Well, astrobiologists don’t know what aliens might look like, because they haven’t observed any. But they do have an idea what conditions on other planets (“at least 100 billion in our galaxy alone”) might be like, and by assuming the principles of evolution and natural selection will apply, they can theorize what kinds of organisms might inhabit them.

Darwin’s aliens
Making predictions about aliens is not an easy task. Most previous work has focused on extrapolating from empirical observations and mechanistic understanding of physics, chemistry and biology. Another approach is to utilize theory to make predictions that are not tied to details of Earth. Here we show how evolutionary theory can be used to make predictions about aliens. We argue that aliens will 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. In particular, we can say something about how complexity will arise in space. Complexity has increased on the Earth as a result of a handful of events, known as the major transitions in individuality. Major transitions occur when groups of individuals come together to form a new higher level of the individual, such as when single-celled organisms evolved into multicellular organisms. Both theory and empirical data suggest that extreme conditions are required for major transitions to occur. We suggest that major transitions are likely to be the route to complexity on other planets, and that we should expect them to have been favoured by similarly restrictive conditions. Thus, we can make specific predictions about the biological makeup of complex aliens.

Cambridge University Press

Shouldn’t the abstract give us some idea of what those “specific predictions” are? Luckily, this is an open access article. There are some fun passages about why my hands are willing to help my testicals reproduce, rather than trying to reproduce themselves. This is because “their interests are aligned”, and my testicles have (or once had, at the risk of providing way too much information to the reader…) the potential to produce more hands at some point. Like a termite colony, I have gone through a transition to a complex organism, and my various parts are not out for themselves but are working together in a harmonious whole. (That’s the theory, although I’m not sure those testicles were aligned with my brain during my teenage years…)

So there might be simple alien life out there, in the form of single-celled viruses, fungi, bacteria, algae, or whatever. What we are really interested in is complex alien life, which would consist of “a nested hierarchy of entities, with the conditions required to eliminate conflict at each of those levels”. They could have complex individual bodies along the lines of ours, or some kind of creepy colonies made up of simpler organisms, more like ants or termites but with more tentacles. There are some pictures in the paper, have a look.

Universities have astrobiology departments, and it seems like a fun thing to study. You get to learn about biology and astronomy, combine theory and observation.