The general characterisation we have developed has a philosophical or semantic benefit: it "defines" the subject matter of biology without circularity. But this is not its only advantage.
We have spoken about "patterns of gene expression" and we have made explicit reference to DNA, proteins and particular parts of cells. However, the living state as we have characterised it does not have to involve DNA or proteins. Our account has been abstract enough to avoid reliance on particulars. Anything that performed the same role as DNA could serve the same purpose: act as a "central library" in which the "documents" can be copied in a controllable way. The term "pattern of gene expression" could still be applied. Anything that could do what proteins do (i.e. be responsible for all the structures and all the activities of the cell!) would serve the same role as proteins. We could still talk about the various aspects of internal state and the mechanisms of response to stimuli. We know of no other material that could take the place of DNA or of proteins; but our characterisation of the living state could in principle apply to life on other planets irrespective of whether that life is DNA and protein based.
Moreover, our account could prove useful for discussing the origin of life on Earth. Conventional approaches to the origin of life are based on the argument about whether nucleic acids or proteins came first. Irrespective of the answer, they presume that increasingly complicated organisation led to the emergence of the first organisms. This might be so, but we can consider a logical alternative: that there were organisms before the first nucleic acids and proteins. A general characterisation of the living state allows us to explore this possibility, even if only to dismiss it.
These are topics for later chapters. What else have we gained so far? Perhaps what you have read in this book so far might enhance your appreciation of the natural world. The oak, the primrose, the beetle and the weasel each consist of countless cells. Each cell contrives, through the interplay of internal state, gene expression pattern and responses to stimuli, to fulfil its ordained part in the proper functioning of the whole organism. Fig. 10-1 applies to every cell in each of these organisms. Myriad microorganisms, invisible to the naked eye, house similar interplays, all described by the same general scheme. Working together, these organisms make up an ecosystem, exchanging and recycling energy and materials, controlling each other's population sizes.
In chapter 1 we said that deeper insight and understanding increases rather than decreases our awe and wonder at the natural world. This is true for us; we hope it might be true for our readers.
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