Life! You know it when you point at it but you find it difficult to say just what it is. I am going to try to pin down the characteristics of an entity that make you say "That's alive.". I want to look at other systems that have similar properties but which we do not normally think of as alive. Then I will look at the differences between these systems and systems which we think of as alive.
In my next post I will look at things which are described as life but are arguably not. These are mostly activities which are dependent on life.
What I am doing in this article is trying to clarify my own thinking on the subject. I hope that readers might find my ideas worthy of contemplation and discussion.
The first question about life is "Is it a process or an entity, an activity or a thing?". Is life a substance or force permeating living beings which is not present in non living entities? Or is life the activities that go on in living beings?
Well, the way you know if something is alive is by what it does. If it is inactive and unchanging one identifies it as being dead. This suggests that one should think of life as what goes on in a living being. One should only propose the existence of a life force if there is no other way to explain what is going on.
In the 1950s and 1960s we uncovered the physical bases of heredity and the energy transport mechanisms. Inheritance turned out to be coded on the nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Energy was found to be transported by adenosine triphosphate (ATP). While much of the mechanism of life is not understood, there seems to be no reason to believe that any unknown principles of physics or unknown forces are involved.
(An aside. In the nineteenth century developments in biology and geology did suggest that there were energy sources unknown to contemporary physics. Evolution in biology and uniformitarianism in geology required a vast age for the Earth. Known energy sources could not explain how the internal heat of the Earth or the heat of the Sun was maintained over such a long period. This problem was solved with the discovery of nuclear energy.)
Life is a property of a system not of any individual part of a system. You cannot point to a molecule in a cell and say "That molecule is alive.". A protein or lipid or carbohydrate molecule is the same inside or outside a living cell. One cannot look at any particular chemical reaction and say "This is life.".
OK. So what is it about a system that has us saying "This cell or this organism is alive."?
One answer I have seen is reproduction. However this by itself is not it. A sterile organism can still be alive. A worker bee is not dead. Neither is a pet that you have had sterilized.
The best candidate that I have seen is autopoiesis. This is self maintenance. An autopoietic process brings about its own continuation and the maintenance of the structure in which the process occurs. An autopoietic system replaces its components with new ones that it has created. This description fits cells and it also fits organisms. However it can be argued that it also fits organizations such as corporations, states and ant nests. These are not exactly what one thinks of when one talks about life.
I think we can say that all living systems are autopoietic but not all autopoietic systems are alive. Systems that we describe as alive are self-contained bounded physical systems rather than systems linked by a network of concepts. Thus life is a property of a definite physical body whose individual components were created by that body. I am not sure just what properties as well as autopoiesis are required for a system to be described as alive.
Reproduction of living systems uses the mechanisms of autopoiesis. New organisms are created by the living organism using components that it has created and organized into another autopoietic system. However a self replicating system does not have to be autopoietic. It can simply create copies of itself without maintaining itself or doing anything much except replication. It can also replicate itself by taking over another system and making that other system replicate it. Examples of this behavior are the activities of viruses and prions.
Living organisms do not live forever. To allow a kind of life to persist some members of a kind have to be able to reproduce that kind. Thus self-replication is a property of all entities that we normally think of as life forms. However not all members of a kind have to be able to reproduce. An organism can produce both sterile and fertile offspring. Bees and ants provide good examples of this.
Living cells are the most obvious examples of living systems. They are elaborate structures made up of organic matter, water and inorganic materials (mostly as ions in solution). They are maintained and replicated by physical and chemical interactions among the components.
However the components of an autopoietic system do not have to be organic molecules or organic structures that are not alive by themselves. (A cell membrane is a universal part of all known living cells. Isolated from a cell it cannot be described as alive.)
The components of an autopoietic can themselves be autopoietic systems. This is the case in multi-cellular animals, plants and fungi. These originated as multi-cellular colonies. The cells became differentiated into many types and organized into structures in new autpoetic systems. The cells can be isolated from an organism and grown by themselves in unorganized tissue cultures. The culture is not an autopoietic system though the individual cells that it is made up of are. A mouse is autopoietic system and the cells that it is made up of are also autopoietic systems.
Thus life can be recursive. It can exist on multiple levels. The life of a mouse is not the life of its cells though it depends on the life of the cells. One can make a case for even higher levels of autopoietic organization. For example a bluebottle (Portuguese man o'war) is a colony of multicellular polyps. There are several specialized types of polyp in a colony and an overall structure.
If autopoietic systems organize into a higher level system the individual components can loose the ability to maintain themselves. They can become dependent on their interactions with the other formerly independent systems. That is, living organisms can organize into a higher level living system and the lower level of life can then be subsumed into the higher level of life. This appears to be what happened when eucaryotic cells were formed. (Life can be divided in procaryotic and eucaryotic organisms. Procaryotes have relatively simple cell internal structures. Eucaryotes have elaborate membrane bound cell internal structures. Procaryotes are the bacteria. The eucaryotes are the protists, the animals, the plants and the fungi.) The eucaryotic cell probably formed as a symbiotic association of several different types of bacteria. Examples of cell organelles that were once independent organisms are mitochondria, plastids, flagella and cilia and the nucleus. I would not regard these as being autopoietic systems in their own right any longer.
Another possible example of a higher level autopoietic system composed of autopoietic sub-systems is the biosphere of the Earth as a whole. The Gaia hypothesis regards the entire biosphere as a self regulating system. The hypothesis is that action of living organisms maintains the planet in a condition suitable for life. A system of feedback loops stabilizes the climate and and atmospheric composition etc. The main components of the system are bacteria. Other living organisms, the atmosphere, the hydrosphere and the outer layers of of the lithosphere could also be regarded as components of the system. Under this hypothesis the Earth would be an example of an autopoietic system that does not reproduce. The Gaia hypothesis is sometimes criticized as being a fuzzy mystical approach. I see it as a useful way of looking at a complex of feedback loops. It is a plausible, probably correct but unproven hypothesis.
All the known examples of life ultimately depend on aqueous organic chemistry. In principle this does not have to be the case. One sees many examples in science fiction of other kinds of life. Life with liquid ammonia or other solvents instead of water, life based on silicon rather than carbon compounds, metallic life forms, life based on nuclear reaction and so on. Some of these alternatives could be possible. Some probably aren't. But this is a problem for another generation to solve.
Most of the material in this article is not original. I suspect that many of the original ideas have also occurred to others. The assembly, organization and assessment is mine.