. Jellyfish have light-sensitive ocelli, which at first may appear to be similar to our eyes. Instead, they are not for sight, they are for light detection only, allowing the animals to orient themselves in the water column.
Depending on the light being emitted by its sun, there may be aliens out there that have organs that are sensitive to the radio or microwave end of the spectrum and others may be able to see X-rays or even gamma rays. Who knows?

There are also differences in the structure of eyes of animals. Many arthropods have what are known as compound eyes which are made up of multiple lenses. Each lens delivers an image of a small part of the surrounding environment. When combined, all of the small images allow good wide-angle coverage which, in turn, allows an extreme ability to detect motion, great for evading either capture or getting squished, but its weakness is in its inability to show detail.

Some arthropods also have what are known as simple eyes (having only one lens) that are normally found in rows or clusters. They simply show shades of variance between dark and light, but very little detail; in some of the arthropods the eyes line the back of their heads and down their backs. Then there are our eyes and those of most higher animals, also simple eyes, which have a high-resolution lens allowing good detail vision both close up and far away.

Humans hear when sound waves are collected in the ear and travel down the ear canal to strike the eardrum. These vibrations are then amplified and transmitted to the brain for interpretation. Depending on the type of animal, this will, in turn, determine the frequency of sounds that can be detected. Cats and dogs, for example, can hear sounds at higher frequencies than humans but also hear at lower levels like we do. Bats, on the other hand, can hear sound at higher frequencies but have a narrower range than humans, and cannot hear at lower frequencies. Cetaceans such as whales and dolphins as well as elephants communicate at very low frequencies below the range of human hearing.

Smell is another sense that is not uniform across the animal kingdom either. In humans, the sense of smell is a rather weak sense, whereas in many other animals, it is a crucial sense for survival. Next time when you take your dog for a walk, observe how his head is usually on the ground taking in the odours. Within those odours are messages such as territorial determination and sexual receptiveness. It is a truly rich world that humans are all but blind to. Even taste is not the same across the animal kingdom. Cats, for example, do not taste sweetness. Other animals detect bitterness and still others do not.


Vertebrates or backboned animals on Earth possess an internal skeleton which supports their bodies. In most cases, the backbone is made up of calcium in the form of bone, and in other cases, such as the sharks and their close relatives, the rays, it is made up of cartilage. Turtles are another interesting group of animals in that they possess a backbone or vertebrae but they also have an external shell, which in most species is hard, that also protects the sensitive and fragile organs of the chest cavity. An alien race may possess a dual system like the turtle. For example, if fragile or important organs are on the appendages of an alien, it is not inconceivable that they would evolve some form of external sheeting, much like a knight’s armour on those appendages.

An external skeleton such as we see on insects and other arthropods or certain molluscs can provide the structural support necessary for an organism. An external skeleton has limits, though, in the size an animal can attain. For example, if insects were any larger, they would be crushed by the weight of their exoskeleton. However, we do have large arthropods such as the Atlantic lobster that can reach sizes of up to twenty kilograms because the crushing weight of an exoskeleton is offset by the buoyancy of the water. A planet or moon with a low gravity could have the same effect and conceivably be home to a large animal with an external skeleton, like a land lobster.

Earth also has animals without any supporting structure at all. In the depths of our oceans, octopi are not only able to function well in a liquid medium, but have actually attained at least rudimentary intelligence. Jellyfish, too, have no support structure but function well in their liquid world; unlike the octopus, they have no true intelligence—after all, they have no organized brain.

Plants and some fungi have a physical structure as well. They maintain their shape due to water pressure pushing against cell walls, also called turgor. Alastair Reynolds in his Revelation Space series has developed an invertebrate animal called a hamadryad that is able to exist on land without any internal or external skeleton. Lack of an internal skeleton is instead supplanted by rigidity maintained by the pressure of circulatory fluids, not unlike plants.


The physical support that organisms have will dictate what form of locomotion can be undertaken. Many higher animals move via a system of moving their limbs. In Alastair Reynolds’ Revelation Space series, he describes scuttlers, which have the added benefit of being able to not only remove their limbs but also replace them with those of other species. In the high gravity world of Mesklin, Hal Clement has the natives of the planet move about in a multilegged centipede-like fashion. In Iain Banks’ Culture series, he describes the mammalian Chelgrians which move about not on two or four legs, as many animals on Earth generally do, but on three legs. The Homomda of the same series are also tripedal, as are the Idirans. Larry Niven in his Known Space series has a tripod alien called a Puppeteer.

