by Don Lincoln
The biota of the Cambrian era is preserved in the Burgess Shale formation, located in the Canadian Rockies in British Columbia, as well as other locations around the world. It contains many creatures that are as weird as any Alien found in science fiction. Opabinia (figure 5.2)—with its articulated body, five eyes, tail like a modern fighter jet, and claw-like graspers that extend on a sinuous snakelike appendage as long as the rest of the body—would not at all look out of place in Hollywood’s next blockbuster, set on a planet circling a distant sun.
This is not a book about the origins of life and the evolution that caused the diversity we’ve seen over the past 500 million years. However, Earth is the only planet in the universe on which we are sure life exists. While alien life is likely to be totally different from Earth-based life, understanding the range of the forms of life that has existed on Earth is the first step in our exploration of what we might possibly encounter “out there.” One thing I want to make clear is, as much as I dearly love the television show Star Trek and its spinoffs, it paints a totally improbable universe. Driven by the pragmatic need to have the characters played by human actors, the races in that universe are overwhelmingly humanoid. The chances are essentially zero that the Aliens we might one day meet in our exploration of the cosmos will be so familiar. Our visit to prehistory gives but the merest hint of how strange an alien world might be.
FIGURE 5.2. The animal life of the Cambrian ocean includes body plans that have long since gone extinct, as shown in the arthropod-like Hallucigenia (left) and Opabinia (right). © 2006 The Field Museum, Chicago. Illustrations by Phlesch Bubble Productions.
Lessons from Earth Life
Since before the days of Linnaeus, scientists have classified forms of life into different categories. Initially there were three classes, essentially animal, vegetable, and mineral (although the mineral class was quickly dropped). This early classification proved to be too limited to organize the staggering array of types of life that has been discovered. There are currently a couple of taxonomic systems in use which, for purposes of our discussion, aren’t very different (no matter how passionately the various proponents debate them).
Biologists categorize living things according to their characteristics. The defining characteristics can be genetic or by form, with the two systems having considerable, although not complete, overlap. As an illustration, I offer a popular classification scheme. At the highest level are the domains, which distinguish life into Bacteria, Archaea, and Eukarya. The first two are grouped together as prokaryotes, meaning their cells do not have a nucleus. Eukarya just means that the cells of the organism do have a nucleus. It includes the kinds of life you see when you look out your window. Plants, animals, and fungi are each a different kingdom within the Eukarya domain. These kingdoms are further subdivided into phyla, classes, orders, families, genera, and species. Just to give you a sense of how each level distinguishes between the alternatives, we can see how our own human species is classified. Beginning with our location in the Animal kingdom, we then belong in the Chordata phylum (indicating that we have a hollow channel through which nerves can run— essentially the spinal cord). The next subdivision is the class, with our being a member of the Mammalia class. This means (among other things) that we are warm-blooded, hair-bearing vertebrates with females that produce milk. This classification is followed by our membership in the order Primates and then the Hominidae family and finally a genus and species of Homo sapiens. Since we are concerned with a broad sweep as we investigate body types, the details of these last few distinctions aren’t so interesting to us.
Any self-respecting biologist would cringe over this cavalier description of an intricate system, hard won by centuries of effort. And they should. The painstaking interlinking of the world’s species, finding which one fits in here and which one there, is a marvelous achievement. Indeed to have understood the tapestry of life and how species come into existence and live and die has to be one of the most successful achievements of science. When one includes the more recent genetic work, one cannot help but be awed by the story of life on this planet and even more by the fact that mankind has been able to figure much of it out.
However, we do not aspire to anything so grand here. We are interested in Aliens, not alien life per se. Recall that, in our context, “Alien” (denoted with the distinguishing capital letter) means a creature that can design, build, and fly a spaceship and with whom, in principle, humanity might someday vie for galactic domination. It is not critical that the technological levels be comparable, nor is it critical that they actually build a spaceship. Creatures who fly UFOs to Earth to invade are an inarguable example of what we mean by Aliens, but so are the Na’vi of James Cameron’s Avatar. So Aliens must be mobile, intelligent, and able to manipulate the world around them. They must be able in principle to eventually pilot a spaceship. It is insufficient simply to be life that evolved on a different planet.
