I am making my way at a sedate pace through NASA astrobiologist Peter D. Ward's book Life as We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life. Astrobiology is the real-world version of what science fiction fans have sometimes called “xenobiology.” Though “astrobiology” means “the study of star life,” Ward doesn't limit the possibility of alien life to other stars or even to other planets. We may not only find alien life on Mars, but even on earth.
The scientific definition of life — little more than an intellectual exercise for most biologists, since they know life when they see it — is the most fundamental problem in astrobiology. We have to know what we are looking for if we are to recognize it.
Ward discusses the history of defining life. Some famous scientists have addressed this question, including more physicists than biologists. One of the first books was What Is Life? The Physical Aspect of the Living Cell by Erwin Schrödinger (yes, the guy who both killed and didn't kill theoretical cats). Ward extensively discusses the criteria developed by Paul Davies, another famous physicist. Davies' book The Fifth Miracle: The Search for the Origin and Meaning of Life lists six qualities that could be expected in any life form: metabolism, organization, development, autonomy, the ability to reproduce, and the ability to evolve. Here are a few thoughts on these qualities.