Aliens Might Evolve Into Jabba The Hutt Covered In Anuses
The biggest assumption we make about aliens is that they'll look at least vaguely like us. Part of that is convenient shorthand; it would be hard to bond with a fictional alien if it had 27 ear-penis hybrids protruding from its giant slug body. But we also figure that we look like us, and we turned out OK, so clearly this is a good evolutionary route to take.
But that forces us to make a lot of other assumptions about the conditions of an alien planet and the chemical building blocks present there. If aliens exist, there's no guarantee that they're living on a planet exactly (or even vaguely) like ours. What is safe to assume is that aliens were subjected to the same laws of natural selection that we were. If you're not adequately built for the salt storms of Xipglor, then get the hell out of the Morplok.
Humans, if you'll forgive the casual existential horror, are just giant masses of cells working together. And human evolution had a few key stages when that work got more complicated. Single-celled organisms became multi-celled organisms, multi-celled organisms saw increasing numbers of parts performing an increasing number of functions in tandem, and eventually all of that cooperation produced Chad, your neighbor who insists that everyone in the African country shares his name. The process that produces that complexity is what we should be looking for in intelligent life, regardless of the resulting physical appearance. And that means real first contact might be Buzz Astronaut going for an uncertain hand-to-maggot-penis shake with the butthole cactus here:
Helen Cooper/Cambridge Univ. PressWe imagine Captain Kirk would've spread a lot less love between worlds if any of those green women had looked like this.