Zombies and the walking dead make for good copy, but do little to advance our understanding of life and death. Unfortunately, neither did the National Geographic with a cover article entitled, "The Science of Death: Coming Back from the Beyond."
The article issues forth just about every misconception of life that permeates our national discussion. Sam Parnia, a critical care physician and author of the book Erasing Death, is quoted as saying that death "is a process, not a moment." So far so good. But then he makes a common but critical error in thinking, which gets to the heart of our problem. In discussing a victim of a whole body stroke, Parnia writes that the patient's organs can continue to function for a period after the heart stops beating. From this he concludes that "for a significant period of time after death, death is in fact fully reversible."
Well, no, it is not. If a patient can be revived, the patient was never dead in the first place. But how can that be if we see no brain waves and the heart has stopped beating? Surely that is dead, isn't it? If someone was revived from that state, clearly we must say he came back from the dead, no? That is certainly what is commonly believed, but no, we can't. The success of reviving the victim means that during that state in which it seemed as if biological functions ceased, the functions essential for life in fact remained viable enough to be resuscitated -- and therefore the patient never died. Reports of Mark Twain's death were exaggerated only because he was not dead. The same is true for "miracles" like toddler Gardell Martin who was "dead" for an hour and a half after falling into an ice-cold stream. We are very happy to have him among the living, but he never left us in the first place.
We find this curious state of suspended animation difficult to accept as anything other than dead because we are asking the wrong question about life and death, without ever clearly defining what it means to be alive. Most of us hold deeply and unquestioned the idea that life is all-or-nothing , on or off, live or dead, one or the other, black and white. I mean, something is either alive or dead, end of story. Nothing could be further from the truth.
We are in good company in failing to define life. Dating back to the early Greeks and across millennia to modern times, 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. The problem seems to be that 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 casual observation, the essence of what makes something alive quickly becomes non-intuitive when we are presented by forms that defy easy categorization such as bacterial spores or crystallized virus capsules that can rest inert for centuries before being reanimated. Those viruses would appear to be no more alive than a pile of salt, but we know that only one can be re-introduced into the kingdom of the living.
Iron is Iron
History has failed to give us a good definition of life precisely because life was viewed not as 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. Invoking "vital forces" to explain life endures today in much of the general public. But vitalism, this 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 circular, 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 that: 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.
A continuum describes a whole, no part of which can be distinguished from neighboring parts except by arbitrary division. The best example is visible light. You know without hesitation when something is green or blue, 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. A pristine lake might be green-blue or blue-green or turquoise, but not clearly green or blue. This nature of the light applies to the idea of living and non-living as well. If we call green "dead" and blue "alive" 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 is an aggregation of atoms, the same in things dead or alive. The simplest and lightest atoms such as hydrogen, helium, and some lithium formed just moments after the Big Bang. A star derives energy from the combining of these lighter elements into heavier elements through nuclear fusion. 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.
This all relates to life; just hold on a few more seconds. 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. Atoms are just atoms, so the old premise that life is made of some special stuff is wrong. But more modern efforts to describe life fall short, too. The most recent edition of the Encyclopaedia Britannica offers a typical definition of life as 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, as has been every previous attempt.
The Characteristics of Life
Broadly speaking, the following characteristics are usually invoked in various orders and degrees to define life: autonomy; reproduction; stability, change and evolution; resistance to entropy; conversion of matter and energy; metabolism; excretion; movement; autopoiesis; homeostasis; complexity; organization; growth and development; respiration; responsiveness; the presence of a genetic code. The fatal flaws in each of these characteristic called upon to define life all fall into just three simple categories: the traits assigned to life are present in some non-living systems (like growth, in crystals), the traits assigned to life are absent in some living systems (movement - think of sponges), or the traits can only be determined or defined across generations (like evolution or reproduction), depriving us of the ability to determine if the beast before us is alive or not. Every single character or trait that has been used to define life suffers from one or more of these three deficiencies.
Take the presence of a genetic code for example. That would seem to be a pretty good way of defining life because DNA is only found in living things. Wrong. We can extract DNA from fossils, and few would argue that an old bag of bones that has been in the ground for 100,000 years is alive. This type of defect in definition can be found in every one of those categories that so many have invoked to define life.
Let's return to the idea of colors. 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 colors of light are absent of any 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, just as we do not know when green becomes blue. A virus could be alive or not, simply depending on your perspective. In some cases, such as viruses, bacterial spores, and prions, 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, aloof to our discomfort.
The region along the spectrum of complexity where non-living transitions to living 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 stuff as non-life. Previous definitions of life have fallen short because of a common commitment to find a unique spark that simply does not exist. Definitions struggled to capture something essential about life that was not found in the non-living world, rather than accept that no such distinction can be found. Definitions of life were meant to reflect something fundamental about nature, rather than serve as a useful tool 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.
These thoughts are not original, just widely ignored by those outside the field of biology. Josephine Marquand suggested in 1968 that we "avoid the use of the word 'life' or 'organism' in any discussion of borderline systems." Norman Horowitz in 1955 and John Keosian in 1964 concluded much the same as here. Even the 1968 Encyclopaedia Britannica stated that "There is no point along the continuum of existence from the simplest atom to the most complex animal, at which a line can be drawn separating life from nonlife." Notice, however that Marquand, Horowitz and Keosian are not household names, nor is the Britannica observation widely cited. The idea of a continuum of complexity, with simple inorganic systems at one end and the highest life forms at the other, is a bit difficult to digest, and does not satisfy the human need for easy answers. The idea also moves against the grain of our intuition about something being alive or not. So we put up some resistance. But resistance is futile.
With a new perspective on the phenomenon of life, we can look with a more jaundiced eye at claims of death and resurrection in cases like Gardell Martin. We can readily reject statements like "death is in fact fully reversible" when we know that life is a continuum along a spectrum of complexity, with no simple on-off switch. Those revived were never dead, the switch was never turned off - just dimmed to below our ability to see, waiting to be re-energized. Let's move away from this rather silly idea of victims coming back from "the beyond" and leave Zombies, the walking dead and resurrection to Hollywood and Sunday sermons. You can't come back from a place you've never been.