Greetings, Carbon-based Bipeds!
Most of us picture alien life the way it's portrayed in movies, where aliens are commonly depicted as human-like forms because they use actors either to play the roles directly in make-up or to be models for computer-generated animation. Also, audiences relate to human-like aliens better than to more exotic, monster-like creatures. However, the human body plan -- bilateral symmetry with one head, two legs and two arms -- stems from when early amphibians and reptiles colonized the Earth's land masses, and it seems unlikely that such a shape would evolve on an alien world. So, let's forget Hollywood for the moment and look closely at the real science of astrobiology.
Astrobiology is the scientific study of life in the universe. Astrobiologists seek to understand (among other things) how life arose and evolved on Earth, what governs the way life is organized and what makes a planet habitable.
Astrobiology combines the disciplines of biology, chemistry, physics, geology and astronomy. Often, astrobiologists must use the information learned about life on Earth as a guide for studying life elsewhere. Let's examine some of the things that we have learned from life on Earth.