what is the simplest form of life?

“merely bags of chemical soup, the sticky residue of the glorious death of a thousand dying suns”

eniteris, on what constitutes a living thing.

It seems like a straightforward question at first, but if we scratch the surface just a little bit the enormous complexity of attempting an answer becomes apparent. However, since hubris is my middle name, I remain undaunted and can promise my beloved readers a complete answer over the next couple dozen paragraphs.

The first complication we encounter is the fact that we do not currently have a good definition of what life actually is.

“Hang on there, ol’ beep ol’ pal,” I hear you say, “what do you mean we don’t have a good definition? Life is obviously the state of being alive and not-dead.”

Firstly, how dare you address me in such a fashion. Secondly, technically you are correct, but only technically. If we define “life” in that way, we very quickly run into the fiendish circular logic trap of defining what it means to be dead (can something that never lived be dead?) and alive (the opposite of dead? the state of having life?). You can see how we may tie ourselves in knots and not in a fun way.

There have been many attempts to define life and all of them seem to be incomplete in one way or another. Persons who paid attention in middle and high school biology classes may remember learning that there are seven criteria for life, as follows:

  1. Homeostasis – the regulation of processes to maintain an equilibrium
  2. Organization – being composed of at least one cell, so called the “basic unit of life”
  3. Metabolism – transforming energy into cellular components (usually chemically)
  4. Growth – expanding oneself through metabolizing resources, i.e. “a higher rate of anabolism than catabolism” (from wikipedia) in order to increase the size of its components
  5. Adaption – the ability to undergo evolution in response to the environment
  6. Stimulus response – the broad term for reacting to aspects of the environment (different from number 5)
  7. Reproduction – the ability to make more copies of oneself, usually described as sexual or asexual

To the first order, this list is a good approximation of what biologists consider to be properties of living things, but it starts to fall apart when we consider the interesting edge cases. Obviously, humans fit all of these criteria and most humans reading this are arguably alive. This also applies without much room for dissent to most mammals, plants, fish, and insects that one might come across outdoors. The interesting arguments happen when we apply this to the more interesting organisms.

One fairly well-known debate with regards to the above list has to do with whether viruses can be described as alive or not. Some people make the argument that because viruses are not capable of replicating outside of their host cells, they cannot be fully alive and instead are merely machines to replicate their own genome. Those on the other side would say that because we can apply the principles of Darwinian evolution to viruses, they must be considered to be a form of life. I am of the opinion that viruses are living things, for two reasons.

The first reason is beautifully illustrated by the name of my favorite class of viruses: phages. These are a class of viruses that infect bacterial (and sometimes viral) cells. The psychology and classic language students among the audience will recognize that the word “phage” comes from the Greek word meaning “to devour”. They were so named because they were initially thought to be “eating” the bacterial cultures in which they were first observed. To me, this is an excellent refutation to the argument that viruses are not alive because they cannot replicate without their host cells. To a phage, a host cell is merely a resource, like nutrients to a bacterium. In my view, it would be just as incorrect to say that because a human cannot replicate without food, they are not alive. The second, and far simpler reason is that viruses can be killed (think alcohol based hand-sanitizers, heat, etc.), which I maintain is a solid heuristic argument, if a bit circular.

As an aside, there is an article by Eugene Koonin and Petro Starokadomskyy in the journal Stud Hist Philos Biol Biomed Sci (which might be my favorite abbreviated journal title ever) that suggests a completely different view to the debate. Rather than viewing viruses as being alive or not, Koonin and Starokadomskyy suggest it is more useful to think of viruses in terms of their position on the selfishness-cooperativity axis, along which all biological replicators sit.

By now, we’ve gotten a bit off track. This article is not about viruses and whether or not they are alive, its about the simplest form of life that we could successfully argue for. So far, all we’ve done is muddy the waters with viruses and talk about how the list of criteria for life is not completely useful. With that in mind, it is worth noting that the definition of life that NASA uses (according to wikipedia) is much simpler: “a self-sustaining chemical system capable of Darwinian evolution.” This definition is decidedly broader and may be more useful for our purposes.

For all the discussion about viruses and whether they are alive and should have rights and freedoms under the law, they are not the simplest form of life, despite being exceedingly simple. In fact, there are three other examples of arguably simpler forms of life that I can think of. Let’s take a look at them one at a time.

  1. Viroids. The first example I can think of as being simpler than a virus is the aptly named “viroid”. Like a virus, a viroid is a small amount of genetic material (i.e. RNA) that can infect other cells to replicate itself. Unlike viruses, however, the viroid lacks a protein shell that protects a virus’ genome during transmission and helps it infect cells. Instead, the viroid is essentially just a loop of RNA that uses its host to replicate itself. Basically all of the same arguments about viruses apply to viroids, and certain classifications may not even distinguish between them. That said, they go on the list because, without the protein shell, they are decidedly simpler than viruses.
  2. Ribozymes. Like viroids, ribozymes are molecules of RNA. The difference here is that a ribozyme is an RNA molecule that is capable of some enzymatic activity (in fact, the name comes from RIBOnucleic acid enZYME). Those of you who recall your biology courses may remember that the molecules that replicate your genetic material (polymerases) are also enzymes. It has been hypothesized that a ribozyme structured in just the right way would be able to replicate itself. While this hasn’t precisely been observed yet, Tracey Lincoln and Gerald Joyce were able to produce a pair of ribozymes that replicated each other. This candidate gets points off for technically existing only in theory (for the moment), but it gains those points back for how nicely it fits into the RNA-world hypothesis.
  3. Prions. The third and final candidate in this list for simplest life form is also the one that will likely cause the most consternation among biologists. In short, a prion is a misfolded protein that is able to induce similar proteins to misfold in the same way. This has significant effects if it happens with any of the proteins we need to live, and is the cause of diseases like Mad Cow and Scrapie (terrifyingly, the section about treatment of prion diseases on wikipedia is very short and begins with the sentence: “There are no effective treatments for prion diseases.”). The main difference between a prion and the other entries on this list is the prion’s lack of genetic material. Instead, it is information about how the protein misfolds that gets transmitted.

This begs an interesting philosophical question: if it is a form of life, is it the prion itself, or the information about how to misfold that is actually alive. If we start classifying information as being alive, we may be obliged to include much of the universe’s phenomena to the point where the label becomes useless. On the other hand, there are likely some people who take a thermodynamic view of life and would argue that the information (genetic or otherwise) is the only defining feature of life to begin with. The whole debate could devolve into a moment reminiscent of Diogenes’ plucked chicken (“Behold, a man!”).

We could spend lifetimes chasing our tails around this question but since this would be a waste of our time and tails, I would rather take the viewpoint that any strict labeling system will break down if you prod the edges enough. In any question of philosophy (and life overall) we must remember the old adage that labels were made for (hu)man, not (hu)man for the labels. We use terms like “alive” and “dead” to help make sense of the universe as we try to describe it to ourselves. The moment that our understanding reaches a point where the labels we use are no longer useful (or, indeed, more of a hindrance than a help), they should be cast aside to make way for new descriptions and ways of thinking.

Anyway, I hope you learned something from reading this, and I think we can all agree that the award for simplest form of life goes to prions. Definitely prions.

/millibeep

Author’s note – This post has more links to wikipedia than I normally include. For those that are unaware, wikipedia is a non-profit that runs off volunteer work and donations. If you use it a lot, like I do, please consider setting up a regular monthly donation of a couple bucks. It helps people all over the world with access to high-quality, encyclopedic knowledge and makes it possible for people like me to write articles like this one.

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