There are also microbes that flow along like a contained liquid. An amoeba moves by what is known as pseudopodia which involve the flow of cytoplasm as extensions of the organism. Other protozoans move by the use of cilia which are like tiny hairs lined up on a cell’s surface. This allows the organism to achieve greater power and speed. Some microbes have even developed a form of locomotion via wheels in the form of a flagellum that rotates in three dimensions similarly to the rotation of a wheel.

What about wheels on a macroscopic level? Why don’t we see them on Earth? Maybe on some alien world they do exist. Actually there is a variation that does exist on Earth. It is limited, however, on Earth to the microscopic world. One anatomist performed a thought experiment and constructed a disk for a large animal which would work. So why don’t we have large animals with wheels? The environment provides a clue. It may be efficient on a level piece of land, but the lumps and bumps of the Earth make it a difficult way to travel. Next time you are on a pothole-filled highway, you will appreciate the efficiency that limbs have over wheels.

Fleas utilize a unique propulsion method in that they have mechanisms on their bodies that actually allow their extra-long hind limbs to act as spring-loaded thrusters. A result of this unique propulsive method allows fleas to jump up to 150 times their own length.
Kangaroos are mammals that have also adopted a jumping mode of locomotion. The mechanics behind their jumping is anatomically very different from that of the flea, but the result is similar in that it allows very efficient movement with a minimal amount of energy consumption. In fact, since it is so efficient, one can wonder why there are not more animals on the planet that have adopted this mode of transportation.

Other creatures such as bats, birds and many insects move about by flying. Closely linked to flying animals are the gliding animals. Some species of squirrels, snakes, lizards, and frogs utilize the drag of air on outstretched skin attached to their limbs, similar to the drag created by an open parachute, to move in their arboreal environments. It is a very efficient way of moving amongst treetops, though in a thin atmosphere world, it would not be efficient at all. Iain Banks describes in great detail gliding aliens, the Nauptrians, in his Culture novels.

Equally efficient in moving amongst the trees on Earth are the gibbons that move by brachiating with their exceedingly long arms. Brachiating is effective in a jungle or heavily treed environment but useless on an open plain.

Bivalve molluscs such as oysters and clams move by the propellant power of a false foot. Other molluscs without the benefit of a shell such as squid and octopi and their relatives move by a form of jet propulsion effective in water that could also be effective in certain gaseous environments such as in the James Tiptree Jr. (aka Alice Sheldon) novel Up the Walls of the World. In Iain Banks’ Culture series, he describes the Affronter which moves normally by walking on limbs or paddling through the air, but also has the ability to vent gas through its anus in order to propel itself forward. That form of propulsion really does not sound overly pleasant.

On Earth, snakes and even some lizards move about very efficiently without the aid of limbs; in fact, they are often able to navigate territory that is not as easily accessed by limbed animals. However, even among snakes there is a great deal of variation. Just watch the gliding movements of a garter snake through the grass as opposed to the apparently erratic jerky movements of a sidewinder rattlesnake across the desert sand. The sidewinder’s unique way of moving, evolved over millenia, is very efficient in that it limits the surface area of the body that is in contact with the hot sand at any one time.

Other organisms such as plants and some animals such as sponges are essentially sessile; that is, they do not move from place to place. Science fiction, too, has created a sessile form of animal life in the form of a Grog, found in Larry Niven’s Known Space series.


In order to sustain a physical body, animals need a circulatory system. Circulation is essential in a multicellular animal in order to transport nutrients and oxygen to interior cells and to remove waste products. Human circulatory systems are known as closed systems; that is, blood is transported by a pumping organ (or organs, as in the case of the earthworm, which has five pairs of hearts that are really just muscular pumping networks). Most higher animals possess a closed system. Insects and bivalve molluscs on the other hand have what is known as an open circulatory system, in that blood is released into tissue spaces where it is recollected and reoxygenated. Some animals such as flatworms do not have circulatory systems but rather feed through a system of diffusion of nutrients from the environment into the cells.

