What is life?
What distinguishes something which is alive from that which is not?
How can we explain all life to a hypothetical being unfamiliar with our categorization paradigm?
These questions have often bothered me, and I decided I would share them with you. We define life (most of us, anyway) through cell theory. The idea that cells are the basic unit of every living thing. Where, though, does one draw the line between “cell” and “organized collection of molecules.” For example, are mitochondria independent cells? They have their own metabolism, DNA, replicate independently of the cells in which they live, and interact with the cell in a variety of ways. They are categorized as a “part of a cell” rather than an independent cell. Chloroplasts are similar in this sense. Viruses are a particularly nuanced problem as some viruses (Mimivirus comes to mind) can be larger than the smallest bacterial cells (Mycoplasma). Mimivirus, for example, only requires ribosomes and energy metabolism of the host cell, providing all the necessary DNA templates for all proteins required for it to replicate.
These problems arise from the fact that biology rarely fits into neat little categories. Strict logical definitions of “life” or “cell” or “organism” don’t fit neatly. Is a biofilm a single organism or a collection of organisms? What about Myxococcus xanthus swarms?
Ultimately, I do not have an answer to this question. In fact, I think I have more questions than I started out with.
“What is life?”
“What is a cell?”
“Is a bacterial spore a cell?”
“What is the threshold level of cooperation to be considered multicellular?”
“How can we go about organizing a system which removes this ambiguity?”