What Astrobiology Might Tell Us About What Aliens May Look Like

But that doesn’t mean that we have nothing to work with. Astrobiologists have attempted to figure out how aliens may evolve given what we know of the evolution of life on Earth, life on Earth in extreme environments, and the environments of other worlds we have studied from a distance.

While sci-fi writers and sometimes biologists have suggested that other lifeforms may be silicon-based, there are reasons to suspect that carbon is at least a much more likely base for life.

“First, bonds between two Si atoms or between Si and C are weaker than those between two carbon atoms, and bonds between Si and a host of common elements (hydrogen, carbon, halogens, and nitrogen) are chemically and photochemically reactive and subject to decomposition,” a book on the topic explains. 

“Second, the chemistry of Si, in general, lacks molecules that contain unsaturation, in particular, double bonds (Si==C, Si==Si, Si==O, Si==N). Molecules containing the analogous unsaturated carbon molecules (C==C, C==O, C==N) are integral parts of almost all biomolecules, including amino acids, proteins, carbohydrates, nucleic acids, fatty acids, and vitamins. Devoid of unsaturated molecules, organosilicon systems could not serve as building blocks for the complex chemistry necessary for life.”

While it’s possible that alien life could potentially form from silicon, carbon can form the necessary bonds more easily, making it more likely that if we do find alien life, it will be in familiar carbon form.

One approach, often seen when trying to study life on Mars, is to look at similar environments on Earth. We know that certain lifeforms, such as Diploschistes muscorum and Cetraria aculeata, can survive in such extreme environments

“Lichens inhabit diverse ecosystems worldwide, but they are particularly crucial in extreme environments like hot deserts and cold Polar regions. They are known as extremophiles, able to survive under extreme temperatures, intense radiation, and prolonged water scarcity,” one team explains in their paper. “The remarkable ability of lichens to endure harsh conditions has led to the suggestion that they are well-suited to survive the extreme environment of outer space.”

Similarly, studying life in our deep oceans, where sunlight barely touches, if at all, could tell us about the potential for life underneath the ice of Europa.

“The ocean may be an ideal environment to harbour primitive life, having conditions similar to those in Earth’s thermal vents in its deep oceans, where life may have arisen on our planet,” Bonnie Buratti, Senior Research Scientist at NASA’s Jet Propulsion Laboratory, explained to New Scientist.

Another approach is to look at traits and features that have reoccurred on Earth several times over. While it is still an assumption, it is a pretty good bet that life elsewhere will go through a process of natural selection.

“A simple replicator could arise on another planet. But without natural selection, it won’t acquire apparently purposeful traits like metabolism, movement or senses,” a review on the topic explains. “It won’t be able to adapt to its environment, and in the process, become a more complex, noticeable and interesting thing.”

Certain features, such as eyes and wings, the form of a crab, a streamlined shape like a dolphin, and echolocation, have evolved several times over since the simplest lifeforms emerged on Earth over 3.5 billion years ago. While these features arose in the environment of Earth, given the laws of physics are assumed to be the same everywhere, life on planets similar to Earth may be subject to similar pressures, and come up with similar features.

In many ways, it might be easier to pin down what more advanced aliens may look like, rather than more simple lifeforms. For example, though you could imagine intelligent aliens living in the oceans of a hycean world, it is difficult to picture them developing the same technology that we have.

“In an underwater world imbued into a fluid, such as water or liquid methane, where sound signals can be heard hundreds of kilometers away, communication between individuals could be feasible without the need for communication devices,” one study explains.  “Telecommunications technology might never emerge on such a world, even though it could be home to a fully developed civilization.”

In order to create technology comparable to our own, aliens would likely need to evolve similar traits, such as legs to move around, digits to carefully manipulate materials, and a good set of eyes to see light. These, or alternatives, may be requirements for any technological civilization.

“You need binocular vision to judge distance (to prey). Elevated head to see predators. Eyes near the brain to reduce the time delay (or degradation) of the visual signal. Sound and smell sensors (ears and nose),” Bernard Bates, physics professor at the University of Puget Sound, told Popular Mechanics. “Your survival chances improve if you can use all of the ways you can to detect food, mates, and predators. Living in an atmosphere means sounds and smells will arrive before the stinky, noisy predator.”

There are some scientists who go further, and suspect that alien life may actually look a lot more like us than we previously thought, given natural selection’s tendency to produce higher-level organisms working towards a common goal out of simpler “units” of life.

“We can’t say what aliens will look like in the sense of specific traits or adaptations, such as big green eyes or arms and legs. Instead, we’re arguing that aliens will be subject to the same rules of natural selection as we are, and as a result will have similar kinds of adaptations,” Samuel Levin from the University of Oxford, lead author of a study looking at potential evolution of aliens, told IFLScience in 2017. 

“Whether or not Ewoks or Na’vi exist, if they did, we can say a lot more than people might guess about their internal components, how their life cycles start, and what their evolutionary history is.”

So, what we know of the evolution of life on Earth suggests that aliens may at least develop similar mechanisms to survive on their own planet. But that doesn’t mean that when we find life out there, it will be boring. For example, we know that life on Earth favors left-handed amino acids, despite the fact that right-handed amino acids are possible, potentially making it incompatible with life on our planet. 

Meanwhile, our study of exoplanets and other suns has given us a sense of how life could be different, if they harbor life at all. Red dwarfs, the most abundant stars in the cosmos, are smaller in mass than our own Sun and are cooler as a result. Looking at life that could survive around such stars, astrobiologists suggested last year that purple life may be more likely than the green we are used to.

“Purple bacteria can thrive under a wide range of conditions, making it one of the primary contenders for life that could dominate a variety of worlds,” the team said in a statement at the time.

“We are just opening our eyes to these fascinating worlds around us,” they added. “Purple bacteria can survive and thrive under such a variety of conditions that it is easy to imagine that on many different worlds, purple may just be the new green.”

By studying life on our planet, and the environments of other worlds, we may be able to get a sense of what life may look like on an alien planet. According to what we know, it could follow similar mechanisms to life on Earth, but don’t expect it to look exactly the same. It being purple may be the least of the surprises the universe has in store for us.