Plants have combination circulation systems. They draw in nutrients and water from the ground via diffusion and osmosis from the soil environment into their roots. From there, nutrients move upward through xylem vessels, the upward draw accomplished through the evaporation of water from the leaves known as transpiration. In turn, the sugars created through the photosynthesis in the leaves of the plant are transported downward through differences in water pressure between the cells to the lower reaches of the plant.

Science fiction authors have imagined other examples of circulation systems. Instead of blood or hemolymph as we have in insects, in Stephen Baxter’s short story “The Sun-People,” an alien life form from the Oort Cloud of the outer solar system is discovered that uses liquid helium for circulation.

Nourishment and Respiration

Nourishment is an essential for life as we know it; hence aliens will probably need organs to be able to consume, digest and eliminate waste products. Earth organisms have an amazing number of ways to gain nourishing sustenance. Some microbes merely absorb nourishment and eliminate waste through a process called diffusion. Normally at a more macroscopic level, diffusion would not work, so what other means of gaining sustenance are there?

Firstly, animals must take food or sustenance in and break it down before it enters the digestive tract. Most animals that we are familiar with have mouths, and most advanced animals not only have mouths but mouths with vertically moving jaws to assist in the breakdown of food when first taken in. Some of the largest animals on Earth, whales and whale sharks, have jaws that do not truly break food down, but instead, the jaws, when opened, draw water across a filter medium, drawing small organisms out of the water to fuel their enormous bodies. Snakes and some lizards swallow their prey whole and are able to perform feats of jaw dislocation that are impossible to imagine happening in any human being.

Some multicellular parasites such as tapeworms have eliminated the need for any digestive organs other than a mouth. However, unlike most animals with mouths, these animals do not have jaws. They merely attach themselves to the wall of the intestine, deriving nourishment directly from their hosts. Other macroscopic parasites such as lamprey eels that feed on fish, too, have jawless mouths, but unlike the tapeworm, do have a fully developed digestive tract.

Some flies, for example, also have no jaws as we do but have a straw-like mouthpart which enables them to consume liquids only, yet they do eat solid food. Regurgitating its stomach juices onto solid food, thereby liquefying it, allows a fly to eat just about anything. Next time you have a housefly on your sandwich, think about where the last meal of that fly came from.

Mosquitoes feed through a syringe-type mouth part that penetrates the skin of an animal. When feeding with their mouthpart, the mosquito injects an anticoagulant and draws the free-flowing blood outwards. Other insects such as ants and termites have jaw-like mouth parts that move horizontally rather than vertically.

Bivalve molluscs such as oysters and clams have no mouths either, and feed through a filter system not unlike that of whales, but the mechanics are very different. They draw water into their bodies and tiny cilia in the digestive cavity filter out particles of food.

Plants gain nourishment through the autotrophic means of photosynthesis, where they convert the energy of sunlight into glucose. Their roots draw nourishment from the soil to gain the other nutrients necessary for life.

What about waste elimination? Most animals take in nutrients at one end and eliminate the waste produced by digestion at the other end, but again it is not universal. Some such as flatworms and starfish have one opening that serves both as a mouth and an anus. Not a very pleasant thought from our human bias perspective.

In order to provide oxygen to interior cells, humans and most animals have lungs which take in air and expel the waste by-product of carbon dioxide. However, not all animals rely solely on lungs. Amphibians such as frogs, toads and salamanders have skin that acts as a breathing organ in addition to lungs. Some salamanders actually are known to breathe entirely through a mucus membrane in the throat or skin. Insects respire through a system of cuticle-lined tubes in their bodies that bring air directly to all parts of the body, much like another circulatory system. They can regulate air flow by opening and closing of special pores on the exoskeleton. Fish and other aquatic creatures, on the other hand, breathe through a system of gills which allows them to extract the oxygen directly from the water. 


Animals and plants reproduce by the exchange of gametes (germ cells) through some form of sexual interaction. Sexual reproduction is a very efficient way to exchange genetic material from unrelated organisms of the same species; though it increases the variability in a population, it has the limit of requiring organisms to find a mating partner. To overcome this difficulty some organisms have options; to breed with another organism of the same species, or if that fails, to reproduce by self-exchanging gametes; there are creatures and many plants on Earth that are hermaphroditic, possessing male and female parts, thus eliminating the need of seeking a partner. Being a hermaphrodite does not increase variability of a species, but it does ensure the transmission of one’s genetic material to a new generation. Certain fish have taken hermaphroditism to the next level and change their sex according to environmental demands; if there are too many males in an aquarium for example, some will change into females. Ursula Le Guin in The Left Hand of Darkness describes humanoid aliens that alternate sexes according to a biological cycle.

