These are creatures alive on planet Earth. And they show us just how different alien life could be from anything we know.
What Does a Martian Look Like?, also known as Evolving the Alien, is a book by biologist Jack Cohen and mathematician Ian Stewart about what life on Earth, in its surprising variety, allows us to guess about what alien life could (and could not) be like. However, it also takes a science fiction author's point of view for imagining the alien, and discusses a lot of the traditional tropes associated with aliens in fiction as well as suggesting more realistic alternatives.
Tropes used and discussed
- Artistic License Biology: Naturally, this gets called out a lot. For example, how did the xenomorphs evolve to be parasites to humans?
- Bizarre Alien Biology: Not only is it pointed out that any real alien biology we might find is going to be pretty unfamiliar, even though it may manifest some familiar universals; it's also pointed out that life on Earth already contains more oddities than some scientists (in the "astrobiologist" camp, as opposed to the "xenoscience" this book identifies with) are willing to allow for alien life.
- Human Aliens, Humanoid Aliens, Intelligent Gerbil, Rubber-Forehead Aliens: Unsurprisingly dismissed as unrealistic, but with thorough argumentation.
- Starfish Aliens: It's not even a question whether they're going to be weird. The main question is how weird they can be.