So we don’t need to know about the equivalent of an alien chimpanzee, nor do we need to know if the alien planet has a creature like a squid. What we want to know is when we encounter an Alien life-form, what are the ranges of possible forms it can take? For that, we must consider much broader questions. What is the skeletal structure of the Alien? Does it have an endoskeleton like us, or an exoskeleton like a lobster? Is it hot blooded or cold blooded? Does it have distinct sexes and how many? It is the answer to queries like this that that we hope to explore by studying Earth-based life. After all, we know on Earth that there are many answers to these kinds of questions. While Aliens are likely to be very different in detail, we can learn a lot about what is possible simply by looking around us.
So we start our investigation by studying the domains and kingdoms. Of the three domains, we will put off discussion of the Archaea until the next chapter. Archaea life utilizes radically different metabolic choices and properly belongs in a discussion about life that exists under very different conditions than the ones with which we are most familiar.
The first of the domains we will discuss is Bacteria, which are typically unicellular and don’t have a nucleus. Could Aliens be formed by evolved bacteria? (And by this I mean forming multicellular life using cells that have the structure of bacteria.) The answer is probably no. It’s a matter of energy. Energy is formed in the cellular walls, and the bacteria structure has a much less complex cellular composition. This results in a much lower amount of energy available to the cell to do the sorts of things that would need to be done to form an intelligent Alien. Even though bacteria can come together to work cooperatively, this particular form of life just doesn’t generate enough energy to be a viable building block of an Alien.
In fact, this is a good time to look at what is necessary to decide whether a particular body plan or biochemical approach is a credible candidate for making Aliens. The most basic consideration is energy. Evolution and environment can and do exert a powerful pressure to shape the direction that life follows, but variation like that can only occur if there is adequate available energy. If there is not enough energy to do something, it won’t happen. It’s a little like cars. There are Model Ts and jalopies and Ferraris. Car designers have come up with a vast array of different types of automobiles. However, one thing common to all of them is the need for an energy source. As we think about the life on Earth and how it might or might not have evolved under different conditions into an intelligent Alien, we should keep in mind the question of energy constraints and cars. There are many car designs and possible energy sources (e.g. gasoline, ethanol, wind, solar, nuclear, etc.), but a car needs some kind of energy to run. No energy means no motion.
So, if the energy source is simply too small to evolve the sorts of properties needed by an Alien—for instance, intelligence, mobility, and the ability to manipulate technology—then is it impossible for that particular Alien to exist.
Eukarya
Since the energy generation mechanisms of Earthly bacteria are simply too low to evolve into a
n Alien, we turn our attention to Eukarya. Eukaryotes are more complex kinds of cells than bacteria. These cells contain within them even smaller structures, themselves embedded in membranes. The central feature of Eukaryotes is the nucleus that gives them their name, which comes from the Greek eu (good) and karyon (kernel). The Eukaryotes contain other organelles that are the source of the energy in the cells. Mitochondria are the organelles that provide energy to animal cells, while chloroplasts provide energy to plants. There is a vast body of knowledge regarding the form and function of eukaryote life, and we will only touch on it in the lightest possible way, dipping into the details only when absolutely necessary. It is important to remember that the details of Eukarya aren’t crucial. However, the boosted energy-making capabilities of Eukarya are.
Since we know that earthly Eukarya can generate adequate energy, it is valuable to explore this type of life a little deeper. Eukaryotes are broken into four kingdoms. They are the three that were mentioned before, animals, plants, and fungi, along with protists. We sort of intuitively understand the first three from our common experiences. Protist is sort of a catch-all category of organisms that don’t fit into the first three. Protists tend to be unicellular creatures and, superficially, seem pretty similar to each other. Indeed it was only in the early 1980s when the diversity of protists began to be appreciated. The understanding of the evolutionary interconnections of the protists is an active area of research, but their unicellular nature makes them unsuitable for making Aliens. For multicellular life, we need to turn our attention to fungi, plants, and animals.
