Can we define ‘life’?


(above: This Aug. 11 false-color image from NASA’s Cassini spacecraft highlights concentrations of ice from geysers that spout water vapor and ice particles. The image shows coarse-grained and solid ice are concentrated along valley floors and walls, as well as along the upraised flanks of the “tiger stripe” fractures, or sulci, which may be covered with plume fallout that landed not far from the sources.)

Here, the discussion veers into the philosophy of science: What is life? How do we define it? If we seek life elsewhere, will our existing definitions help us find it?

Carol Cleland, professor of philosophy

Carol Cleland, professor of philosophy

Carol Cleland, a professor of philosophy at CU, grapples with these questions. She and Christopher Chyba of Princeton University are recognized authorities on the difficulty of adequately defining “life.”

Cleland and Chyba wrote a chapter in Planets and Life: The Emerging Science of Astrobiology, which was published by Cambridge University Press last year. As they had done in a previous journal article, they argued that we not only lack a broadly accepted definition of life but also that the scientific project of defining life is fundamentally mistaken.

“To answer the question ‘What is life?’ we require not a definition but a general theory of the nature of living systems,” Cleland and Chyba write. “In the absence of such a theory, we are in a position analogous to that of a 16th-century investigator trying to define ‘water’ in the absence of molecular theory.”

As Cleland and Chyba note, attempts to define life founder on counter-examples – living things that are wrongly defined as non-living, and non-living things that are incorrectly included in a flawed definition of “life.” Philosophers as old as Aristotle attempted to define “life” as something that could reproduce itself.

By that definition, mules (which are sterile) would not qualify as being alive.

Other attempts to define “life” are similarly flawed, Cleland and Chyba observe. For instance, life has been defined by metabolic standards: something with the ability to consume or convert energy to move, grow or reproduce. By that measure, fire and perhaps even cars might be deemed alive.

A thermodynamic definition might describe life as a system that takes in energy to create order locally. But, as Cleland and Chyba note, that would seem to encompass crystals, “which, like fire, would not generally be considered alive.”

Contending that life reveals itself via certain biochemicals assumes that all forms of life employ those substances. The fact that life on Earth contains proteins comprising 20 amino acids does not, in itself, tell us whether all forms of life do.

“Without access to living things having a different historical origin, it is difficult and perhaps ultimately impossible to formulate an adequately general theory of the nature of living systems,” Cleland and Chyba write.

That problem is not unique to life and reflects a simple logical point, they add: One cannot generalize from a single example.

Nor, they contend, can one define life by stipulating arbitrary characteristics of living things, by specifying examples of living things, or by listing “operational” functions of living things.

Defining life by listing examples of living things, for instance, runs the risk of incompleteness: If you cite a German shepherd as an exemplar of a “dog,” what does this tell you about the nature of a Chihuahua?

Operational definitions are similarly defective: The fact that litmus paper turns red when placed in a liquid tells us nothing about the nature of acidity; it reveals only that a liquid with this property can be called “acid.”

NASA’s “working” definition (a chemical Darwinian definition) posits, for instance, that life is a “self-contained chemical system capable of undergoing Darwinian evolution.” That definition presupposes chemistry, self-reproduction, genetic variation and natural selection. As Cleland notes, there are many potential problems with this definition. The restriction to chemistry rules out informational or mechanical (robotic) artificial life; whether such entities are alive shouldn’t be merely a matter of definition.

Furthermore, many individual organisms do not have the capacity to reproduce, and hence would not qualify as “life.” This is true for sterile individuals as well as sterile hybrids (which constitute a population) such as mules. Neither would be deemed “alive” by this definition (as it is stated), and attempts to accommodate them by modifying the definition lead to different problems.

“Definitions blind you,” Cleland observes. Rather than relying on a demonstrably imperfect definition of life, Cleland suggests employing “empirically well-founded, albeit provisional, criteria that increase the probability of recognizing extraterrestrial life while minimizing the chance of being misled by inadequate definitions.”

Those criteria could incorporate a wide range of biosignatures, which would help scientists identify anomalies that could indicate life. Importantly, Cleland and Chyba write, “our strategy is deliberately designed to probe the boundaries of our current concept of life. It is only in this way that we can move beyond our Earth-centric ideas and recognize genuinely weird extraterrestrial life, should we be fortunate enough to encounter it.”

Jakosky praises Cleland as a leader in the field of astrobiology, but he takes a different view of the desirability of defining—or propounding provisional criteria—that delineate life from non-life.

Jakosky likens attempts to define “life” to the scientific debate over what can and cannot be called a “planet.” In our solar system, Pluto has lost its planetary designation and is officially called a “dwarf planet.” In the universe, there is a range of objects from dust particles to planets to stars. Within that range, there are clear instances of each type.

But, Jakosky notes, there are “gray areas” such as Pluto, or the larger satellites such as Titan, about which well-informed scholars can disagree.

In the case of life, Jakosky notes there also is a gray area in between clearly living and clearly non-living. Viruses, for example, fall in that gray area. Similarly, there was a gray area in time: Earth did not have living things at its inception. Sometime in its first half-billion years, the fossil record shows evidence of life. In the next half-billion years, the evidence proliferates.

As he writes in his book: “Was there a distinct moment when Earth went from having no life to having life, as if a switch were flipped? The answer is ‘probably not.’” There were probably entities that had some but not all characteristics we would view as evincing life. These would have fallen in that gray area, in which they could arguably be placed into either category.

Given the impossibility of having a single, unique boundary that defines “life,” Jakosky suggests that an entity be judged by scientific consensus “as to whether it meets most or all of the characteristics of life.”

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