Excuse the semi-random title, but, this question has been bugging me for a while. Ever since I was a kid and saw movies about space and aliens, I asked the question, “would real aliens really look like that?” Let’s face it, the mainstream film and art culture tend to portray aliens as humanoid life forms. How much of a humanoid form can be drawn from reasonable, educated guesses? And how much of it is just plain fantasy?
NOTE: I did not do any real research before writing this post, and am making educated guesses based off the top of my head. If you are an expert at any of the stuff (astrobiology in particular) please critique this post in the comments!
Before we examine what intelligent aliens could look like, let’s first look at the most basic conditions of where life can arise, if at all. There are some very intelligent guesses as to the cosmic ingredients of life. Let us examine each ingredient in turn.
The habitable zone
All life, as far as we know, have 1 function: change one form of energy into another. There must be a steady supply of energy that can be consumed. For us, it’s the Sun, mostly, although there is also life at the ocean depths that draw energy from thermal vents (and even these vents draw their energy from the heat of the Earth’s molten core, which is also due to radioactive decay). I think it’s safe to say that there must be either a good source of steady radiation for there to be life. Now, the best way to get a steady source of radioactive energy is by orbiting a star. Stars, if of the right size, have very good lifepsans (in the order of billions of years!). But orbiting a star also has another tremendous benefit: you get to stay at a stable location in space. If there was no Sun to orbit, the Earth would be hurling across space in some random direction (and it would be frozen over rather quickly).
So, there needs to be a sun-like star. The next thing necessary for life is probably a planet-sized blob of rock with some water and an atmosphere. The planet-scale size, water, and atmosphere really go hand-in-hand and can’t exist meaningfully without the other. Let us start with water. Water is necessary for life because it is the best chemically neutral “solvent” where many different chemical reactions can take place freely. Water also has the special property of being less dense as a solid than as a liquid, which keeps it from freezing over too easily; e.g., the icebergs of the Arctic float around, and melt, when they reach warmer waters — if the ice was to simply sink the moment it froze over, such melting processes would never occur. The rocky composition of the planet is required because it provides the only way to keep the water in a stable place (what we call oceans).
Now, water itself is a very precious substance — if you are designing a planet, you’d need a way to stop water from evaporating away into space. This is because the radioactive rays from the nearby sun could slowly “boil” the water, one molecule at a time, away. So, first, you’d need some sort of protective shield around the planet to keep the water safe. The Earth uses a magnetic field to do this, and thankfully the Earth is large enough to generate a strong enough magnetic field. This is the reason why our alien world would need to be planet-sized. Also, a planet can, by virtue of its size, retain most of the water with its gravitational field; if water molecules turn into separete Hydrogen and Oxygen gas moledules, their escape into outer space could be slowed down significantly, so that such evaporation would take many millions (or billions?) of years.
So we’ve established that you need water, and a planet to keep it. What about atmosphere? Well, the atmosphere plays a very important role on Earth: it shields us from too much radiation. Yes, the Earth’s magnetic field protects us from harmful radiation, but not all of it. You still get a lot of radiation from the sun itself. For us on Earth, the ozone layer plays a big role in protecting us from direct radiation. And so it is with our hypothetical alien world: it, too would need an atmosphere of some sort (and plus, if you have large bodies of water (oceans) with an atmosphere, you could get a water cycle, so that the water gets spread across into dry land, to allow for land-dwelling organisms).
Sun, rocky planet, water, and atmosphere. OK. Now comes I think probably the most controversial point: the intelligent alien would be a land-based animal. To support this hypothesis, I will first get rid of the other two alternatives: flying (winged) animals and marine (ocean-dwelling) animals. First, why can’t winged animals become intelligent? My guess is that intelligence evolved from the ability to manipulate nature in accurate, reproducible ways — i.e., we could create tools with our hands, and this separated the stupid and the less-stupid. Flying animals, by the laws of physics that govern our universe, cannot be too large. In fact, the smaller the better. This is why most flying organisms are insects. Also, because nature strives for brutal simplicity whenever it can, chances are that if you find a winged creature, it will only have wings, not wings and arms. And if you only have wings, then, you won’t be able to grab things in the accurate manner required to create tools.
Well, what about marine animals? Well, the problem is basically the same as that of winged animals: you will have fins, not arms or hands, because you need to swim. Dolphins of the future could have IQs that are 5000x higher than us, but without a means to manipulate nature around them with precision instruments (e.g., hands, opposable thumb, etc.), they will be forever doomed to the same patterns of behavior as their 50 IQ ancestors. If you think about it a marine animals are just like winged animals, except that their “wind” is the water. And as for land-dwelling marine animals, I also think that this category cannot produce any intelligent life, because of the following reason: the presence of water shortens the distance between all potential predators and prey. You need as big a distance as possible between you and the predator to survive as an intelligent life form, because this is the only way you can show Nature your intelligent decision-making skills and collect enough evolution points. If sharks around a 2-mile radius can sense your presence, and there are no trees to “climb up” to to avoid them, there is very, very little time to make any decisions. This is probably why the dominant evolutionary design of underwater land dwellers (other than fish) are crustaceans, with their natural armored shells. This short predator-prey distance, or PPD, explains why dolphins cannot truly sleep like we do.
