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Life's Working Definition: Does It Work?


By: Astrobiology Magazine staff

Is it alive?
A crystal can grow, reach equilibrium, and even move in response to stimuli, but lacks what commonly would be thought of as a biological nervous system.
Image Credit: National Ignition Facility Programs

How to define "life" is a sweeping question that affects whole branches of biology, biochemistry, genetics, and ultimately the search for life elsewhere in the universe.

Comparing the semantic task to the ancient Hindu story of identifying an elephant by having each of six blind men touch only the tail, the trunk, or the leg, what answer a biologist might give can differ dramatically from the answer given by a theoretical physicist.

However, some initial agreement is possible. Living things tend to be complex and highly organized. They have the ability to take in energy from the environment and transform it for growth and reproduction. Organisms tend toward homeostasis: an equilibrium of parameters that define their internal environment. Living creatures respond, and their stimulation fosters a reaction-like motion, recoil, and in advanced forms, learning. Life is reproductive, as some kind of copying is needed for evolution to take hold through a population's mutation and natural selection. To grow and develop, living creatures need foremost to be consumers, since growth includes changing biomass, creating new individuals, and the shedding of waste.

To qualify as a living thing, a creature must meet some variation for all these criteria. For example, a crystal can grow, reach equilibrium, and even move in response to stimuli, but lacks what commonly would be thought of as a biological nervous system.

While a "bright line" definition is needed, the borderline cases give life's definition a distinctly gray and fuzzy quality. In hopes of restricting the working definition at least terrestrially, all known organisms seem to share a carbon-based chemistry, depend on water, and leave behind fossils with carbon or sulfur isotopes that point to present or past metabolism.

If these tendencies make for a rich set of characteristics, they have been criticized as ignoring the history of life itself. Terrestrially, life is classified among four biological families: archaea, bacteria, eukaryotes, and viruses. Archaea are the recently defined branch that often survives in extreme environments as single cells, and they share traits with both bacteria and eukaryotes. Bacteria, often referred to as prokaryotes, generally lack chlorophyll (except for cyanobacteria) and a cell nucleus, and they ferment and respire to produce energy. The eukaryotes include all organisms whose cells have a nucleus - so humans and all other animals are eukaryotes, as are plants, protists, and fungi. The final grouping includes the viruses, which don't have cells at all, but fragments of DNA and RNA that parasitically reproduce when they infect a compatible host cell. These classifications clarify the grand puzzle of existing life, but do little to provide a final definition.

Defining life takes on a more bewitching character when extended beyond the Earth's biosphere. The recent addition of extremophiles (archaea) to the tree of life underscores the notion that life is defined by what we know, what we have seen before, and often what we have succeeded in domesticating to a laboratory petri dish.

Astrobiology Magazine sought out expert opinion on this important question from Dr. Carol Cleland, who teaches philosophy at Colorado University in Boulder and is a member of NASA's Astrobiology Institute. While on sabbatical in Madrid, Spain, at the Centro de Astrobiologia (CSIC-INTA), she shared her thoughts on the power of definitions to shape science and philosophy.

Interview with Carol Cleland

"I am interested in formulating a strategy for searching for extraterrestrial life that allows one to push the boundaries of our Earth-centric concepts of life." -Carol Cleland
Image Credit: University of Colorado
Q: What is your opinion of attempts to define of "life?"

In a recent paper in Origins of Life and Evolution of the Biosphere, Christopher Chyba and I argue that it is a mistake to try to define 'life'. Such efforts reflect fundamental misunderstandings about the nature and power of definitions.

Definitions tell us about the meanings of words in our language, as opposed to telling us about the nature of the world. In the case of life, scientists are interested in the nature of life; they are not interested in what the word "life" happens to mean in our language. What we really need to focus on is coming up with an adequately general theory of living systems, as opposed to a definition of "life."

But in order to formulate a general theory of living systems, one needs more than a single example of life. As revealed by its remarkable biochemical and microbiological similarities, life on Earth has a common origin. Despite its amazing morphological diversity, terrestrial life represents only a single case. The key to formulating a general theory of living systems is to explore alternative possibilities for life. I am interested in formulating a strategy for searching for extraterrestrial life that allows one to push the boundaries of our Earth-centric concepts of life.

Q: In the category of what is "alive," would you exclude what you call the "borderline" cases - viruses, self-replicating proteins, or even non-traditional objects that have some information content, reproduce, consume, and die (like computer programs, forest fires, etc.)?

This is a complex question. Language is vague, and all terms face borderline cases. Is an unmarried twelve-year-old boy a "bachelor?" How about an eighteen year old? How many hairs does it take to turn a "bald" man into a man who is "not bald?" 20 or 100 or 1,000 hairs?

The fact that there are border line cases -- that we can't come up with a precise cut-off -- doesn't mean there isn't a difference between a bachelor and a married man, or a bald man and a man who is not bald. These difficulties don't represent profound difficulties; they merely represent the fact that language has a certain degree of flexibility. So I don't think that entities like viruses provide very interesting challenges to definitions of "life."

