Schrödinger asks, in What is Life? The Physical Aspect of the Living Cell’s chapter Living Matter Evades the Decay to Equilibrium, “What is the characteristic feature of life? When is a piece of matter said to be alive?” Few would claim that a single atom is alive, or even a single molecule. It is necessary, then, when observing collections of molecules, to discern what structure and activity is present to warrant the claim of life.
The systems that we call alive are ordered in a way that is distinct from systems that we do not call alive. A periodic crystal is highly ordered, a sometimes perfect tessellation of its structural pattern, but it doesn’t act in a way that we would correspond with life. A rock does not act in such a fashion either.
We look at lions and we see motion, we see intent to devour. They can be an active threat to us. We look at cows and come to understand that they eat and can be eaten, providing sustenance. Though their motion is achingly slow, we observe similar traits in plants—predation, the possibility of sustenance, etc. Even the ‘intent’ to grow towards the light. No such action do we observe in things we would not call alive, such as a rock.
It makes sense why this distinction (life versus non-life) would be useful to us. A rock on the ground need not pose a threat in the way a prowling lion certainly would, and it cannot provide sustenance in the way a juicy flank or orange can. We observe a whole class of entities that seem to share many aspects of action, and we begin to delineate them based on structure and operation, giving us categories of animal, plant, fungus, etc. As our awareness and knowledge of our environment expands, we uncover more entities, often with novel aspects, yet we still fit them within our categories with relative ease.
So it is that we come to a set of ‘requirements’ for what is life. Self-replication, growth, adaptation, etc. Schrödinger poses the question: “How does the living organism avoid decay? The obvious answer is: By eating, drinking, breathing and (in the case of plants) assimilating. The technical term is metabolism.” If we were to find an entity that possesses only some of these aspects, many people would claim that such an entity was “not alive” because it did not take part in every aspect of our definition. However, how is this intellectually honest? We observe many entities, and from the gamut of our experience, we create a delineation informed by the aspects that we observe in the entities. If we come to a new entity that does not share in all of those aspects, we say it is not alive. We anticipate nature when we create a definition and then apply it to nature, rather than letting nature inform the definition. In this example, the virus is an entity that does not take part in every aspect of our definition of life. It does not metabolize. Yet it reproduces and adapts. However, these aspects are not where we should look. To gain a sense for the relatedness of entities, emergent operation should not be the rubric. Instead the structure and how it produces the observed actions should be the rubric for relatedness.
We possess a general understanding of the presence of nucleic acids in ‘living’ entities and how their operation culminates in the larger action of the ‘living’ body. We say that many aspects of the entity result from the action of its nucleic acids, which in turn result from the structure of the nucleic acids themselves.
We look at viruses and come to understand that their action is a result of the presence of nucleic acids with a genome that specifically determines their cycle of action. The whole species of nucleic acids, spanning plants, animals, microorganisms, and viruses, act similarly because they are of a like structure. We came up with a definition for life that was not informed by all available entities, and when we observe an entity that falls outside the definition, we discard its plausible inclusion into the hallowed ranks of life. This is not an honest way of defining. What even is defining, with regard to Nature? Are some delineations more valid than others? Surely this is so. We can observe the properties of a gram of mercury versus a gram of argon and be able to delineate them. The definition of life, on the other hand, is more suspect. For millennia, every time we found a new animal, it appeared and acted similarly enough to animals already held as alive to be itself considered alive. Such is the case with plants and fungi. These macroscopic entities shared a like nucleic structure, which in turn determined their like macroscopic structure and action, which was the basis of their being lumped into the same category of ‘living things’.
Once we became able to observe scales previously invisible to us, we found entities (microorganisms: bacteria, protists, etc.) that shared like action with macroscopic entities. This enabled their quick inclusion in to the ranks of living things. However, once we came to viruses, their unlike action (in some respects, namely their lack of growth and metabolism) had us claiming that they could not be alive because they did not share in all the characteristics seen elsewhere. Yet when we looked, we came to know that they possessed similar nucleic acids, and their nucleic operation was nigh identical to macroscopic entities. We came to understand that every entity we called alive shared the same foundation of operation, the nucleic acid. The fact that the operation of nucleic acids can produce an entity that does not need to metabolize is grounds to dissolve the definition for life.
What do we want a definition for, to begin with? We wish to delineate thing from thing, to find the basis of similarity and difference among what we observe. We can now look at all of ‘life’ around us as the product of the long-term evolution of nucleic acids. It’s astonishing how variable this species of molecule is. Its successful self-replication for billions of years has produced structures from viruses to aspens, multi-celled organisms from which a single cell can be separated and grown in isolation, massive colonies of insects that function cohesively, patterns of intelligence emerging from the summation of simple parts.
It’s possible that in the future we could find things that would externally appear “alive” as we recognize it today, whose structure does not utilize nucleic acids. However be the structure of their functioning, if it was locally anti-entropic and could self-replicate, we would still be able to distinguish them from our earthy ‘living things’ because their structure was distinguishable.
By relinquishing our definition for life that was created prior to our larger knowledge of the operation of ‘living’ entities, our understanding of the world around us can become fully a result of what we observe, rather than our applying ideas past their plausible relevance.
The distinction between rock and giraffe is a real one, and we now exist in a world where we are acutely aware of what makes them different. Gone are the days when the only observations we had were macroscopic. Understanding the physicochemical makeup of our objects of interest directly informs a more exhaustive understanding of their macroscopic action. The distinction lies in their makeup, not in their large-scale action. Convergent evolution produces similar large-scale actions, but the nucleus of similarity is derived from the hereditary chain, and the hereditary chain lies in the evolution of nucleic acids.