by Don Lincoln
In summary, the chances of us encountering a plant-based Alien who evolved in an environment similar to Earth’s is improbable due to physical limitations. A mobile Alien that absorbs the bulk of its energy from sunlight is not impossible, but it will require a different chemical to transform sunlight to metabolic energy and possibly a higher energy environment to supply the sunlight. Mobile plants with alternating mobile and sessile phases are also possible.
We should keep in mind that heterotrophs (creatures that consume other creatures) have an advantage in terms of being able to simply exploit the energy gathered by others. Like on our Earth, we can imagine that there will be plants that consume and transform sunlight or chemical energy (discussed in the next chapter) and creatures that take advantage of that ability and consume the plants. Remember that a blade of grass works hard to convert light into grass, but a sheep can consume many blades of grass, thereby benefitting from solar energy gathered over a large area. Effectively the grass has become an extension of the sheep’s energy-gathering area, without the penalty of having to carry it around with them. Animals can simply consume a lot of the energy that the plants have produced. This might be an insurmountable advantage, even on a planet where plant mobility is energetically possible. After all, if the plants have more energy, this just supplies more energy to the things that eat them.
Animals
Following our discussion of the limitations of plant-based Aliens, we now turn our attention to animal-like life forms. Almost certainly any Aliens will be based on different biochemistry, with a different “genetic” encoding scheme. However, we know for certain that (1) Earth-based animal life could produce an Alien-equivalent and (2) that animal life on Earth has taken a vast variety of different forms. So we can take a look at the range of life observed on Earth to learn something of the possible.
The Animal kingdom consists of several phyla. The phylum including humans is Chordata, which, roughly speaking, means “has a backbone or spinal cord.” There are other phyla that do not have a central nervous system. Some (like sponges) do not have differentiated cells.
When considering which of the phyla of the Animal kingdom might have evolved into an intelligent, tool-using, species, there seem to be a few crucial attributes. Differentiated tissue would seem to matter, as well as some ability to manipulate the environment. A central nervous system protected by a spine like we have doesn’t seem to be crucial. For instance, the octopus, which has no bones at all and a partially dispersed nervous system, can exhibit remarkably intelligent behavior. They can be taught shapes and patterns. They can be trained to open jars with food in them. In 1999, scientists filmed octopi in the wild digging halves of coconut shells out of the seafloor. They then carried the shells with them and used them to form a protective shelter. This behavior was invented by the octopi and not trained into them by humans. This highly intelligent tool usage should totally destroy any vertebrate-centrism one might have.
Even insects can show evidence for types of intelligence. Honey bees exhibit considerable ability to communicate. Using a kind of dance, a lone forager bee can return to the hive and tell other bees where a food source is located. The other bees can then go directly to the food source. This could be considered an extremely complex instinctual behavior, but researchers have found that the ability of bees to communicate depends on their getting enough sleep. By depriving bees of sleep, their communication dance becomes less accurate. This suggests a type of intelligence that could in principle grow into something more akin to human intelligence, as it does not appear to be purely instinctual behavior.
The phylum Chordata is the most familiar to us, consisting of fish, birds, mammals, reptiles, and amphibians. These are the classes of animals that exhibit the behaviors most consistent with intelligence. So, for the rest of the chapter, we will explore the spectrum of body types, mobility types, object manipulation strategies, and other ways in which organisms interact with the environment. As we will see, there are an amazing number of options. However, during this discussion, we must guard against Chordata-centrism and keep in our minds the fact that nonvertebrate animals exhibit capabilities that perhaps could have led to intelligent life in an alternate history of Earth.
Alien Considerations
There are many properties one might consider when thinking about what an Alien might look like, things like body symmetry, number of limbs, and size. The next few pages discuss about twenty such considerations, using lessons taught us by earthly life.
