Defining Life

I’ve had this conversation a couple of times recently, because it poses an interesting question: can we create a definition for ‘alive’ that encompasses not only known biological life, but also any theoretical lifeforms we can imagine? This might include alternative biochemistry, artificial life (nanites?), and even digital lifeforms.

Obviously there is an inherent problem in this discussion; we are assuming everyone shares a similar definition of life. However, even a skin-deep probing can reveal divisive philosophical questions. Are computer viruses alive? How about self-replicating structures of dust particles in a plasma? Is the Earth alive? We can’t truly resolve this problem without first clearly setting a boundary for what things are alive and what things aren’t alive. For example, scientists seem to have resolutely decided that biological viruses are not alive. Similarly, its clear to our human sensibilities that a car engine is not alive, even if it is highly advanced and has all sorts of sensors and regulatory mechanisms.

For the sake of discussion, I’m going to skip over this roadblock and dive in. Wikipedia gives these criteria for calling something ‘alive’:

1. Homeostasis: Regulation of the internal environment to maintain a constant state.
2. Organization: Being structurally composed of one or more cells.
3. Metabolism: Converting chemicals and energy to maintain internal organization.
4. Growth: A growing organism increases in size in all of its parts, rather than simply accumulating matter.
5. Adaptation: The ability to change over time in response to the environment.
6. Response to stimuli: A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

There are some good ones in there, but a few need to go. Let’s throw out Organization (this can almost be seen as tautological — things made of cells are alive because they are made of cells — and exclusive of otherwise potential candidates for life), Growth (one can imagine an organism which is artificially constructed, but then maintains itself perfectly, or a mechanical organism that starts life by being constructed externally, and slowly grows smaller as it sacrifices components to stay operational), and Reproduction (again, imagine a constructed organism that cannot reproduce). This leaves Homeostasis, Metabolism, and Adaptation/Response to stimuli.

However, its clear that Metabolism is important: an organism must take something from its environment and consume it to maintain an internal state. Metabolism and Homeostasis are where biological viruses fail the ‘life test’. While some advanced viruses meet the Adaptation and Response to Stimuli (arguably the same thing, just at different scales), no virus can use resources from its environment to perform internal upkeep. It requires the hijacked machinery of a cell to do that.

Unless you say that living things are part of a virus’s ‘environment’. Then you could argue that in some sense of the word, viruses are alive, because they use resources present in the environment to perform internal upkeep. This raises an important question about context. Indeed, all definitions of life seem to hinge on context. For example, a computer virus’s environment is the computer system. Resources would be computing time and memory, perhaps.

Is a computer virus alive? Advanced viruses can modify their own state (metamorphic code), respond to stimuli (anti-virus, user activity, etc), and metabolize resources from their environment. They also reproduce, although we cut that criterion so the point is moot. If a computer virus meets the requirements for life (albeit unconventionally), then do we have to accept it as a lifeform?

Moreover, there are things we wouldn’t normally call a single entity that fulfill the requirements for life. These are often termed “living systems”. The Earth is a prime example. It has systems that regulate its interior, it absorbs sunlight and that helps fuel the regulatory cycles on the surface. It’s debatable whether the Earth responds to stimuli. Sure, there are feedback loops, but the Earth doesn’t really respond accordingly to changes (say, changes in solar luminosity or meteoric impacts) in order to maintain homeostasis. Quite the opposite, in fact. For example: a decrease in solar radiation produces more ice, lowering albedo, thus lowering albedo further.

So maybe the Earth isn’t alive, but we have to consider nonetheless that systems can be alive. In fact, its questionable whether humans are single organisms. Several pounds of our weight are gut bacteria, independent organisms which share no DNA with us, but on which we rely for survival. We are a system. Call it a colony, call it symbiosis; the entity that is a human is in fact a collection of trillions of ‘independent’ organisms, and yet that entity is also singularly ‘alive’.

Can we trust our initial, gut reaction that tells us what is alive and what isn’t? Moreover, what use is there in classifying life in the first place? We treat cars that are definitely not alive as if they are precious animals with a will of their own, and then squash bugs without a second thought. Is it important to define life at all, rigorous criteria or not?