Sense organs and appearance
So that leaves us with land-dwelling animals. We can take all the clues on our own planet to make some very good guesses in this final category. My only lament is that these guesses make the hypothetical intelligent alien very, very boring (and very humanoid). First, our alien must have limbs. All land-dwelling creatures on Earth have limbs, because limbs provide the best way to move about across a hard surface inside a low-density medium (the atmosphere). Let’s throw in hands here as well, because hands, as stated above, are the best natural tools to manipulate Nature in an accurate way. Since the two-hand, two-feet design is probably the best (and simplest) way to have hands and limbs for mobility, we’ll just adopt the “two hands, two feet” design. Next, the alien must have eyes. Eyes are one of the most primitive and basic organs (many bacteria have eyes), and provide the best bang-for-the-buck in terms of the information it can gather. Light is the fastest medium of information, and also the most prevalent (the alien Sun would provide a constant stream of light, as would any moons orbiting the planet). Light allowed our own human ancestors to maximize their PPD, because they could spot predators a mile away by just their color (this is impossible to do in the ocean), and it also allowed early hunter humans to communicate with abstract symbols to each other in a manner that only they, not the prey, could understand (what we now call hand signals) to maximize their intelligent decision making skills.
Hands, feet, and eyes. OK. What about a mouth, and nose? Well the mouth is certainly mandatory: how else can you eat food? There must be at least one opening for the nutrients to enter, and so there must be at least one mouth. As for the nose, for us humans it allows us to keep a well-salivated mouth, because we can breathe through our nose (which is dry by default); if we had to always breathe through our mouths, we would have a much harder time keeping our saliva flowing and ready to eat a meal when we chanced upon it. Our alien’s mouth would also have to be salivated with some chemicals to help it chew things and swallow the fine pieces as a single whole (imagine swallowing dry leaves with zero saliva… not pleasant). The nose also functions as a primitive poison detector (e.g., rotting food); if a species had to use their mouth to test something out every time they sat down for a meal, it would die out pretty fast.
The alien will also have ears, probably. This is because they allow the organism to sense things when the eyes can’t (e.g., at night, for example). Awareness of the environment is 99.9% of the evolution game, so anything to maximize information-gathering senses helps to keep the species from going extinct.
Eyes, mouth, nose, and ears. Amazing! Our alien probably has all of these things. Now comes an interesting question: does the alien have a face like we do? Chances are, I think, yes. First, the mouth has to go at the very bottom, because this is the only way of keeping the eyes above it. Why do the eyes need to placed above the mouth? It’s because of gravity: the alien, like us, we will pick up food from the ground (a freshly killed prey animal, any collection of food from multiple sources, etc.) more than from trees or other high places. Also, water will almost always be at ground-level. Having the eyes above the mouth allows us to look out for potential threats or opportunities while we eat, simultaneously. The alien nose will also probably find a spot between the eyes and mouth, because it can’t be below the mouth or above the eyes. If below the mouth, then it would simply get in the way when eating. The nose can’t be above the eyes because of gravity again: anything expelled from the nose would hinder the eyes. So the nose goes between the eyes and mouth. As for the eyes themselves, there would be at least two of them, because having two eyes provides highly accurate depth perception, and also acts as a primitive form of “insurance”: you can lose one eye but still go on your way. Lastly, the ears are located probably on the sides, on opposite poles, because that’s the best way to capture as many sound waves as possible from different directions. Since our alien has a “face”, the ears would have to go on a different axis — the simplest would be to have the ears on the left and right, not the front (face) and back.
Our hypothetical alien probably as a face and ears like ours. But does it also have a brain behind the face, on a “head”? Could it instead have a face and brain in the “torso”, if it has one? Well, I think the alien would also have a brain inside a “head” with a face, much like ours. Why? First, keeping the bulk of the sensory organs (eyes, nose, mouth, ears) in one place is great because it allows us to protect them from harmful threats in the most efficient way. If we had our sensory organs spread apart everywhere, it would be very difficult to protect them all at once. If we just cover our heads with our hands and arms, we can protect almost all of our important sensory organs quite effectively. By covering your face alone with one hand, you protect your eyes, nose, and mouth — impressive, don’t you think? Thus, the alien will also have a head for the face. Having a head provides another benefit: it allows maximum use of the other limbs and torso — you can put them underwater, inside mud, or whatever, and it has no detrimental effect on your main senses (eyes, ears, etc.).
Now, what about the brain? Is it also inside the head of the alien? Most likely, yes. This is because the center “torso” area of the alien will already be filled up with older, more primitive organs such as a heart, digestive organs, etc. And, chances are that the brain of its more stupid ancestors would have been very close to its major sensory organs such as its eyes, for reasons of simplicity (why have extra-long neurons that go from the eyes to some far-flung brain when you can have short neurons with a brain close by?).
My conclusion is that intelligent aliens will look very humanoid in form. And chances are, their Earth will look very similar to ours, with an atmosphere and oceans. I don’t think this prospect is boring at all. Just imagine — they too will have a word for “water”, and “Sun” and “Earth”, as well as “face”, “eyes”, etc. The more you think about it, the more fascinating it gets.
The only unpleasant part about it is that, if we ever do meet such aliens, they will freak us out with their warped humanlike appearance. But hey, at least the freakout will go both ways, right?