Body Symmetry
The most familiar symmetry is called bilateral symmetry. This symmetry means that the left and right side are mirror images of each other. This particular body shape is favored by most of the higher animals. However, it is not the only possible choice. Spherical symmetry, where the body is like a ball, is possible in a water environment but difficult to imagine on dry land, where gravity would distort the body, unless it was hard. Another common symmetry is radial symmetry. This is the symmetry of jellyfishes, anemones, and starfish. Starfish have five or more arms, demonstrating a special form of radial symmetry, and many jellyfish have a four-way symmetry.
A final form of symmetry is no symmetry at all. This would be a life-form with some kind of lumpy structure, with protrusions and blobs here and there. An example of Earth-life with this body type is the sponge. Given the range of types of body symmetries seen here on Earth, it is hard to guess what symmetries an Alien might have.
Number of Limbs
There are a great number of choices here. Tetrapods, as their name suggests, have four limbs. This includes mammals, birds, and most lizards. Snakes have no limbs at all, although they evolved from a tetrapod ancestor. Insects have six limbs, while spiders and octopi have eight. Hallucigenia had fourteen. Centipedes have 20 to 300 legs, while millipedes have 36 to 400, with one rare species having 750 legs. Prehistoric Opabinia had but a single appendage.
There appears to be little Earth life can tell us about the number of appendages a life-form can have. However, our restriction that this be an Alien to compete with humanity for galactic domination makes it seem likely that it must have at least one appendage to manipulate the world around it. This is not a restriction caused by life, but a restriction caused by the need to invent and manipulate advanced technology.
Size
Our experiences on Earth can’t tell us much about the size we can expect an Alien to be. The size of animals ranges from tiny insects to giant whales. Other restrictions suggest that intelligent Aliens are unlikely to be wholly water dwellers, although an amphibious lifestyle or even semiaquatic species, such as seals and penguins, are possible. While whales and dolphins are intelligent, we must recall our definition of Aliens. Underwater species cannot exploit fire, which is necessary for a species to attain the technology level to qualify as an Alien.
The requirement of mobility on land makes very large animals unlikely. So whale-sized Aliens are improbable. We do know of rather large dinosaurs. This might set a reasonable upper limit on the size of Aliens.
At the smaller side, the issue is neurology and intelligence. Too small a creature and there is no possibility for individual intelligence to develop. The situation is confused somewhat by the concept of a hive mind. Individual bees or ants seem to have minimal intelligence, yet the collective behavior is actually quite complex.
Individual creature intelligence is observed in octopi, small primates, raccoons, and animals of similar size. This sets a rough limit on the likely minimum size of an intelligent Alien using Earth-based neurology; in the range of a small cat. With a different brain structure, this restriction might be removed.
Obviously any discussion of size is dependent on the gravity of the planet on which the Aliens formed and the type of skeletal structure that supports the equivalent of muscle tissue. A planet with a lower gravitational constant will allow larger creatures.
Skeleton
Any land animal will likely need a skeleton of some kind. The boneless octopus would have
considerable difficulty with locomotion on land compared with an animal with some sort of structure. Common animal skeletons are endoskeletons (inside the body like birds, mammals, and lizards) or exoskeletons (surrounding the body like insects and lobsters). I can think of little reason for one versus the other, except that a creature with an exoskeleton will have to molt to grow. However, there are other options, including a mixture of both technologies, or a young form of the life that has bones which dissolve after maturity, when an exoskeleton is formed. While not having an exoskeleton per se, the turtle combines a hard outer shell with a traditional skeleton. And, of course, a skeleton needn’t mean bone. Cartilage, chitin, and other substances could be employed.
Nervous System
According to legend, if you’re ever attacked by zombies, you always go for a head shot. It’s the only way to be sure. The reason for this is the central nervous system observed in mammals. We have a brain that is connected to the rest of the body first through the spinal column and then a branching network of nerves. This particular design has some convenient features, as it centralizes thinking and the motor control that governs the body. However, there is no a priori reason why a creature couldn’t have a distributed nervous system, with bits of their equivalent of a brain spread out over the body. If we ever encounter such an Alien, we better hope that they don’t become zombies.
