Posted by: Dan | September 6, 2007

Meanings and Theories of Life

As ‘Ivy Privy’ reminds me, Carl Zimmer has an article in SEED on The Meaning of Life. Zimmer writes:

“There is no one definition that we agree upon,” says Radu Popa, geobiologist and the author of Between Probability and Necessity: Searching for the Definition and Origin of Life. In the course of researching his book, Popa started collecting definitions that have appeared in the scientific literature. He eventually lost count. “I’ve found at least three hundred, maybe four hundred definitions,” he says.

Indeed, defining ‘life’ is difficult. Is the essense of life self-replication or propagation? Is it metabolism, or some sort of trophism? Chemical gradients across semi-permeable membranes (homeostasis)? Organization (of what?)? Growth? Adaptation? Response to stimuli?

Carl Sagan had an interesting essay on this, as have many others, including Erwin Schrödinger’s famous essay What is Life?. Most of these examples are incomplete or unsatisfying in one way or another. Perhaps, as some who Zimmer interviews assert, we will have to wait until we find non-terrestrial life to better understand how to define life. Or, perhaps, there is no “magic bullet” explanation.

But maybe ‘life’ is not different enough from other chemical and physical processes to warrant a simpler definition than something that fits the “all of the above” category, relying on a checklist of criteria. That’s my bet. Life does appear to do nothing more than utilize physical-chemical processes that can be found in non-life – it’s merely the arrangement that is unique.

Zimmer continues:

Recently, a new voice has entered the debate. Carol Cleland, who teaches philosophy at the University of Colorado and works with NASA’s National Astrobiology Institute—essentially as their philosopher-in-residence—is making a more radical argument: Scientists should simply give up looking for a definition of life. They can’t even begin to understand what life really is, she claims, until they find forms of life profoundly different from those we know here on Earth. Only when we can compare alien life with life on our planet will we understand the true nature of this ubiquitous, ephemeral thing.

Cleland believes biologists need to build a theory of life, just as chemists built a theory of the elements and physicists built a theory of electromagnetism. Definitions, she argues, are concerned only with language and concepts, not true understanding. By taking the semantics seriously, Cleland is calling for nothing less than a scientific revolution. Only when we change the way we think about life, she argues, will the true study of it begin.

I thought I just did that. Okay, I’ll formalize it…

Maybe “Life” is merely any self-propagating, homeostatic, and trophic arrangement of chemicals upon which stochastic natural selection can act.



  1. Then even the stars themselves are alive!

  2. I’m not quite sure how they could be considered homeostatic, as stars do not have membranes, osmotic gradients, or response to stimuli. Also, stars aren’t literally reproductive.

    They’re certainly trophic or metabolic though. And regardless, the ability to raise even the possibility that stars would overlap with a definition of life just goes to highlight the difficulty here…

  3. Larry Moran muses about “the phage group,” which included a number physicists who ventured into biology in hopes of discovering new physics.

    Waiting for the paradox


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