Life's Working Definition: Does It Work?

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, or 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 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 is considered to lack what would commonly be thought of as a biological nervous system to guide it.

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's classification descends into or from the four great biological families: archaea, bacteria, eukaryotes, and viruses. Archaea are the recently defined branch that often survives in extreme environments as single-cells that resemble bacteria. Bacteria, often referred to as prokaryotes, generally lack chlorophyll (except for cyanobacteria) and a cell nucleus, since they ferment and respire to produce energy. Humans are eukaryotes, since our cells have a nucleus, a feature shared with plants, animals, protists and fungi. The final grouping includes the viruses, which are not cells at all, but fragments of DNA and RNA that parasitically reproduce if a compatible host cell gets infected. These classifications into kingdoms clarify the grand puzzle of existing life, but do little to bound the problem of defining life.

As an unbounded problem, 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 so far.

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?

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.

As an example of how one's definition of life can directly shape exploratory science, 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 directly for evidence of life on Mars. The scientific payload highlights the common features thought to classify what might previously have indicated life growing there. 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 when matched against the prevailing environmental conditions like martian temperature, pressure, wind speed, UV flux, oxidation potential, and dust environment.

For life to flourish, the prerequisites are nearly as lengthy as the definition itself.

I refer everyone to Karl Popper's essays on the demarcation problem, in which he argued that categories such as 'life' and 'non-living' are merely a convenient shorthard way of avoiding long lists of individual examples. Seen in this light, arguments such as 'what is life?' are stripped of their (speciously) fundamental character. Before arguing whether a definition of life should written to include, say, viruses but not prions, one should ask whether the question matters. If the shorthand fails - if not everyone, say, agrees that life does not include prions - then simply expand the shorthand slightly to say 'non-prion life'.

The definition of 'human life', on the other hand, is a legal and moral question as much as a scientific question. The scientific answer (according to Popper) is human life begins wherever it's useful to draw the line. It would be utterly inappropriate to try to translate that almost radically anti-epistomological approach to ethics or jurisprudence.

As to what spacecraft should search for - search for interesting molecules and interesting chemistry, and if you get results that make it look some of the chemistry has aspects in common with ourselves, narrow down the scope of our search. Looking for big, complicated molecules is comparatively easy; a mass spectrometer will do it nicely. That's the way science generally works; first do a very broad search for data, then look at the data, and decide what more specific questions you want to ask.

The most compelling concept for testing the hypothesis that there's extraterrestrial intelligent life comes from looking at the problem from a different perspective.

Astro physicist rely on laws of physics as a roadmap to what takes place in the Universe. They know that humans can manipulate the laws of physics to create any number of things that nature alone cannot account for. From anvils to linoleum to skyscrapers to zidovudine, conscious interdiction and control of nature's laws are an everyday part of human life on Earth.

Would it be possible to identify the telltale mark of extraterrestrial conscious life interdiction or control of the laws of nature?

For example, say a conscious being in another part of the Milky Way galaxy or other part of the Universe was looking through a highly-advanced-technology telescope at Earth. And the area they were looking at was the Hover Dam that creates Lake Meade or were looking at Hong Kong or New York City. They would readily identify that there was conscious life on that planet--Earth.

Considering the present state of the conscious humans on Earth, harming their fellow conscious beings in wars and by government force against their own citizens, if there is extraterrestrial life it is almost certainly exponentially more advanced than Earthlings. Perhaps a billion years or more advanced. Early on in their existence they will have out-competed and abandoned war and weapons of mass destruction as a gross contradiction as a means to fulfill their purpose--to live happily. For those technologies in the hands of conscious beings can annihilate all conscious beings and would be literally unthinkable to them. On the other hand totally natural and almost with out second thought to them would be their own immortality.
As you now read this, research scientists are knocking on the door of biologic immortality for Earthlings. Some will complain but that is to be expected of conscious beings that still live in a world that believes there's no benevolent replacement for wars and governments that initiate force, threat of force and fraud against their own citizens.

Extraterrestrial conscious beings with their exponential advanced technology reaching out to new frontiers they begin controlling outer space in close proximity to their planet. They continue reaching further into the cosmos.

Astro physicist on Earth with a physics roadmap can use mathematical theories, predictable expectations and computers to identify events that occur in other areas of the Milky Way galaxy or other parts of the Universe that fall outside of nature's natural course of events. Leaving the only plausible explanation that conscious beings controlled or interdicted nature's predictable patterns.

Thus, we don't need to wait passively for radio signals from deep space to reach our radar screens. Nor do we have to create a super advanced technology telescope. In fact, the answer to the hypothesis may be sitting in the several terabytes of data that has already been collected. Requiring a different perspective to know what to look for in the mountain of data. As you now read work on that is well underway.

That's interesting. There are a couple of sci-fi books that explore these hypotheses in some detail: Manifold: Space, Manifold: Time, and Manifold: Origin. Stephen Baxter, I think.

If a civilization is big enough to engineer more of their environment than a few scratchlines on the surface of their homeworld, they will be doing things on a larger scale, perhaps altering suns, moving suns into better position, things that can be seen. If there is some data, techniques of data mining might notice other meta-analytical effects: topology, crystallography, cointegration, computational subtraction, maybe gravitational lensing of an artificial nature. Even the chemical nature of their nearspace would be changed as they develop and mine resources.

In a few years a new space telescope will be in orbit, a telescope capable of seeing earthsized planets around other stars. Already base data is being collected using remote sensing of the entire earth so we can recognize oceans, biological modification of land and sea, characteristics of planets with lifeforms like us.

It's a big deal and a massive effort at our present rudimentary stage of interstellar capability; imagine the effect on civilization if another earth is found. Or conversely, if it is not. Either way, we will be changed.

imagine the effect on civilization if another earth is found.

Though our distant cousins in the Universe may be wielding technology a billion years more advanced than ours, meaning it is well beyond or ability to utilize because all advanced technology requires step by step integrations to achieve the advances -- there could be some basics that they could share. Like, how did our Universe come to be; was it an ex-nilo quantum flux from a gravity unity or perhaps an advanced technology civilization in another Universe set the initial conditions that caused the Big Bang. If it's the latter it's highly probable that our distant cousins may be creating or controlling specific dynamics of entire galaxies. Perhaps even creating their own universes.

Equally important it points to conscious beings being an integral part of existence. The controllers of existence by obeying nature they harness the laws of nature--conscious beings a part of nature are by nature benevolent benefactors of all conscious beings. But not the creators of existence because existence simply exists. For what's the alternative -- the complete contradiction that existence doesn't exist?

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