Locomotion
There are a tremendous number of locomotion strategies employed by Earth life. There is walking, flying, swimming, slithering, hopping, tunneling, and brachiating. There are also animals that move on the surface of the water.
For swimming, there is the motion of a fish (with a tail side to side) or a dolphin (tail up and down). There is the use of flippers like a turtle and the propulsion of squids and cuttlefish. Swimming capabilities have independently evolved several times, resulting in similar, streamlined body shapes imposed by the need to move quickly through the water.
Flying has evolved on Earth at least four times, with birds, pterosaurs, bats, and insects, suggesting that this is a fairly common locomotive adaption. A flying Alien is entirely plausible.
There is little reason to select any particular form of locomotion for our Aliens.
Speed
The speed of an animal is tied to many other things. For instance, a heavily armored animal is typically slower than one without armor. Predators tend to be fast. On the other hand, humans aren’t particularly fast in the Animal kingdom. There is little the animal world can tell us about Alien speed.
Color
The color of animals spans the rainbow. Aliens could have any color.
Defenses and Offenses
The natural defenses and offenses that Aliens could have are quite broad. Humans are actually rather unimpressive in their offensive and defensive skills, but make up for it in their ability to utilize weaponry to overcome their structural limitations. Any Alien capable of building a spaceship will have similar skill. However, there is no reason that the Aliens will not have other abilities. In nature, animals exploit a myriad of defensive and offensive strategies, from the camouflage of a leafy sea dragon, a tiger, or a cuttlefish to the venom of a cobra, a scorpion, or a male platypus. Mammals tend to not be venomous, perhaps because they are quick enough to kill with tooth or claw, whereas venom takes time.
Shells, horns, and spines provide protection, for example the tortoise and the ankylosaur or the porcupine and the blowfish. And, of course, simply avoiding the conflict through a burst of speed is a wise defensive choice. Rabbits, the spine tailed swift, and the gazelle are able to move extremely rapidly.
Temperature Regulation
The internal temperature of an animal can be regulated by the body’s own metabolism (endothermic) or can depend on the environment, as is the case for insects, fish, and reptiles (ectothermic). In general, internal temperature regulation is a safer evolutionary choice, given that ectothermic species can be sluggish when the environmental conditions are colder. However, there is no reason to expect that the Aliens come from a planet as cold as Earth. Perhaps their planet is sufficiently warm that there is no need to evolve endothermy. Since metabolism depends on enzymes that tend to operate best in a fairly narrow temperature range, endothermic animals generally have a considerable advantage, but, in the right environment, the selection pressure may be small.
Blood
Blood is not necessary for all animals. Some insects use a fluid called hemolymph to transport oxygen to their tissues. However the higher animals use a substance that enhances the oxygen-carrying capacity of the liquid inside them. The most familiar type of blood contains a compound called “hemoglobin,” which contains iron and gives blood its red color. Each hemoglobin molecule can attach to up to four oxygen molecules and increases blood’s oxygen-carrying capacity to more than seventy times what it would be if the oxygen were just dissolved in water.
However, the iron-based hemoglobin molecule isn’t unique. There are other options. For instance, some insects have a copper-based blood, using a compound called hemocyanin. Hemocyanin transports oxygen about a quarter as efficiently as hemoglobin, so it is more suitable for creatures with lower metabolic requirements. Oxygenated blood containing hemocyanin is blue. And sea squirts and sea cucumbers carry a vanadium-based protein in their blood called hemovanadin. There is still some controversy on the role of this protein in oxygen transportation. When oxygenated, it turns mustard yellow. It is pale green otherwise.
