What is the definition of life? I remember a conference of the scientific elite that sought to answer that question. Is an enzyme alive? Is a virus alive? Is a cell alive? After many hours of launching promising balloons that defined life in a sentence, followed by equally conclusive punctures of these balloons, a solution seemed at hand: "The ability to reproduce--that is the essential characteristic of life," said one statesman of science. Everyone nodded in agreement that the essential of a life was the ability to reproduce, until one small voice was heard. "Then one rabbit is dead. Two rabbits--a male and female--are alive but either one alone is dead." At that point, we all became convinced that although everyone knows what life is there is no simple definition of life.
If I were forced to rush in where angels fear to tread, I would offer "a living organism is an organized unit, which can carry out metabolic reactions, defend itself against injury, respond to stimuli, and has the capacity to be at least a partner in reproduction."
English: Phylogeni of living organism. Česky: Fyl...
Aphrodita aculeata (Sea mouse), taken at the Marin...
The Earth seen from Apollo 17.But I'm not happy with such a brief definition. When allowed more extensive reflection, however, I think the fundamental pillars on which life as we know it is based can be defined. By "pillars" I mean the essential principles--thermodynamic and kinetic--by which a living system operates. Current interest in discovering life in other galaxies and in recreating life in artificial systems indicates that it would be desirable to elucidate those pillars, their operation, and why they are essential to life. In this essay, I will refer to the particular mechanisms by which those principles are implemented in life on Earth, while reserving the right to suggest that there may be other mechanisms to implement the principles. If I were in...