Fungi
Due to their superficial resemblance to plants, Fungi were originally classified simply as part of the Plant kingdom. Further study revealed considerable differences; for instance they do not photosynthesize and their cell walls can contain chitin rather than the cellulose of plants. Chitin is the material that forms the exoskeleton of many arthropods and insects. In fact, recent genetic work has revealed that fungi are more closely related to animals than to plants, although they are a distant cousin indeed. Unlike plants, fungi eat other things.
So with regard to our question, is it likely that fungi might have evolved into an Alien? The answer to this is no. Fungi get their energy from very energy-poor methods. There is simply not enough energy available for fungi to adapt the behaviors necessary to be an intelligent Alien.
Plants
The question isn’t so clear a priori when one considers plants. The kind of Alien we’re talking about will have to move in some way, and plants are generally immobile. However, the science fiction and fantasy literature abounds with examples of moving plants, from the “Feed me Seymour” plant in Little Shop of Horrors, to Tolkien’s Ents, to the Whomping Willow of Harry Potter fame, the triffids from The Day of the Triffids, and the monster from The Thing from Another World. Is a mobile plant actually possible?
The kingdom of plants is exceedingly rich, filled with towering sequoias, annoying dandelions, prickly cacti, and languorously waving cattails. The range of body plans is quite diverse. Surely motion has evolved somewhere in the past? Does the phototropic ability of plants to move toward sunny environments count? Or the sudden snap of the Venus flytrap? Could these simple behaviors evolve toward more energetic mobility?
I think the answer to these questions is actually pretty clear and can be answered on energetic grounds. But before we discuss that, we need to talk a bit about the differences between plants and animals (which we know could evolve into an Alien). Both are eukaryotes, with a nucleus. Plants have a cell wall, typically made of cellulose, which gives plants their structure in the absence of skeletons. In contrast, animals have a cell membrane. Plants are autotrophs, which means they make their own energy, while animals are heterotrophs, which means they consume energy from plants and other animals and adapt it to their needs. The power source of animals is their mitochondria, which are tiny structures inside the cell, while the corresponding source for plants are similar objects called chloroplasts. Chloroplasts are structures inside plant cells in which photosynthesis occurs, converting light into metabolically useful energy. Chloroplasts contain the chlorophyll that gives plants their characteristic green color.
Do carnivorous plants tell us anything? If plants can eat animals or insects, surely we can believe that the more outlandish and fictional plants are at least possible? Actually, it might surprise you to know that the Venus flytrap and other similar plants do not get any energy from their prey. They get nutrients only, in contrast to other plants, which extract nutrients through their roots. In fact, nearly all carnivorous plants have evolved to live in extremely low-nutrient environments. If these plants are moved to a more nutrient-rich environment, they typically die. The calcium from ordinary tap water can kill a Venus flytrap … essentially because the plant grabs and stores the minerals it needs like a starving person might gorge on the roasting pig it finds at an abandoned luau.
But carnivorous plants are quite rare. Of the half a million or so plant species, only a few hundred are carnivorous. This is because the driving focus of all life is to acquire enough energy to reproduce. Since the parts of the plants involved in predation are poor energy collectors, the plant pays a price by turning leaves (solar energy collectors) to other uses. Essentially these plants evolve this way out of necessity. Just like cacti have unusual specializations to live in a place with very low water access; carnivorous plants have evolved their unique capabilities in order to exist in a “nutrient desert.”
In order for plants to evolve to have animal-like properties, they would need to gain a nervous system, sensory ability, and mobility. This takes an awful lot of energy. Since plants only get their energy from sunlight, we can do some quick estimates of how much sunlight is necessary to power a human. It’s not that an Alien must be human, but it gives a sense of the kinds of energy requirements that are necessary for a creature “sort of like us.”
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, wit
h 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?