by Don Lincoln
Some species produce many offspring, knowing that many will not survive to reproduce themselves. An example might be frogs or rabbits. Other species produce fewer offspring but spend more time with them to ensure that they survive. This is the evolutionary tactic taken by humans.
For some species that have sexual reproduction, there are hermaphrodites, whereby a creature has the reproductive organs of both sexes and can both impregnate others and bear the young of their species. There are also species with tremendous sexual dimorphism, like the angler fish, in which the male fuses itself to the female and then atrophies away until he is nothing more than a sperm source.
An unusual adaptation in a few species actually has more sexes than the usual two. There are species in which individuals change from male to female and back again. There are species in which there are large “alpha” males with harems and smaller males of the same species with coloration that mimics the females. They hide in the harems and reproduce that way. There are insects in which a single dominant female lays the eggs and the other females are reproductively neuter. Even on Earth, sex can be complicated for a species. There is no reason to believe the male/female dichotomy will apply to Aliens.
Senses
What senses will our Alien have? It seems that a sense of touch is crucial to essentially all living organisms. Having a tactile awareness of your environment is important, whether you are predator or prey if, for no other reason than to know if something is biting you. Hearing is similar to touch, although there is a broad variation in how well species can hear. Taste or something similar allows organisms to decide if something is food or not. Vision is a very important sense and has evolved independently several times. Vertebrates, cephalopods (e.g., squids), and cnidarian (e.g., box jellies) have “camera-like” eyes, and each followed a separate developmental history.
There are at least ten different “eye technologies” that probably originated from a small spot of photoreceptive proteins on a unicellular common ancestor. However the details vary, from the human-type eye, in which focus is accomplished by changing the shape of the lens, to another choice in which the lens doesn’t change, but the shape of the eye does. Then there are the multiple lenses of insects, the reflective eyes of scallops, and many other designs. Thus, while the details of the vision might be quite different, we can conclude that it is probable that our Alien will be able to see. It is simply too valuable an adaptation in a lighted environment to do without.
Of course, by “see” we don’t mean just “see what we can see.” Some snakes are able to detect infrared. Birds, reptiles, and bees can see some ultraviolet. So the possibilities of Alien vision are quite diverse.
It is important to recall that much of the vision of Earth creatures is optimized to see light where the sun is brightest. Aliens evolving on another planet would likely evolve the ability to see best using the brightest light available on their world. Thus it is possible that they could see the kind of light we do only poorly.
Senses that some Earth-life has that humans don’t include the echolocation of bats and dolphins (useful in low-light environments), the ability to sense electric fields like some fish and sharks, and the magnetic sense of many migratory species (e.g., some birds, tuna, salmon, sea turtles, and more). We can also imagine Aliens developing a sensitivity to radio waves.
Obviously it is not mandatory that Aliens will have all of the senses we do. For instance, a subterranean species would have no need to develop sight. Tactile and auditory senses seem like they would be universal, as they would be helpful in any environment. A sense of smell or taste provides a method of chemical analysis; for example, some poisons taste or smell bad. Both senses might not be crucial, but having one or something similar would probably provide an important survival advantage.
Communication
The communication between Aliens will be aligned to their senses. Here are some options that Aliens might exploit: motion, smell, light, sound, or radio. Imagine trying to talk to an Alien who uses scent to communicate. (Given how slowly smells travel and dissipate, this is an improbable scenario, but it helps one think about how difficult human-Alien communications might be.)
Life Span
This is difficult to generalize from Earth life. Mice live only a few years, while some tortoises might live to about 200 years. There appears to be no strong correlation with metabolism rates on Earth. But, given the many factors that go into determining longevity, it is difficult to predict an Alien life span, except to state that an Alien must live long enough to learn the technology of previous generations.
Social Structure
Animals spend their time in many ways, from packs, to herds, to a solitary lifestyle. It is likely that Aliens will be social creatures in a way at least somewhat analogous to humans. The need for communication and retention of technical knowledge over the generations almost guarantees that the individuals will work together.
Wrap Up
These attributes of life are certainly not intended to be encyclopedic but rather to give a flavor of the kinds of variation possible should alien life evolve using carbon as the basic building block and with a biochemistry that is similar to our own. Of course, on a different planet, with different sunlight and chemistry, life might be quite different. Exploring some of these other options is the goal of the next chapter.
In summary, the study of biology on Earth certainly teaches us something of what is possible when discussing what an Alien might be like. Surely this brief survey has not explored all the possibilities. It is also clearly very Earth-centric. However, it does show some of the range of what we might encounter. While we realize that our conversation here does not span all possibilities, we might close with the following thought: Knowing something is better than knowing nothing, as long as you know it’s not everything.
