Being among the living, all of us have a bias toward life. We are not to blame, for our first reference point is our own existence. We view life as special, something unique, divine, sacred. We naturally consider life as inherently superior to inanimate objects that do not enjoy our vital fluids. Being dead is less interesting than being alive.
But our fealty to the living, while understandable, obscures some uncomfortable truths. As comedian George Carlin observed with this typically offbeat take on the sanctity of life: "If everything that ever lived is dead, and everything that's alive is gonna die, where does the sacred part come in? I mean, life is sacred? Who said so? God? Hey, if you read history, you realize that God is one of the leading causes of death."
Our preconceptions, our biases, and our self-serving tendency to view life as sacred derive in part from the fact that life is mysterious and notoriously difficult to define with precision. The difficulty arises because every characteristic that was supposed to be unique to life has ultimately been found in non-living systems. Each new description of life seems to exclude some form of existence that can reasonably be deemed alive or to include something inanimate.
Defining life matters because one must first have a refined understanding of the essence of life in general to comprehend human life in particular -- or more to the point, human existence in relation to other forms of life and our supporting physical environment. Wrestling with the difficult task of defining life is necessary if we are to reveal god's superfluous role and understand humanity's humble place on Earth.
Historically, the effort to distinguish life from non-life has proven so difficult partly because the quest has been based on an erroneous assumption. From the early Greeks to modern thinkers, great minds have recoiled from the notion that life might be a matter of degree, because our intuition so strongly demands that something be alive or not. But our intuition serves us poorly here.
Most people would logically reject the conclusions from quantum mechanics that an electron will exhibit the properties of either a particle or wave, depending on how we measure its activity. But experiments have proven that duality with incredible degrees of precision. Similarly, the more rigorously we attempt to define life the more we encounter ambiguous cases that test our assumptions, stretch the limits of our definitions, and demonstrate where intuition and common sense falter. With even modest scrutiny, the essence of what makes something alive quickly becomes non-intuitive when we are presented with forms that defy easy categorization, such as crystallized virus capsules or bacterial spores. Then we have prions: nothing but raw protein containing no genetic code. Yet these proteins self-replicate and cause horrible brain-wasting maladies such as mad-cow disease, Scrapie, and Creutzfeld-Jacob disease.
While philosophers and biologists have failed to meet the challenge of defining life, in their labors we find clues to how this impasse can be overcome. The fact that scientists have discovered no simple quality that defines life, after endless years of effort, is an important hint that life is not something materially different from non-life but instead represents a natural place in a continuum from simple to complex.
History has failed to give us a good definition precisely because life was viewed not on this continuum from inanimate to animate, but as a huge leap from one to the other. To be alive meant having a special essence, something beyond the normal mechanisms that governed inorganic chemistry and physics. The tendency to invoke "vital forces" to explain life endures today in much of the general public. But vitalism, the principle of endowing the living with a life force, is tautological and explains nothing. If something is alive, it must have a life force; if it is dead, a life force must be absent. That is not helpful.
We now know that no life force exists. The laws of physics and chemistry are indifferent to our struggle to define life, and operate identically on the same principles whether we deem something to be living or dead. The carbon, nitrogen, phosphorous, iron and other atoms that come together to form our bodies are just atoms; they are the same elements that are found in the iron skillet in our kitchens and the nitrogen in the soil fertilizing our gardens. The atoms in our bodies are not special or endowed with any properties different from the atoms in every object around us. Iron is iron is iron, whether attached to hemoglobin in our blood or flaking off the hull of a rusting ship.
Digression for Nerds
But before going any further, let's pause to clarify two concepts critical to the definition of life that have already been introduced: a continuum and atoms.
Stuff Without Borders
A continuum describes a whole with no part that can be distinguished from neighboring parts except by arbitrary division. The best example is visible light. You know without hesitation when something is green (Astroturf) or blue (the newest M&M candy), but cannot say exactly when one color yields to the next. Any attempt to define where one color ends and the other begins becomes arbitrary because green turns to blue across a smooth gradient of frequencies with no inherent boundaries. This nature of the light spectrum applies to the idea of living and non-living as well. If we call green "inanimate" and blue "animate" we see that no boundary exists between the two because they transition one to the other with no intervening gap.
