We are facing a complicated exercise of scientific speculation. Will they have two eyes or more? Nose and mouth surely, because from somewhere they must introduce food to stay alive and, perhaps, they have something similar to breathing. But how many noses and mouths will they have? We, in a blatant design flaw, eat and breathe through the same tube, but evolution on your planet need not have made such a mistake. By the way, the alien’s body will have to get rid of leftover food products, so something like an anus must also have.
The height of the possible alien is easier to predict, because both it and the size are determined by the gravity of the planet. The greater the gravity, the greater the sensation of weight . This implies that if human beings had appeared on a planet with 10 times the gravity of Earth, our bones would have to be thicker to support the weight of the body. But that works against us, because if they are larger, their mass will be greater and, therefore, we will be heavier. Therefore, our muscular system should be really powerful to be able to move, which also implies more mass. Then, in places of high gravity, there is no choice but to reduce the size if we want our imaginary ET to survive.
What can the theory of evolution tell us about what an ET would be like? In January 2011 the journal Philosophical Transactions A published a series of articles on extraterrestrial life. Among them was one from Cambridge University paleontologist Simon Conway Morris where he argued that with everything we know about evolution there are a number of restrictions to apply to what aliens look like.
On the one hand there is your biochemistry. It will be based on carbon and cannot differ too much from ours, since organic chemistry is the same everywhere in the universe ; the molecules may be different, but the underlying chemistry will be the same. The same can be said of the cells, which must have them. It is impossible to believe that the complex machinery of life does not need a closed and well-defined space. And its appearance? Here the thing is not so because in the definition of the body environmental issues come into play. Evolutionary novelties arise from ancient characters and chance must also be taken into account: mammals dominate the Earth because chance put an asteroid on a collision course with our planet.
According to Morris, although the appearance cannot be defined with propriety, they will have some characteristics of which we can be reasonably sure. One of them is bilateral symmetry because it is a simple way to build bodies . Radial and triple symmetries are more complicated, and on Earth they have never gone very far. Now, it is not ruled out that they may have a modular organization, like arthropods. A second characteristic is that, if we want them to be intelligent, they must have some free appendage to be able to manipulate the environment , and two is better than one. Another characteristic is a high metabolism, capable of generating the large amount of energy that the brain consumes (in humans it is 30%), which suggests that they are warm-blooded beings.
On the other hand, an intelligent being needs to have a good information storage system , a brain. Similarly, it must have sensors to be able to relate to the environment, similar to our eyes, ears and noses, and located close to the brain, so that information arrives quickly and can be processed quickly. Now, the range of vision can vary, but not much: maybe they see more in the infrared part of the spectrum, or maybe in the ultraviolet. Further below, for example in the wavelength of microwaves or radio waves, it would be impossible for purely physical reasons: they would need eyes the size of our satellite dishes. And you can’t go too far above ultraviolet either, because this radiation is lethal to the organic molecules of life; in the case of X-rays, being very penetrating, there would be no retina to stop them. Of course you must have two eyes: binocular vision is essential to perceive depth. What type? The best option is those of mammals, with a central lens. They could have compound eyes, like insects , but lacking a lens they would have to be too large to obtain a similar quality of vision. Another option would be the schizochroal, a type of compound eye made up of several lenses separated by a white membrane, the sclera. This type of eye was unique to the Phacopina , a suborder of the extinct trilobites, and appears to have been useful in low-light conditions. Will it work for worlds with low light intensity?
Morris thinks that life on planets similar to ours could evolve in a similar way to ours. To justify his opinion, he argues that on other planets life has been able to follow a convergent evolution, that is, that different species reach similar solutions independently . On our planet there are many examples of this, such as the eye, which is believed to have evolved more than fifty times. Thus, convergent evolution makes the appearance of proteins, eyes, limbs, and intelligence inevitable.
Now, not all scientists think that there are humanoids swarming through the universe, as if it were an episode of Star Trek; the diversity of life on Earth is enormous and that we humans look the way we do and that it is not intelligent spiders that use smartphones is just a matter of pure evolutionary fluke. “Why should they look like us? No other creature does, except apes,” says SETI Institute scientist Seth Shostak. The famous science fiction writer Arthur C. Clarke found humanoids on other planets to be a ridiculous idea: “Nowhere in the galaxy are there creatures that we would mistake for human beings, except on a very dark night.”
