First of all, it is important to understand that life is not something on its own. There is no system, matter, energy or force we can call life. The only reason we consider a system alive is because it behaves in a certain way. If there is no living system, there can be no life, since it is the behavior of the system which makes it alive. So when an organism dies, it is because its structure is altered in such a way that the behavior required for life is no longer possible, not because something is removed or leaves the system.
Life cannot be transferred from one living system to a non-living system because, for a system to behave in a certain way, it requires a specific prebuilt structure and information code which controls its behavior. If an identical copy of a systemís structure is copied in such a way that the copy starts behaving exactly like the original, then a new living system is built (a clone), life is not transferred.
Therefore, the real question here is not what life is but what does a system do to be considered alive? What is this behavior we call life? This is actually not a hard question to answer. The reason it has been difficult to answer in the past is because of our ďbiocentrismĒ, in other words, believing that features found in cellular life are what define life just because we are made of cells. But all those features have evolved with time, after the first proto-cells appeared; they are not what is really required to be alive. Let us analyze some of those features which have been considered essential for life and see if they are actually indispensable or just random evolutionary adaptations to the environment:
Metabolism: is a process by which living systems manipulate energy and resources to remain alive. Some kind of metabolism must be required for life, but also any process could work, as long as it fulfills the same main objective.
Homeostasis: is the ability of living organisms to preserve their life through internal regulatory mechanisms after the external environment threatens to change the system's internal conditions. Homeostasis also seems to be a requirement for life because living systems have to preserve their internal environment if they want to stay alive.
Response to stimuli: is a method used by living systems to react to their changing environment when it threatens their life. As with homeostasis, organisms need to respond when their external environment changes if they want to remain alive. The difference is that homeostasis works internally while response to stimuli works externally.
Complexity: all living systems must have a certain degree of complexity, but we do not really know how complex they must be to be able to remain alive since this complexity depends on the environmental conditions disturbing the system. The harsher the environment, the harder it will be to stay alive and, as a result, complexity increases.
Reproduction: is a method used by some organisms to build one or several new separate individuals with their same structure and information code when the original organism will not be able to stay alive much longer. If an organism finds a way of not dying, it might not need to reproduce anymore, so reproduction is not essential for life. Besides, if an organism is born sterile we still consider it alive. Even a cell that can no longer divide, as long as it can metabolize and move, is still living.
Replication: is the process by which an organism creates more of the structure he is built from, to develop, grow or repair damage. It is important to make a distinction between reproduction and replication. When we grow, or a wound is healed in our body, our cells are actually replicating, not reproducing, since they are not separate organisms but part of a multicellular organism, but when bacteria divide into daughter cells, they are reproducing because a colony is not a multicellular organism. When a baby is born and it becomes a separate being, we are reproducing but if a new born human becomes part of a connected meta-organism like the Borg in Star Trek TNG, and they cannot separate from each other because they die, just like the cells in our body, then these humans will no longer be reproducing but would be replicating.
Death: is just a consequence of harsh environmental conditions and, when genetically preprogrammed, it comes as a side effect of reproduction to favor the survival of the offspring in exchange for the life of the parent. Since death is the termination of life, the purpose of life is actually to avoid death so any behavior that ends up in death is definitely a destructive behavior, not a requirement for life.
Evolution: is how a group of organisms Ė a species Ė adapts to their environment through mutations, reproduction and death. Evolution does not work at the individual level; species evolve, not organisms, but it is the organism which is alive. Furthermore, if a species stops evolving because it has reached an optimal state of adaptation to its environment, their members are not considered dead. Therefore, evolution is not a requirement for life but a consequence of death, which in turn favored reproduction as the only way to preserve life, and mutations which inserted randomness into the system to create better or worst adapting behavior.
Development: is how living systems adapt to their environment through structural change. Most organisms which reproduce also develop to be able to reach a mature state after being born. If an organism has reached, or was born with an optimal adaptation to its environment, it would not have to develop but would still be considered alive, thus, development is not a requirement for life. It is the only way, though, for an individual organism to improve its adaptation. Development can be considered, in a way, an organismís structural evolution by means of replication instead of reproduction. It is directed, however, by the organismís information code, which makes it more predictable than evolution.
Growth: is actually a kind of development to improve size, strength or complexity when the environment demands it. It is also the way organisms which reproduce can reach optimal adaptability or maturity after being born. Again, if a living system already has an optimal size, strength and complexity, growing is not needed to remain alive, thus, growth is not a requirement for life.
Therefore, there are two ways for an organism to adapt: by evolving (which requires mutation, reproduction and death, and only works at the species level) and by developing (which can be done at the individual level, does not require mutation, reproduction or death, works by replication and can be programmed).
