The very first concept a child learns in science is usually the distinction between living and nonliving things. The seven characteristics that define life are movement, nutrition, growth, respiration, excretion, reproduction, and response to stimuli (as stated in several school textbooks). As a biology teacher, this is first concept that I am expected to teach. As a biologist, this is one concept I am yet to understand!
If we go by this set of necessary features that characterize an entity as 'living', then what about those who cannot reproduce. Let alone the sterile plants that grow in my garden, what about those human beings who are biologically unable to give birth to a little one? Are they not living? And what about those lying in a coma who have lost most abilities to respond to external stimuli? Are they nonliving?
Maybe I understood it wrong; perhaps it is that not all these characteristics are essential to define an entity as living. It is just that these are the characteristics that are unique to living beings and the presence of one or more of them in an entity mark it as living. However, if that is the case, then the old Honda Dio in my house that drinks fuel like crazy (nutrition), utilizes that fuel for energy (respiration), uses the energy to move (movement), and coughs up exhaust fumes like a forest set afire (excretion) is a fully competent living being!
Computer viruses that can replicate, stars in our ever expanding universe that grow, and the new AC with 'intelligent sensors' that Katrina endorses these days, all appear to share characteristics of a 'living being'.
So then, what is 'LIFE'? The more I read the more I have begun to realize that the concept of life is one that baffles both philosophers and scientists. At which point did the first organism supposedly evolve from inanimate matter? What is it truly that distinguishes us from nonliving things? If we discover a colloidal mass of molecules on mars (let’s say) that trap solar energy and convert it into another form of energy, would we say we have discovered life?
Many scientists have come up with varied theories and definitions each trying in their own way to identify a distinct basic property to define life. Current theories identify cellular nature, heredity and evolution as some of the components that define life. Physicists too, are trying to make sense of the complex system using their understanding of the physical world. Theories of emergent behavior and self-assembly are some of the many examples. But as someone who dabbles in molecular biology, again, all these theories only indicate chemistry. When did this chemistry evolve into 'bio'? Honestly, I am not desperate for an answer at the moment.
I just wish I knew how I could, in all justice, make a third grader understand – what defines life.