Authors: Don Lincoln
The resting energy usage of an adult human is about 60 watts, about that of an ordinary incandescent light bulb. That’s just sitting there, doing nothing but having your heart beat, lungs fill and empty, and all those squishy organs in your midsection doing the sorts of things they do to get you through the day; getting up and moving around takes even more energy.
So how much sunlight does it take to power the average coach potato? The amount of sunlight hitting the Earth’s surface at the equator is about 1000 watts for every square yard (assuming the energy receiver is always hitting the sun face on). So that would mean that our hypothetical, equatorial, plant-biology-based, human-like, coach potato Alien would need about a square foot, always facing the sun. Of course, the sun doesn’t shine 24 hours a day. It’s not like our heart stops at night, nor does sunlight always hit straight on. So we would need perhaps twice as much sunlight-grabbing area to store up the energy for the night, plus a little extra to account for inefficiencies in storing the energy for their midnight snack. In fact, accounting for night and the fact
an Alien wouldn’t always be facing the sun, the average amount of sunlight a creature could expect to see is 200 to 300 watts per square yard. Therefore, including the most basic considerations, we might think in terms of having maybe a few square feet to collect sunlight just to live and not move. In order to gain enough energy to move around, maybe we’d need a bit more. A square about 2 feet on each side is a reasonable amount of area, so this sounds promising. Maybe mobile plant-Aliens are possible?
But there’s a problem. Chlorophyll doesn’t absorb energy with 100% efficiency. Chlorophyll can, theoretically, collect about 10% of the sun’s energy. However, plants typically achieve an efficiency of only about a third or half of that. Thus a hypothetical plant-Alien would need to have a surface area of about a square 10 to 12 feet on a side. But, of course, a solid animal that size would have a much higher volume and therefore weight (and correspondingly higher metabolic needs). If you sit and mull this over for a short while, you begin to appreciate why trees and bushes have the shape they do, with a compact trunk and then branches and twigs to simultaneously minimize the mass and maximize the sun-collecting potential.
We shouldn’t forget the fact that plants also need to have a deep root system to get at the water and minerals below the ground’s surface. Uprooting, moving, and rerooting would be an energetically prohibitive affair. Over the hundreds of millions of years of evolution, no plant based on Earth biology has evolved animal-like locomotive abilities (or at least we see no evidence for such a plant in the fossil record). This suggests that the ability to move is not consistent with the limitations of gathering energy from sunlight.
However, the numbers mentioned above give us some idea as to what kinds of factors might change this conclusion. For instance, chlorophyll, with its 3 to 5% efficiency in collecting sunlight, isn’t up to the task under an Earthlike sun. If some other (and more efficient) chemical accomplished the task of collecting sunlight, that would change the calculation. Another factor that might make mobile and intelligent plant Aliens more credible would be to evolve in an environment in which there is simply more energy in sunlight to absorb. Of course, more sunlight comes with increased temperature, which means one starts to need to worry about boiling the water in the plant’s tissues. Finally, there is another option, which would be plants that were sessile for a long time, gathering energy and storing it in (perhaps) sugars or lipids. The plants might spend a week, a month, or a whole growing season collecting energy that would be used either to let the plant move or to give mobility
to offspring. (Visualize a tree that drops a walking orange or something.) This sounds outlandish, but is it qualitatively different from the sleep or hibernation of animals?
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.