The concept of life belongs to the basic cognitive endowment of every human. This deep anchoring makes it difficult to define life. Everyone has an intuitive understanding of how a living object should look and how it should behave, but for scientific purposes it is necessary to make this implicit and vague knowledge explicit and clear. How else should we identify extraterrestrial life on other planets—or artificial life in the computer?
A classical strategy of defining life, which can be traced back to Aristotle, involves listing characteristics of living beings. A modern biologist would enumerate at least three essential properties: metabolism, self-reproduction, and mutability.
- Organisms metabolize. There is a permanent exchange of matter and energy going on between organisms and their physical environment to build and preserve their physical structure.
- Organisms reproduce themselves. Organisms transmit at least a part of their genetic information to their offspring. This information is necessary for the structural and functional development of an organism.
- Organisms are mutable. Their self-reproduction is not completely perfect, so it is probable that even offspring of asexually reproducing organisms show genetic variation if compared with their parents.
Like any list of characteristics of life, our proposal has two fundamental defects. First, there is at least one important borderline case that turns out to be very difficult to classify. Viruses have no metabolism of their own, so they need a host cell to be supplied with energy for their self-replication. Do they alternate between life and death—alive whenever they have invaded a host cell and dead whenever they are outside a host cell? Second, there is a class of nonliving objects that possess all of the characteristics on our list. Crystals grow in saturated solutions by replicating their highly symmetrical spatial structure. This process is not perfectly exact, so variations can occur in the crystal structure. In addition, one can detect a bidirectional metabolism; the building blocks of crystal growth are molecules coming from the solution, and the growing crystal radiates heat into the solution.
Both examples indicate that our list of characteristics of life contains only necessary, but not sufficient, properties for the identification of living systems. This is no surprise if we adopt an evolutionary perspective, which explains organic life as a result of natural history on Earth.
Because evolutionary theory will also address the problem of the origin of life, it must construct models for the prebiotic chemical evolution of the first self-reproducing macromolecules. By so doing, evolutionary theory presupposes that there is not an ontological cut between living and nonliving natural objects; rather, there is a physicochemical continuum. Consequentially, the evolutionary biologist cannot draw a sharp conceptual distinction between living and nonliving systems. But a complete list of necessary and sufficient characteristics of life would assume the possibility of both a clear ontological cut and a conceptual cut between prebiotic and biotic natural objects.
Darwinian evolutionary theory proposes that organisms are objects that are shaped by natural selection. A way out of the difficulty of defining life, then, is to characterize it by the natural dynamics of its origin and evolution. To do so, we must state the laws structuring this dynamics and the boundary conditions under which these laws act.
Natural selection is a dynamic process in systems in which struggle for survival occurs. That happens whenever a population of varying elements (for example, macromolecules, organisms) self-reproduce under restrictions on growth (on primitive Earth, in the ecological environments of today). We thereby take up the characteristics of life listed earlier but now integrate them into an evolutionary perspective.
How complex the resulting elements of the systems should be to call them “living,” then, is clearly a normative decision that cuts into a continuous natural history according to our respective research interests. So a list of characteristics of life is meaningful but in a specific research context. If we are, for example, interested in the origin of genetic information, the takeoff of life will be the appearance of self-replicating macromolecules.
A famous example of how to follow this strategy of embedding characteristics of life into an evolutionary perspective is Jacques Monod’s Chance and Necessity (1970). This French molecular biologist defined two principal properties of living systems that he called “invariance” and “teleonomy” (from ancient Greek telos aim, and nomos law). Whereas invariance is the ability of a system to reproduce its structure correctly, teleonomy is the property of a system that is structured so as to realize a project by its behavior. Monod saw no dualism between invariance and teleonomy because the behavioral teleonomy of an organism involves, in the last analysis, transmitting the genetic information that is needed to reproduce its structure. Monod explained this functional dependence of teleonomy on invariance by assuming the temporal precedence of invariance over teleonomy both in the general evolution of life and in the specific development of individual organisms.
Modern biology has investigated both the material and functional aspects of this priority of invariance to teleonomy. The concept of biological information captures the functional dimension of this priority because it designates all processes in living systems in which a biochemical structure instructs the construction of another biochemical structure by means of a code. Taking the example of the gene, its functionality as hereditary information relies not only on its material stability in evolutionary processes but also, and above all, on the existence of a genetic code that relates the biochemical structure of a gene to the biochemical structure of a protein. Thus, the material structure of a gene is biologically relevant only insofar as it is, by its hereditary function, an information carrier between an organism and its offspring and, therefore, subjected to natural selection.
The concept of information is used here not in a metaphorical manner that would, if taken literally, impute an intention or even a consciousness to all living systems. Instead, information must be understood in a naturalistic way as a concept that can describe the functional architecture of the physicochemical structures that build up a living system. If Monod was right in giving self-reproduction priority over any other property of organisms, the concept of information is basic to every list of characteristics of life.
- Küppers, B.-O. (1990). Information and the origin of life. Cambridge, MA: MIT Press.
- Monod, J. (1970). Chance and necessity: An essay on the natural philosophy of modern biology. New York: Alfred A. Knopf.
- Schrödinger, E. (1992). What is life? The physical aspect of the living cell. Cambridge, UK: Cambridge University Press.