On the other hand, I don't think that defining "life" is a very useful activity for scientists to pursue since it is not going to tell us what we really want to know, which is "what is life." A scientific theory of life (which is not the same as a definition of life) would be able to answer these questions in a satisfying way.

As an analogy, the medieval alchemists classified many different kinds of substances as water, including nitric acid (which was called "aqua fortis"). They did this because nitric acid exhibited many of the sensible properties of water, and perhaps most importantly, it was a good solvent. It wasn't until the advent of molecular theory that scientists could understand why nitric acid, which has many of the properties of water, is nonetheless not water. Molecular theory clearly and convincingly explains why this is the case: water is H2O -- two hydrogen atoms and one oxygen atom. Nitric acid has a different molecular composition.

A good theory of life would do the same for the cases that you mention, such as computer programs. Merely defining "life" in such a way that it incorporates one's favorite non-traditional "living" entity does not at all advance this project.

Q: What is your favored theory for how life could have arisen on Earth -clay crystals, RNA world, membranes, or some other option?

Freeman Dyson, founder of the "double origin theory."
Image Credit: Trustees of Dartmouth College
It seems to me that all theories of the origin of life face two major hurdles. The biggest one is explaining the origin of the complex cooperative schema worked out between proteins and nucleic acids -- the controlled production of self-replicating catalytic systems of biomolecules. All of the popular accounts of the origin of life strike me as side stepping this issue. Instead, they focus on the other hurdle: producing amino acids and nucleotides, and getting them to polymerize into proteins and nucleic acids (typically, RNA). But it seems to me that none of them have provided us with a very satisfying story about how this happened.

All the scenarios that have been proposed for producing RNA under plausible natural conditions lack experimental demonstration, and this includes the RNA world, clay crystals, and vesicle accounts. No one has been able to synthesize RNA without the help of protein catalysts or nucleic acid templates, and on top of this problem, there is the fragility of the RNA molecule to contend with.

But I still think that the more serious problem is the next stage of the process, the coordinating of proteins and RNA through a genetic code into a self-replicating catalytic system of molecules. The probability of this happening by chance (given a random mixture of proteins and RNA) seems astronomically low. Yet most researchers seem to assume that if they can make sense of the independent production of proteins and RNA under natural primordial conditions, the coordination will somehow take care of itself.

I suppose that if I had to pick a favorite theory, it would be Freeman Dyson's double origin theory, which postulates an initial protein world that eventually produced an RNA world as a by-product of an increasingly sophisticated metabolism. The RNA world, which starts out as an obligatory parasite of the protein world, eventually produces the cooperative schema, and hence life as we know it today. I like the fact that this account attempts to deal with the origin of the cooperative schema.

Q: Do you think there could have been multiple origins of life, or that life could have come to Earth from somewhere else?

Life arising more than once from nonliving materials could occur elsewhere than Earth, but it could also have occurred on Earth. It is possible that extraterrestrial life exists and that all life nonetheless has a common ancestor. Scientists now believe that microbes can survive interplanetary journeys ensconced in meteors produced by asteroid impacts on planetary bodies containing life. In other words, we could all be the descendants of Martians -- or Martians, if they happen to exist, could share a common ancestor with us! In short, the mere discovery of extraterrestrial life doesn't guarantee that life had more than one origin.

Q: As one of the great mysteries and challenges in science, do you think we can determine the origin of life through experimentation?

I hope so! But until we have an adequate theory of life to drive the formulation of the right experiments, it will be difficult to tell. I suppose it is always possible that life is not a natural category, and thus no universal theory of life can be formulated. But I doubt it.

It is also possible that life on Earth is the product of a very complex historical process that involves too many contingencies to be readily accessible to definitive experimental investigations. An adequately general theory of life would make this clear, however. Besides, historical research is quite capable of obtaining empirical evidence that can resolve historical questions of this sort-evidence that is just as convincing as that provided by classical experimental research! So even if we can't produce life in the lab from nonliving materials, it doesn't follow that we will never know how life originated on Earth.

What's Next?

The European Space Agency will launch a Mars mission in early summer 2003. Current plans are for its lander, Beagle 2, to perform biological experiments designed to search for evidence of life on Mars. As an example of how the definition of life can directly shape exploratory science, the scientific payload on Beagle 2 will investigate the common features thought to indicate life. For instance, Beagle 2 will look for the presence of water, the existence of carbonate minerals, the occurrence of organic residues, and any isotopic fractionation between organic and inorganic phases. Each of these will provide clues to the likelihood of life on Mars when matched against the prevailing environmental conditions, such as temperature, pressure, wind speed, UV flux, oxidation potential, and dust environment.

Abstract from Cleland, Chyba (2002): "There is no broadly accepted definition of 'life.' Suggested definitions face problems, often in the form of robust counter-examples. Here we use insights from philosophical investigations into language to argue that defining 'life' currently poses a dilemma analogous to that faced by those hoping to define 'water' before the existence of molecular theory. In the absence of an analogous theory of the nature of living systems, interminable controversy over the definition of life is inescapable."

Cleland, Carol E.; Chyba, Christopher F., Origins of Life and Evolution of the Biosphere, v. 32, Issue 4, p. 387-393 (2002).
NASA's Astrobiology Magazine