Some science fiction authors have taken sex to the next level. Isaac Asimov’s The Gods Themselves has aliens made up of three sexes. The Azadians of Iain Banks’ Culture series also have three sexes, with the male described as having a penis, with sperm then fertilized by the apex sex having ovaries and a reversible vagina, which then deposits the fertilized eggs into the female sex. 

Many microbes, on the other hand, which survive very well, reproduce by cell division; that is, they make exact duplicates of themselves after division. This, at first, appears to limit variability. However, many bacteria have come up with methods of exchanging genetic material by depositing it in the environment, making it available to other bacteria, often unrelated species, for incorporation into their genome. This principle is behind the current situation that modern medicine finds itself in, fighting a constantly evolving battle against bacteria that quickly adapt to new antibiotics.

Aliens, like the organisms on Earth, are a true product of their environment. The possibilities are endless. Change the variables and you change the outcomes. If Earth was closer to the Sun or further away, if it had a heavier gravity or a lighter one, a thicker or thinner atmosphere, two moons or none, if it was an ice world or a desert, the outcomes on life on Earth would have been very different or life may not have arisen at all. We can only speculate on what may have been. That’s what science fiction is all about, asking the crucial question “what if?”

Further Reading

Alcamo, I. Edward. 1997. Schaum’s Outline of Microbiology. McGraw-Hill.
Alexander, R. McNeill. 2013. Principles of Animal Locomotion. Princeton University Press.
Ball, Philip. 2011. Shapes. Oxford University Press.
Barlow, H. and Mollon, J., eds. 1982. The Senses. Cambridge University Press.
Bejan, Adrian and Marden, James. 2006. "Constructing Animal Locomotion from New Thermodynamics Theory." American Scientist 94:342-349.
Blakemore, R. 1975. "Magnetotactic Bacteria." Science 190:377-379.
Bardach, John E., Magnuson John J, May, Robert C. et al., eds. 1980. Fish Behavior and Its Use in the Capture and Culture of Fishes. Proceedings of the Conference on the Physiological and Manipulation of Food Fish. International Specialized Book Services.
Cressey, Daniel. April 2012. "Pigeons May “Hear” Magnetic Fields." Nature News. 
Gould, J.L. 1984. "Magnetic Field Sensitivity in Animals." Annual Review of Physiology 46:585-598.
Hall, John E. 2010. Pocket Companion to Guyton and Hall Textbook of Medical Physiology, 12th ed. Saunders. 
Nilsson, Goran E. 2010. Respiratory Physiology of Vertebrates. Cambridge University Press.
Marshall, Alan John, Parker, Thomas Jeffery, Haswell, William Aitcheson et al. 1972. Textbook of Zoology: Invertebrates Vol. 1, 7th ed. Macmillan. 
Reiber, Carl L. and McGaw, Iain J. 2009. "A Review of the “Open” and “Closed” Circulatory Systems: New Terminology for Complex Invertebrate Circulatory Systems in Light of Current Findings." International Journal of Zoology. Article ID 301284, 8 pages.
Romer, Alfred and Parsons, Thomas S. 1985. The Vertebrate Body. Holt Rinehart & Winston.
Sherwood, Lauralee and Kell, Robert Thomas. 2009. Human Physiology: From Cells to Systems. Nelson College Indigenous.
Stewart, Ian. 2007. What Shape Is a Snowflake? Weidenfeld & Nicolson.
Takami, Shigeru 2002. "Recent Progress in the Neurobiology of the Vomeronasal Organ." Microscopy Research and Technique 58:228-250.
van Kleef, Joshua, Berry, Richard and Stange, Gert. 2008. D"irectional Selectiviy in the Simple Eye of an Insect." Journal of Neuroscience 28:2845-2855.
Wu, Le-Qing and Dickman, J. David. 2012. "Neural Correlates of a Magnetic Sense." Science 336:1054-1057.