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Aliens and Origins: What is life? Six criteria

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I am making my way at a sedate pace through NASA astrobiologist Peter D. Ward's book Life as We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life. Astrobiology is the real-world version of what science fiction fans have sometimes called “xenobiology.” Though “astrobiology” means “the study of star life,” Ward doesn't limit the possibility of alien life to other stars or even to other planets. We may not only find alien life on Mars, but even on earth.

The scientific definition of life — little more than an intellectual exercise for most biologists, since they know life when they see it — is the most fundamental problem in astrobiology. We have to know what we are looking for if we are to recognize it.

Ward discusses the history of defining life. Some famous scientists have addressed this question, including more physicists than biologists. One of the first books was What Is Life? The Physical Aspect of the Living Cell by Erwin Schrödinger (yes, the guy who both killed and didn't kill theoretical cats). Ward extensively discusses the criteria developed by Paul Davies, another famous physicist. Davies' book The Fifth Miracle: The Search for the Origin and Meaning of Life lists six qualities that could be expected in any life form: metabolism, organization, development, autonomy, the ability to reproduce, and the ability to evolve. Here are a few thoughts on these qualities.

Metabolism and organization

I grouped these together because they are inseparable. Metabolism is the processing of energy to reduce entropy, and entropy is the opposite of organization. So metabolism preserves organization. And since metabolism requires enormous complexity, organization enables metabolism.


Development is the change in an individual organism over time. I am unconvinced that development, separate from metabolism, is absolutely necessary in the definition of life. Some microbes develop little, if at all, between their asexual “birth” and their reproduction; why should we require it of a microscopic alien creature?


This is the slipperiest of the concepts in the Davies definition of life. Not only is it difficult to pin down exactly what it means (something about “self-determination”), but also exactly how it applies. For example, is a bee hive with a single queen an autonomous unit? Or does that apply to each individual bee in the hive? Or to each cell within each bee? The bee's cells, and the hive's bees, depend respectively on the bee and the hive for their “self-determination.”

Ability to reproduce

Organization means information, so reproduction requires a means to transmit information. Therefore, for any potential life form to reproduce, it needs both a way to reproduce its physical substance and metabolism and the information necessary to organize it. In other words, life needs a genome.

Ability to evolve

I am not sure this item is necessary for the list, not because it is inaccurate, but because of parsimony. The other five qualities can all apply to single organisms, but evolution is something that happens to populations. Metabolism and organization imply variations in efficiency (and therefore competition among members of a population). If reproduction involves any errors or changes in the genome, then logic shows that any population would evolve through natural selection.

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