SIX
ELEMENTS
The third planet is incapable of supporting life … Our scientists have said there’s far too much oxygen in their atmosphere.
Ray Bradbury, The Martian Chronicles
In the previous chapter, we looked at what sorts of lessons familiar Earth life might tell us about what an Alien might be like. These observations were not meant to be exhaustive, as they were based on a very limited range of biochemistry. Animals breathing oxygen and converting glucose into energy and plants converting sunlight doesn’t even span the range of observed biochemistry here on Earth, let alone the range of the possible. There are creatures on Earth that use methane to exist and others who extract energy purely from chemicals, rather than exploiting (directly or indirectly) light from the sun. Then there is sulfur respiration and fermentation, just to name a few alternatives.
At the end of this chapter, we will talk about more “exotic” forms of Earth life. Our real interest is about Aliens who could potentially visit our planet, but their story is inextricably tied up in the question of non-Alien alien life. One must have the second to have the first. Accordingly, we will spend some time exploring what we know about alien life and the limitations placed on such life by simple considerations of chemistry and physical law.
The reader should be aware that any writing on this subject is bound to be incomplete. As noted popular science essayist and pioneer geneticist J. B. S. Haldane wrote in his 1927 book Possible Worlds and Other Papers, “The Universe is not only queerer than we suppose, but queerer than we can suppose.” It is quite reasonable to suppose that the universe will have a trick or two up its sleeves and we will be surprised more than once. Still, we can talk about what we know about the relevant chemistry. If nothing else, we will learn what the important considerations are for modern astrobiology.
What Is Life?
This question is seemingly so simple, and yet it has vexed some of the most knowledgeable scientists and philosophers for decades. While hardly the first writing on the subject, physicist Erwin Schrödinger’s (of Schrödinger’s cat fame) 1944 book What Is Life? is one such example. It is an interesting ear
ly attempt to use the ideas of modern physics to address the question. Both James Watson and Francis Crick, codiscoverers of DNA, credited this book as being an inspiration for their subsequent research.
The definition of life is not settled even today. Modern scientists have managed to list a series of critical features that seems to identify life. A living being should have most, if not all, of the following features:
It must be able to regulate the internal environment of the organism.
It must be able to metabolize or convert energy in order accomplish the tasks necessary for the organism’s existence.
It must grow by converting energy into body components.
It must be able to adapt in response to changes in the environment.
It must be able to respond to stimuli.
It must be able to reproduce.
These features distinguish it from inanimate matter.
While these properties can help one identify life when one encounters it, they don’t really give us a sense of the limitations imposed by the universe on what life might be like. The purpose of this section is to get a sense of whether a would-be science fiction writer is being ludicrous when he or she bases a story around an Alien with bones made of gold and liquid sodium for blood. So what does our current best understanding tell us that life requires? A combination of theory and experimentation suggests that there are four crucial requirements for life. They are (in decreasing order of certainty):
A thermodynamic disequilibrium;
An environment capable of maintaining covalent interatomic bonds over long periods of time;
A liquid environment; and
A structural system that can support Darwinian evolution.
The first is essentially mandatory. Energy doesn’t drive change, rather energy differences are the source of change. “Thermodynamic disequilibrium” simply means that there are places of higher energy and lower energy. This difference sets up an energy flow, which organisms can exploit for their needs. It’s not fundamentally different from how a hydroelectric power plant works: there is a place where the water is deep (high energy) and a place where the water is shallow (low energy). Just as the flow of water from one side of the dam to the other can turn a turbine to create electricity or a mill to grind grain, an organism will exploit an energy difference to make those changes it needs to survive.
The second requirement is essentially nothing more than saying that life is made of atoms, bound together into more complex molecules. These molecules must be bound together tightly enough to be stable. If the molecules are constantly falling apart, it is hard to imagine this resulting in a sustainable life-form. It is this requirement that sets some constraints on which atoms play an important role in the makeup of any life. Hopefully after this discussion, you’ll understand the reason for the oft-repeated phrase in science fiction “carbon-based life-form.”
Requirement number three is less crucial; however it’s hard to imagine life evolving in an environment that isn’t liquid. Atoms do not move easily in a solid environment and a gaseous environment involves much lower densities and can carry a far smaller amount of the atoms needed for building blocks and nutrition. Liquids can both dissolve substances and move them around easily.
Finally, the fourth requirement might not be necessary for alien life, but it is crucial for Aliens. Certainly multicellular life or the equivalent will not be the first form of life that develops. The first form of life that develops will be of a form analogous to Earth’s single-celled organisms (actually, most likely simpler … after all, modern single cell organisms are already quite complex). In order to form species with increasing complexity, small changes in the organism will be necessary. Darwinian evolution is the process whereby a creature is created with differences from its parents. The first thing that is necessary is that the organism survives the change. After all, if the change kills it, it’s the end of the road for that individual. Once there are changes that both allow the daughter organism to survive and possibly confer different properties, selection processes become important. Creatures who subsequently reproduce more effectively will gradually grow in population until they dominate their ecological niche.