Atoms deserve special attention since everything we know, alive or not, is an aggregation of atoms. A quick story about these basic building blocks of nature and their astronomical origin will help demystify the stuff of which we are made and make the concept of life's ambiguity more accessible.
The simplest and lightest atoms -- such as hydrogen, helium, and some lithium -- formed just moments after the Big Bang. A star derives energy from nuclear fusion, a process that combines these lighter elements into heavier elements. Our own sun is currently fusing hydrogen to helium, a process that will occupy most of its lifetime. After the hydrogen supply is depleted, the star will burn helium to form progressively heavier elements such as carbon, oxygen, silicon, sulfur, and iron. Up to a point, fusion releases energy and is therefore self-sustaining, which is why we see the sun shining every morning (unless you live in Seattle).
But the creation of elements heavier than iron requires the input of energy, and is not self-sustaining. Some other source of energy is needed, and that comes from the explosion of a supernova. A massive star will eventually deplete its energy source of lighter elements. The star will collapse into itself when no longer supported by the release of nuclear energy through fusion. If the original star was sufficiently massive, the collapse will release a huge amount of energy in a spectacular explosion. The resulting supernova supplies the energy necessary to support fusion of nuclei heavier than iron. The explosion also causes a blast wave that ejects the elements into interstellar space. Some of this dust is eventually gathered up in planets, like Earth, as new solar systems form. Every single carbon atom in your body, and every carbon atom in the charcoal at the bottom of your barbecue, comes from such interstellar dust.
Derived from stardust, the elements in your body exhibit no special properties. Carbon is carbon. Nitrogen is nitrogen. As far back as 1828, Friedrich Wöhler proved the point when he synthesized urea during his attempts to make ammonium cyanate, demonstrating that compounds once considered the provenance of life (like urea) could be made from ordinary inorganic materials, all derived from the detritus of spent stars. Atoms are just atoms; the stuff of life is the same as the stuff of non-life. All of us and all the things around us are quite literally star dust.
Definitions Found Wanting
This understanding both informs and complicates our efforts to define life, exemplified by the most recent edition of the Encyclopaedia Britannica. The venerable lady suggests that life is a "state characterized by the ability to metabolize nutrients (process materials for energy and tissue building), grow, reproduce, and respond and adapt to environmental stimuli."
At first, that sounds perfectly reasonable. But the Britannica definition is in fact completely inadequate, incorporating only a small number of traits from a suite of primary characteristics that have been included in previous definitions of life:
• Stability, Change and Evolution
• Resistance to Entropy
• Conversion of Matter and Energy
• Growth and Development
For our purposes here, we do not need to go into detail about what each of those terms mean. But we know that every trait in that list has a fatal flaw as a means of defining life. Every trait fails as a prerequisite for life because:
1) It is present in some non-living systems (crystals exhibit growth, for example); or
2) It is absent in some living systems (movement cannot define life when living things like sponges don't move); or
3) It can only be determined or defined across generations (evolution, reproduction), depriving us of the ability to decide if the creature before us is alive or not.
Every single characteristic or trait that has ever been used to define life suffers from one or more of these three deficiencies. But perhaps two additional categories, information and artificial life, will help us escape from this dilemma.
Every living things carries within itself a set of instructions for how to develop into a specific type of living thing. Every cell in your body has a complete set of DNA, which acts like the blueprint for an architect building a house. Your DNA contains all the instructions for making you. But somebody needs to read the blueprint to convert the plans into bricks and mortar. That is the role of RNA, which translates the code embedded within DNA by converting the information in the DNA into proteins, the building blocks of all life. If DNA is like an architect with a complete set of blueprints, then RNA is like the general contractor, who reads and translates the architect's plans to build the structure. In this analogy, the structure is a set of proteins, which in fact is what you are.
Think of DNA and RNA as information that each organism uses to build itself. Richard Dawkins has succinctly captured this idea: "There is nothing special about the substances from which living things are made. Living things are collections of molecules, like everything else. What lies at the heart of every living thing is not a fire, not warm breath, not a 'spark of life.' It is information, words, instructions."