Diet
It is, of course, very difficult to know the diet of an Alien, but here on Earth, there are three options: carnivore, herbivore, and omnivore. Carnivores eat meat, herbivores eat plants, and omnivores eat both. There are pros and cons to all three. Herbivores have the greatest access to food, as plant life is ubiquitous. However, plant food tends to be lower in terms of calories available, leading to frequent eating. Carnivores have fewer food choices, as they must catch and eat other animals. This places constraints on their bodies, from the trapping of spiders, the “lurk and grab” of alligators, the “stalk and pounce” of cats, and the “communal attack” strategies of wolves. Carnivores get substantial nutrition influx with a successful attack, but they don’t have as reliable a food source as herbivores.
Omnivores (of which humans are a member) get the benefits of both food sources. It is hard to imagine an intelligent Alien that doesn’t have at least omnivorous capabilities, although it may opt to utilize one food source more often. We should also keep in mind that it is possible that the Aliens might need certain minerals or other substances, similar to Earth-life’s need for water and salt. It is therefore possible that the Aliens might need to ingest materials directly from the ground, like deer around a salt lick. Given that the Aliens will evolve in an ecosystem with a common biological heritage, it is likely that some of this mineral collection will be done by plants for subsequent ingestion by Aliens.
Respiration
Respiration is the intake of vital gases from the environment (oxygen in the case of most animal life on Earth) and the removal of waste gases (predominantly carbon dioxide). As we will discuss in the next chapter, Aliens might elect to use different molecules in their metabolic processes, but the mechanisms to exchange gases with the environment are likely to be similar, as the phenomenon must satisfy basic physical constraints. These constraints include collecting gases from the outside and dispersing them to the tissues of the body. The respiration system is likely to be internal; otherwise something could block the ability to breathe. (For instance, imagine if your lungs were on your outside and you somehow got splattered with mud.)
Small insects have the simplest respiratory system, by exploiting the diffusion of gases into and out of the circulatory system. Recent research has shown that insects have a diverse range of respiration techniques, with some using muscles to expand and contract their respiratory systems in ways that are not terribly different from higher animals.
Land animals usually use a lung system, with an intr
icate system of branching pathways. The inside of a higher animal’s lungs looks a little like a tree and for essentially the same reason. This design maximizes the area to exchange gases in the minimum volume. While birds, reptiles, and mammals differ in detail, the basic structure is similar.
Water-breathing animals, like fish and mollusks, use a gill system to extract oxygen from water. Extracting oxygen from water is a tricky business. Water contains about 3% the oxygen held by an equivalent volume of air. Consequently, fish have evolved highly efficient gills to extract approximately 80% of the oxygen from the water. (This can be contrasted with an approximate 25% extraction efficiency for mammals breathing air.) Still, this scarcity of oxygen might make it more difficult to evolve highly intelligent Aliens under water. Amphibians have a split system, breathing both through lungs and their skin. This ability to breathe through their skin is of great value when submerged in oxygenated water.
Environment
Does the Alien live on the ground, under the earth, under water, or in the air? This is one of the questions for which we can likely exclude some options. While animals exist in all of these environments, it is essentially impossible for our Alien to be purely a water breather. The reason is that we impose the need to have an ability to build a spaceship. While it is clear that intelligence can exist underwater (e.g., dolphins and octopi), building a spaceship requires technology, specifically manipulation of metal. It is very difficult to imagine an advanced technology that doesn’t shape metal. Forming metal requires heat, which means fire. Since fire is impossible under water, it seems that our Aliens cannot be (solely) water breathers. An underwater alien caveman is possible. An Alien in the sense we mean in this book is not.
Reproduction
The number of reproductive strategies employed by animals is astonishing. There is the sexual reproduction of higher animals and the asexual reproduction often seen in microscopic organisms. Some creatures can do both, that is, reproduce sexually or asexually, depending on the environment. Asexual reproduction creates clones of the parent, which have the same susceptibility to disease or environmental change. Sexual reproduction ensures the genetic material is mixed. This results in a more diverse gene pool and is a guard against a change in the environment that might kill one individual but for which others might be better adapted. And, of course, for sexual reproduction there is external and internal fertilization, as well as egg-laying versus live birth.