So let’s talk about these ideas in a little more detail.
Thermodynamic Disequilibrium
The most important consideration for any form of life is the need for thermodynamic disequilibrium. This mouthful of an idea is simultaneously intuitive and counterintuitive.
If you tell someone that energy is necessary for life, you likely won’t get any argument. Plants absorb sunlight, people eat food; the need for energy is self-evident. Yet the reality is a little more subtle. Energy has a technical meaning in science. Energy can be found in a thrown ball, a coiled spring, and a stick of dynamite.
However what life needs is not energy but rather an energy difference. If the energy is the same everywhere, this is not useful. What’s useful are energy differences. To illustrate this subtle difference, consider a water reservoir held back by a dam (figure 6.1).
On the water side, everything is equal. While the pressure changes with depth, the uniformity keeps the water from moving around. It tends to stay put. However the water has a kind of energy that scientists call “potential energy.” (Potential energy is the kind of energy where something would move if we let it, like how the water would move if we broke the dam or how an arrow would fly from a stretched bow if the string were released.)
Now imagine that there is a hole in the bottom of the dam. Water would rush out from the water side to the air side. This is in fact how hydroelectric power stations work. The moving water turns a turbine, which generates electrical power.
The crucial takeaway point here is that an energy difference (and a subsequent flow from high energy to lower energy) is central to the creation of electrical power and that this is true in a more general sense. This is what is meant when we say “thermodynamic disequilibrium.” Thermodynamic means energy and disequilibrium means “not equal” or different.
FIGURE 6.1. Water being held back by a dam is an example of an energy difference, and this energy difference can be converted into high-pressure water flow that can turn an electrical turbine. Although the energy differences of biology and biochemistry stem from concentrations of chemicals being held back by a cellular membrane, or in the interatomic bonds within molecules, the principle is the same.
Life works the same way. Energy differences allow energy to flow and make the kinds of changes that permit life to exist. For life, it is important to be able to store these energy differences for use when convenient. That way, an organism can move around, carrying its energy source with it. This provides protection against random occurrences that might restrict access to energy.
To get a sense of why this is important, consider a hypothetical alien cow that has to constantly eat to survive. If the cow exists on an ever-growing and ever-present patch of alien grass, there is no problem. However imagine a drought. With the death of the grass, the cow would immediately die, unable to move to a fresh patch of grass. Or imagine a plant that uses sunlight like Earth ones do but that can’t store energy. It would live during the day, but die each night. Without a guaranteed, never-ending energy source, life of these forms is very vulnerable. Energy storage is necessary for life to exist.
It seems likely that life made of atoms (as we are) must exploit energy storage in molecules. Certain atoms can be combined together using available energy (as plants do with sunlight). Later, the energy can be extracted by converting molecules containing a lot of energy into lower energy ones and using the extra energy to live. We do this when we eat a cookie and metabolize sugars or fats. Perhaps an even more intuitive example of this would be when we burn wood. Cellulose combines with oxygen through a series of chemical reactions, resulting in carbon dioxide and water. We know that a fire releases heat—that’s typically the point of fire after all—but what isn’t so obvious is that what we’re seeing when we toast ou
r marshmallows is the transformation of molecules with lots of energy stored in their bonds into ones with less energy.
The Constraints Imposed by Atoms
Scientists know a lot about chemistry, how atoms interact and the properties of the matter they form. Surely this knowledge can tell us a lot about what elements are crucial for life. We are “carbon-based life-forms,” as they say in science fiction. But science fiction also talks about other possibilities. The Horta in the Star Trek episode “The Devil in the Dark” was a life-form built around the silicon atom. Larry Nivens’s Outsiders from his Known Space series have a biochemistry that includes liquid helium. Given the imagination of science fiction writers, both professional and amateur, I could imagine that sitting in somebody’s drawer is a story about mankind’s encounter with an intelligent race, with platinum bones and molten gold blood, who excreted diamonds. (If someone steals that idea and writes a story, I want a cut of the royalties.) So what does science tell us about the range of atomic combinations that is physically possible? For that, we need to think about some simple molecular requirements of life.
Life cannot exist without atoms combining together to make more complex molecules. Thus the way in which these atoms interconnect is a crucial consideration. While it may be obvious that the rules of chemistry are a defining aspect of any form of life, that statement is pretty vague. We can actually do better than that and discuss in the following text some detailed considerations.