What is life? This is a question which has plagued philosophers and scientists alike for centuries. Fundamentally, this is a question about the nature of life – what most basic property or function must a system have to be considered “living”? Over 2000 years ago, Aristotle proposed an answer to this question. He described life as having two essential abilities: self-nutrition and self-reproduction. In more modern terms, self-nutrition is essentially metabolism – the ability of a system to sustain or feed itself. Self-reproduction is just what it sounds like – the ability of a system to reproduce, thereby making more generations of itself. Since the time of Aristotle, many have similarly tried to define life, with efforts ranging from the scientific to the supernatural. Today, there is still no consensus as to what makes something living, but many in the astrobiological community go with the definition decided upon by NASA.
According to NASA, “Life is a self-sustained chemical system capable of undergoing Darwinian evolution”. Now, I think an entire book could be written dissecting this definition in gory detail and analyzing whether it is actually useful as a working definition, but that is not the goal here. Rather, I would like to point out a few things. First, this definition encompasses two essential features of life: life must be a self-sustained chemical system and it must be able to undergo Darwinian evolution. Being a self-sustaining system means, by definition, that the system can feed/sustain itself, i.e. has some sort of metabolism. Having the ability to undergo Darwinian evolution implies a genetic code coupled with some form of physical expression upon which natural selection can act (a genotype/phenotype distinction) and hence the ability to pass on that genetic system to future progeny, i.e. the ability to reproduce. Isn’t it interesting that over 2000 years later we are still defining life, albeit with some new terminology, in the same way Aristotle did, as requiring both metabolism and reproduction.
So, we now know that both metabolism and reproduction are common properties of life – but is one more fundamental than the other? You may be wondering why we would even attempt to separate the two most fundamental properties of life as we know it. The problem is that many definitions of life do just this – they focus on one property or the other in an attempt to whittle away at what we know of life until we reach its most fundamental core – always striving to answer the question of what makes living things different from non-living ones. Now, it is important to note that there is little scientific data to suggest that this is warranted. As far as we know, by observing the examples of life that we have on Earth, BOTH reproduction and metabolism are essential to life. And yet, the question is still being asked – which is MORE fundamental. Asking this question pushes what we know of the nature of life to its limits. Can you have a living system comprised of only a genetic code? Would a metabolic network be considered alive?
Oddly enough, these questions are not being pursued by those who study the nature of life, but rather by those who study its origins. Look at the two leading origin of life theories: the RNA world and the small molecule world. The RNA world theory asserts that the ideal molecule at life’s origins was a self-catalytic RNA molecule, and that this molecule spontaneously formed on early Earth with the ability to replicate itself. The RNA world presupposes that the most essential feature of life is the ability to reproduce, i.e. the possession of a genetic code. The small molecule world on the other hand asserts that autocatalytic networks of chemical reactions emerged from a random assemblage of small molecules, and that this transition marks the jump from non-life to life. The small molecule world is thus presupposing that metabolism is more essential to life. These two “camps” are bitter rivals in the scientific community, with each side being 100% sure that they have the origin of life figured out. Isn’t it interesting though that the line drawn in the sand in theories on the origin of life represent the same two things concluded to be essential to the nature of life? Are the nature of life and the origin of life necessarily intertwined? Must we understand one to understand the other?
Strictly speaking, even if we fully understand the nature of something, we cannot infer its origin. Take a simple example: a chair. Let’s assume it is a simple wooden chair. Now, we can determine from examining its composition that the chair is first indeed made of wood. Then, we could identify the type of wood and hence the type of tree that the wood came from – possibly even the area of the world that it originated. But, can we determine exactly how it originated? Can we know from only its composition today how its seed found the ground? Or what tree its seed came from? Or how far the seed travelled before finding its home in the ground? Or, from only a study of the nature of the chair itself, can we even know it was formed from a seed at all? This simple, rather naïve example illustrates that although we can study something through careful examination of the specimen in front of us and determine its nature, we are unable to infer its origin.
Now, following along this same example, what if we wanted to find out the trees origin? How would we go about doing this? First, we must know as much as possible about the nature of the tree that was used to fashion the chair. Then, using our background knowledge of how other trees have been formed (which we are able to directly observe), we would assert that the tree of interest was formed in a similar way (this brings back the idea of historical vs. operational science, the subject of one of my previous posts). From there, we can continue to reach deeper and deeper to extrapolate the process by which our particular tree came into existence. So, although a thorough knowledge of the nature of something does not imply an understanding of its origin, it is an essential part in the process. Understanding the nature of life therefore gives a goal for origin of life researchers upon which to base their research.
Let’s return to the two conflicting origin of life camps: RNA world vs. small molecule world. Why are scientists asking an origins question drawing lines in the sand on a nature question? This should not be the case. There has been no scientific conclusion showing that either metabolism or reproduction is MORE essential to life, so there should not be any origins theories claiming one or the other as the end goal of their research. We know that BOTH are essential to life. Therefore, a legitimate origin of life theory at this point in time, aiming to find an origin for life as we know it on Earth today, should include both a metabolic and genetic (reproduction) component. Rather than fighting with each other about whose theory is more valid, we should be attempting to soften the lines between these theories to create one which incorporates both of the components we know to be essential to life on Earth.