What is life, anyway? Despite how often it is used, biologists have had difficulty coming up with a coherent definition which would distinguish between life and non-life. The line between the two is not nearly so clear and easy to find as people often assume.
The fact that the line separating life from non-life isn't always clear is one of the reasons why objections to the idea that life might naturally develop out of non-life are misplaced. These objections, typically religious in origin, tend to assume that there is something "special" about life which creates an impenetrable barrier between it and non-life. People sarcastically ask how human beings could possible "evolve" out of rocks or mud.
Obviously the differences between humans and rocks are extreme, but this is not the comparison which should be made. If we are to determine what the essential differences between life and non-life are, we must travel to the other end of the spectrum — to "living" things which barely fit the definition of life and "non-living" things which almost appear to be life.
What sort of definition would include viruses, but not fire? What sort of definition would include sterile mules, but not self-replicating crystals? In The Fifth Miracle, Paul Davies offers a set of characteristics which tend to be found among living things:
- Autonomy
Reproduction
Metabolism
Nutrition
Complexity with Organization
Growth and Development
Information content
Hardware/software entanglement
Permanence and Change
Davies sees two as being crucial and fundamental: metabolism and reproduction, but even these aren’t as clear-cut as we might imagine.
Some bacteria can go into deep hibernation in rocks — with metabolism and of course reproduction ceasing — so we’d have trouble saying that those bacteria aren’t alive. If we were to take that position, then we would have to accept that a creature can move from a living to a non-living state and then back again, depending upon the conditions. Would this non-living state qualify as “dead” or something else?
