What is Life? – A Brief History of Definitions

My friend Anna made the mistake of asking me a few “simple” questions about the meaning of life. It provoked a very brief summary of my work, which appears in this and the next post.


“What is life?” through the ages:

Hebrews: to have God’s breath/spirit in you, or to have blood. Circulation was a big deal.
Greeks: to be able to influence the world.
Aristotle: to be able to turn not you into you (nutrition).
    Formally to have one cause that was formal, final, and efficient, but nutrition is the simplest version of that.
Neoplatonists: to be closer to the One than inanimate things.

By the time you get to the Scholastic period, monotheists will have confused these activities reified as “souls” for substances that possessed these properties. To be alive is to have a soul, or perhaps more accurately souls are always alive, bodies are only alive when possessed by souls.

Michel Foucault (The Order of Things) argues that there was no category “life” before the enlightenment. There were only categories of animals, vegetables, and minerals, or other such typologies. I think this is an overstatement, however. They thought there was something significant about having souls in that gave you the power to grow and reproduce. The great chain of being or scala naturae linked all beings from dust to crawling things to amphibians to birds, beasts, humans, angels, and eventually God. For some this was eternal harmony, for others, more of an escalator, with higher arising from lower. The properties we associate with life were important, but it was unclear how to map them onto concrete sets.  Truthfully, it’s not that much easier, now.

The Enlightenment created a chasm. Life was thought of until this point as a property (or set of properties) of a system, but the mechanists (Descartes, Gassendi, Boyle) would have it that the physical world was only particles operating under universal forces. There wasn’t any concept of emergence (good or bad). Kant argued that “organisms” had unifying functions (being made up of organs with a common goal) which we attributed to them, but which they did not have empirically objectively. Others (no good type for this yet) argued that their simply were no functions, or goals, only chance and necessity.  The biologists were left with a conundrum.  Is life empirical? Is it a category? Can you study it?

Throughout the 19th and early 20th century, we tried to reshape “biology” in the image of a “modern science.”

Could life arise from non-life? No. Through a series of experiments, usually listed as starting with Redi (1626-1697) and culminating in Pasteur (1822-1895), we convinced ourselves that life cannot arise within sterile environments. It must depend on some life-specific antecedent.

Could life be made up of life-specific (“organic”) matter? No. A research program championed by TH Huxley (1825-1895), Alexander Oparin (1894-1980), and JBS Haldane (1892-1964) culminated in the Miller-Urey experiment (1953). Huxley was one of the first to observe that the same elements went into protoplasm and inorganic matter. Oparin and Haldane explored the energetic considerations of organic molecule. Miller and Urey definitively showed that the chemistry necessary to form common organic compounds from abiotic precursors was possible in the (then believed) early Earth environment.  Subsequent experiments have demonstrated that organic molecules fundamental to nucleic acids, proteins, fats, and sugars can form in numerous abiotic environments, including deep space, asteroids, clay surfaces, and geologically active aquifers, just to name a few examples.

Could life be caused by a life-specific (“vital”) force? No. Theories abounded between 1800 and 1950 for a progressive life force, that brought about and directed organisms. Some of the most famous proposals were called orthogenesis, élan vital, and complexification. Darwin (1809-1882) demonstrated that natural selection could fit organisms to their environment and give rise to new species. Mendel (1822-1884) provided a mechanism for the transmission of biological information between generations. In the early 20th century, numerous mechanical/chemical processes were discovered, requiring no unique “vital” force. In short, no life-specific force could be discovered which added to the predictive power of physical forces.

Meanwhile, advances in cosmology intensified the problem. Big Bang cosmologies suggest a time when the universe was so hot and dense that no life – as we know it – could have existed. Between then and now, life must have arisen from non-life.  This is the focus of “origin of life” studies.

It’s unclear whether “life” represents a good empirical (scientific) category. Most of us (even in the sciences) operate with the “pornography definition” – I can’t tell you what it is exactly, but I know it when I see it.  Attempts to come to a more satisfactory definition have focused on

Evolution: being involved in a process of natural selection producing adaptations
Metabolism: being involved in a self-sustaining, self-perpetuating network of chemical reactions
Anentropy: resisting decay toward low energy, disordered states by taking energy from elsewhere
Information: having a level of order impossible or at least highly improbable on the basis of chance and necessity.

All four succeed only by being vague in at least one area. When we try to define adaptation, complexity, order, and information in ways that exclude all non-life while including all life, we have failed to find even low level consensus among scientists.  Currently I suspect that the last three are logically (or at least empirically) impossible. The first, I have hopes for, but I think we need to do some hard thinking before we have anything rigorous enough to call a scientific definition. (What are the units of selection? How do you handle prospective fitness? What time scales are appropriate?)


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