The reason we associate all these features with life is because life is required in order to have these features, not because we need these features to be alive. We cannot have reproduction, replication and death if we do not already have a living system. These features came as a consequence of life, a side effect. The same with evolution and development; we need reproduction and death in order to have evolution, and we need life in order to have reproduction and death, so life is a requirement for evolution, not the other way around. Adding any of these features as part of the definition of life is arbitrary: take a living organism, see what features it has and assume they are required for life. This makes no sense, because, if we require life for these features to exist, then some kind of living system had to exist before these features were developed or evolved for the first time.
So, from this list, we can conclude that the only features that are essential for living systems to remain alive are: any number of homeostatic processes which regulate the system internally and depend on how the system behaves in the first place, some kind of metabolism to obtain the energy and resources needed to operate, but not necessarily at the chemical or molecular level (weíll see why later), and response to stimuli which depends on the environmental forces acting on the system. If we do not take into account the environment, then we can define a closed living system as having the following two requirements to remain alive:
∑ First, living systems are information systems, meaning that they have an information code which controls their behavior;
∑ Second, their behavior allows them to preserve their information code.
A Closed Living System
Thatís really it! Life is a homeostatic autopoietic system and nothing else is needed, so we could say that life is just any behavior that is capable of preserving the information code which controls that same behavior. But we could also say that life is that information code which controls a behavior that is capable of preserving the information code. Then, is life the behavior or is it the information code? I believe it is the behavior, and there is a simple way to prove it: prevent the information code from preserving itself and the system dies! Even if the information code is still present, the behavior we call life is no longer there. On the other hand, alter the information code and, if the system is still capable of preserving itself, it will remain alive. Altering the information code is even required for the system to evolve but it is also not a requirement for it to be alive. So we can say that a living system requires both the information code and the behavior, but what we call life is that behavior which is capable of preserving the living system, because when the system can no longer preserve itself, it dies.
This takes us to the next feature missing from our simple closed system definition of life: the environment. Why is it that a living system needs to be able to preserve itself? We could have the information code producing some behavior and we could say that the system is alive. That is theoretically true, but the universe does not work that way. We have entropy, and the second law of thermodynamics states that any system that produces work will increase its entropy and, therefore, its energy will be dissipated in the form of heat because heat will always move from hotter regions to colder regions. So if an information code produces a behavior, this behavior will increase the systemís entropy and heat will be generated. This heat will flow out of the system making its total energy decrease. For the system to continue working, more energy has to be obtained. If the system is closed (not isolated since no system can be 100% isolated), then no energy is available and the system will end up losing all its energy and will stop working. Here is where the preservation part is needed from our definition. For systems to be alive, they need to be open systems so energy can be obtained from the environment to allow the system to continue working. For more complex systems, other resources are also needed and, once used or converted, they need to be disposed of. So an open living system must include the following:
∑ First, living systems are information systems meaning that they have an information code which controls their behavior;
∑ Second, they are open systems, which means that they exchange energy and resources with their environment;
∑ And third, their behavior allows them to use the available energy and resources to successfully preserve their information code.
Combining these three points above, we can obtain a general definition that fits all possible life forms: A system is alive when it has an information code which controls how to exchange energy and resources from its environment in order to preserve this information code. As we can see, life turned out to be a very simple cycle: information code ŗ use of energy and resources (exchanged with the environment) ŗ preservation of information code.
An Open Living System
So, in reality, there are only two things that the information code has to be able to do to remain alive: (1) exchange energy and resources with the environment and (2) preserve itself in such a way that the behavior continues. This definition is general enough to include all known life forms and to allow for other living systems not yet created or discovered. As long as they have an information code which controls how to preserve itself, they are alive and they will come up with new ways to remain so, or they will die.
So letís apply this concept to cellular life on Earth, for example, a eukaryotic cell. Eukaryotic cells have a nucleus where their genetic code, made of DNA, is located. This genetic code controls how the cell works through a very complicated process which starts with the coding of RNA from DNA. This code is sent outside the nucleus where ribosomes read it and produce proteins out of amino acids obtained from the environment. These proteins will be used as resources to maintain the cellís internal components in good health, allowing it to obtain energy and resources form the environment, get rid of waist and repair any damage produced by the environment.
A Simplified Open Eukaryotic Cellular System
In the end, the whole purpose of all this work is to protect the DNA from being altered significantly. Metabolism, homeostasis and response to stimuli are essential, but complexity, reproduction, replication, evolution, development and growth are all simply side effects or random alterations caused by environmental forces, which could end up being helpful in maintaining the cell alive, could result in its destruction and death, or could even just be neutral to its survival. Of course, destructive behaviors vanish together with the perishing organism while helpful and neutral behaviors are preserved, adding up to the complexities we see in cellular organisms today. Sometimes even destructive behaviors survive when the organism manages to reproduce before dying and their behavior is inherited to their offspring. That is why reproduction has been such an efficient driver to evolution: behaviors that are not helpful today are still inherited and, when the environment changes, turn out to be helpful in the future. Sexual reproduction is even more efficient because the information code for non-helpful behaviors are combined and generate new behaviors that end up being helpful in the future.