This appeal to information to help us define life solves some immediate problems. We can safely say that any live thing contains a genetic code that describes how to make that thing, indirectly in the case of viruses. So perhaps we have finally found the answer to the definition of life. We might conclude simply that the presence of DNA (or RNA) defines life. How elegant! We are so close, but no, once again we have been foiled, because not every thing that contains a genetic code is alive. An animal that has recently died retains a full complement of DNA, but is still dead, just as dead as Monty Python's parrot. We might salvage the situation by saying that life is defined by the presence of DNA or RNA and the ability to use the information to create new copies of the genetic material. That solves the dead animal problem. But then we come to the problem of dormant life. Bacterial spores and crystallized viruses are not capable of using their genetic material to make copies during their periods of dormancy.
Using information content to define life runs up against another problem. The genetic code is digital. Instead of being a binary system of 1s and 0s as in the computer world, the genetic system is quaternary, having four distinct states rather than two. Either way, we have an immediate and obvious non-living example of information stored digitally. Computer code can contain enough information to replicate itself, and the computer has the ability to use the information to do so. The existence of digitally stored, self-replicating information cannot define life because examples are found in non-living systems. Also, as storage media become denser and smaller, the discrepancy between the enormous amount of information contained in an incredibly small space in a cell and the information content on a computer chip becomes slightly less dramatic every year. Although the gap in information storage in biotic and abiotic systems remains gargantuan, one can imagine that the discrepancy will disappear one day. And that leads us to artificial life.
Artificial life is the simulation of biological functions through the use of computer models, robotics, or biochemistry. Many businesses and books are devoted to the topic. But however fascinating this pursuit is, it offers no assistance in our journey to define life. Any clever creation arising out of this field of study could not be defined as alive or not in the absence of a good definition of life. The field of artificial life would benefit from a clean, distinct, unambiguous definition of life, because experts in the field would then know if they had succeeded in making something life-like. But, inversely, progress in the field of artificial intelligence does nothing to advance a definition of life. Even if Aibo the robotic dog advanced to the stage of speaking English, making your breakfast, and driving you to work, we still could not say if Aibo were alive or not without a good definition of life.
As noted earlier, nobody would deny the existence of green or blue, yet nobody can define when one color becomes the other. That inability to draw a clear line between them does not diminish the reality of the two colors. We accept the existence of clearly identified colors even when the transition between frequencies in the light spectrum lacks a clearly delineated boundary. Life is no different. We know at the extremes when something is alive or not, with no ambiguity, just as we know something is green or blue. Other cases are ambiguous. A virus could be alive or not, depending on your perspective. In some cases, such as viruses, bacterial spores, prions and Rotiferans, defining matter as alive or not becomes arbitrary, an exercise in semantics rather than a window into the deeper workings of nature. We might be obsessed with attaching a label of "living" to something, but that something simply sits somewhere along a continuum of complexity regardless of the label finally affixed, indifferent to our discomfort.
The region along the spectrum where abiotic transitions to biotic is a zone of ambiguity that exists because life is not an all-or-none phenomenon, and because the stuff of life is the same as the stuff of non-life. Previous definitions of life have fallen short because of a common commitment to find a "spark" that simply does not exist. Definitions sought to capture something essential about life that was not found in the abiotic world, rather than accept that no such distinction can be found. Definitions of life were meant to reflect something fundamental about nature, not to serve as useful tools for categorizing complexity. That is why all have failed.
There is no single unambiguous definition of life. Most examples of life are complex; most metabolize, grow, reproduce, and evolve over time. But not all do, and not all have all of these functions present. Some physical systems also share these same characteristics. That fact is not troubling; it reflects the reality of nature. "Life" is an arbitrary label we apply to distinguish extremes of complexity along a continuum. We know that a block of pure quartz is not alive and that a screeching kid in the restaurant is; whatever label we paste on all those cases in between is a convenient convention, but in no way reflects any fundamental break or division between the living and non-living.
With a healthy perspective on the phenomenon of life, we prepare ourselves to explore how humanity fits into the big picture. God never enters into the equation; invoking a higher being is not only unnecessary to understand life but one of the biggest obstacles. Without god, humans have no claim to special status. And being humble about who and what we are is easier when we recognize our kinship not only with our cousins in the animal kingdom, but also with the dirt under our feet and the charcoal in our barbecue.