Life, Characteristics of

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 struc tural 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 [Page 1472]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.

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