When speculating on what the future might look like in his 1927 book "Possible Worlds and Other Papers", geneticist and evolutionary biologist J. B. S. Haldane wrote "Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose." This sentiment encapsulates the expectations of many scientists engaged in the search for extraterrestrial life, in that we have to be careful in handling potential evidence, to avoid the possibility of not recognizing a valuable find when it presents itself — alien life may very well be truly alien, to the point where it might be unrecognizable.



However, a research team from the University of Oxford is suggesting that we also tackle the problem from the opposite viewpoint: what if evolutionary pressures on other planets has caused life there to appear more like ourselves than we might otherwise assume? In their paper, "Darwin’s Aliens", the Oxford team argues that, just like on Earth, biological processes on other planets are likely to "undergo natural selection – something that should not be taken for granted but that rests on firm theoretical grounds. Given aliens undergo natural selection we can say something about their evolution."



The paper builds on the idea that if complex organisms begin to form in an extraterrestrial environment, the structure the individual cells form will need to be "made-up of a hierarchy of entities, which all cooperate to produce an alien. At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation, and keep the organism functioning." The paper also offers a number of examples of what these organisms could look like, including a Lovecraftian tentacled septapod.



"We still can’t say whether aliens will walk on two legs or have big green eyes," the paper cautions. "But we believe evolutionary theory offers a unique additional tool for trying to understand what aliens will be like, and we have shown some examples of the kinds of strong predictions we can make with it."



Building on the ideas presented in "Darwin’s Aliens", we might look to our own evolution as humans to explain the otherwise humanoid appearance of entities from reported encounters: evolution on Earth appears to be centered around two main philosophies: redundancy and intelligence. Organisms that followed an evolutionary path that exploited redundancies in their biology survive through sheer numbers, such as individual creatures bearing multitudes offspring and/or multiple body parts, such as it is in the case of insects — basically, if there’s enough of something that’s comparatively vulnerable, a certain portion of it has to survive.



Intelligence, on the other hand, relies on evolving an organism that can outwit its environment, through avenues 
such as technological use, the benefits that a complex social structure can provide, or even simply being able to keep one step ahead of one’s pursuer or prey. This means that evolution will put a priority on evolving the brain in some species, but our gray matter is an energy-intensive computer-organ, meaning that over time unnecessary limbs that would otherwise take up precious brain power will be evolved out. And for many millions of years, creatures with four limbs have arguably made up the majority of the most intelligent species we know of.



In the case of humans, a species that’s taken brain evolution to very high degree, we’re down to the minimum number of limbs needed to survive in our environment: we’ve forsaken the ambulatory aspect of our forelimbs in favor of using them as a technological extension of our brains, and any fewer legs would not be practicable. It would be reasonable to assume that the bodies of another technologically-advanced species from another planet, provided they aren’t the intelligent descendents of slug-like gastropods or octopuses, would also follow a similar path toward a more efficient body that would better facilitate a more powerful brain — and wind up appearing as upright-walking bipeds much like ourselves.