From Non-Life to Life: The Unity of Evolutionary Processes
The origin of life. If there is a more controversial (or complex) scientific problem I have yet to encounter it. Well… the origin of everything, or why there is anything at all is perhaps a little more controversial and complex. But the origin of life is certainly in the top 5. I know it is a scientific problem that has consistently perplexed me. But I shouldn’t feel too bad because it seems to have stumped even the brightest scientific minds. However, a study by chemists Addy Pross and Robert Pascal published last month laid out one of the most impressive working hypotheses I have seen to explain the transition from non-life to life. The paper is boldly titled: The origin of life: what we know, what we can know and what we will never know. It is open access and a tremendous read.
For several decades evolutionary theorists have been working hard to extend the concept of biological evolution into the realms of physics, chemistry, culture, and technology. In my mind this extension is imperative because it will help us more clearly understand major system transitions and the processes that drive change in our universe. The most important of these major system transitions is the transition from non-life to life. We know that biological evolution via well-understood mechanisms (e.g., selection, mutation, gene flow, genetic drift, etc.) allows for the existence of a complex and diverse biosphere. But we do not know how inorganic matter becomes organic matter.
In the recent publication by Pross and Pascal, they first outline what they feel we will never know about this transition: a) the precise historic pathway of inanimate to animate and b) the specific materials present on the prebiotic earth. I agree with them and I can’t overstate how important it is that they recognize this. I feel like attempting to re-create the environment of prebiotic earth is the biggest theoretical and methodological flaw scientists make when investigating the transition from non-life to life. Those experiments are admittedly interesting, but they are not falsifiable.
What we need to do is build an understanding of the relationship between chemistry and biology. Pross and Pascal believe that they have successfully elucidated this relationship. They state that:
In the context of the [origin of life] debate, there is one single and central historic fact on which there is broad agreement - that life’s emergence was initiated by some autocatalytic chemical system.
It follows that the study of autocatalytic systems in general may help uncover the principles that govern their chemical behaviour, regardless of their chemical detail. Extending Darwinian theory to inanimate chemical systems: The recognition that a distinctly different stability kind, dynamic kinetic stability (DKS), is applicable to both chemical and biological replicators, together with the fact that both replicator kinds express similar reaction characteristics, leads to the profound conclusion that the so-called chemical phase leading to simplest life and the biological phase appear to be one continuous physicochemical process, as illustrated in scheme 1.
Under this working framework it does not necessarily matter what organic molecules were present on the prebiotic earth. What matters is that we can understand how replicating systems work, whether they be chemically based, biologically based, or some grey zone between these two replicating worlds.
A theory to unite how replicating chemistry forms the basis of biological systems has been long in coming. Addy Pross suspects there has been such a lack of progress on this unification because chemists have a much better grasp on the static “regular” chemical world. However, he contends that there are “two chemistries”: one static and one dynamic. And both of these worlds produce stability (i.e., persistence over time) in very different ways.
Pross believes that there is enough empirical evidence from the study of systems chemistry to conclude that replicating molecules can persist via Dynamic Kinetic Stability (DKS). This type of chemical stability is vastly different than regular chemical stability. With regular chemical stability molecules lack reactivity. A good example of this is the molecule H2O, which is a hydrogen-oxygen mixture that forms a stable bond over time (it persists as a “thing” and we call that thing water). This hydrogen-oxygen mixture can form rivers, lakes, and oceans that can persist as a stable entity for an indefinite amount of time. However, replicating chemistry has a different type of stability that must operate on the population level because they are highly reactive. DKS essentially is the product of a group of replicating molecules that can be stable over time as a “population” even though their individual members are constantly changing (which is very different from how a “population” of H2O molecules achieve stability). These systems tend to drift from less stable to more stable over time non-randomly. The quantitative level of stability for the replicating system is dependent on a) its overall size and b) the amount of time it has existed. Again, this is very different from something like water that can possess the same level of stability regardless of its size or how long it has existed.
If this is difficult to conceptualize you could apply the same concept to a biological species and it should come into clear focus. Think of the human species. We have persisted for over 150,000 thousand years as a single biological system, and yet our individual members are always changing (at least for the time being #singularity). Other biological systems have achieved even greater stability. For example, cyanobacteria have remained essentially unchanged for 2.5-3.5 billion years. This ancient form of life, a dynamic system, has achieved greater stability than Mount Everest! And within this analogy resides the key to the discoveries within modern systems chemistry: replicating chemical systems essentially “behave” in the same way that replicating biological systems do. This means that abiogenesis - chemical process by which the simplest life emerged from inanimate beginnings - may have an underlying physicochemical continuity with biological evolution that had previously been unrecognized. A non-random selection for stability and complexity.
For me this research is incredibly fascinating for two reasons: 1) systems chemistry reveals that evolution operates at deeper, more fundamental levels of reality via potentially analogous mechanisms and 2) we are now theoretically able to build models of understanding that the origin of life is a non-random evolutionary process.
This research has very deep implications for how common we should expect life to be in our universe. If life is a product of replicating chemical reactions that acquire stability and increase in complexity via selection mechanisms, we should expect molecular life to be ubiquitous.
This discovery could represent a critical reformation of how we understand and conceptualize the universe. If studies of systems chemistry had revealed that at the molecular level there was only random chemical reactions, then our existence would begin to look extremely bizarre. I mean really, really bizarre. The chances of random chemical processes leading to the complexity we find at even the simplest biological levels is essentially zero. Pross and Pascal eloquently end their paper stating as such:
There is good reason to think that the emergence of life on the Earth did not just involve a long string of random chemical events that fortuitously led to a simple living system. If life had emerged in such an arbitrary way, then the mechanistic question of abiogenesis would be fundamentally without explanation — a stupendously improbable chemical outcome whose likelihood of repetition would be virtually zero. However, the general view, now strongly supported by recent studies in systems chemistry, is that the process of abiogenesis was governed by underlying physicochemical principles, and the central goal of [origin of life] studies should therefore be to delineate those principles.
I am very excited to see what future studies in systems chemistry reveal about these underlying principles. I am already formulating my hypotheses! It seems likely to me that the basic evolutionary mechanisms that have been so profoundly useful for describing all life, will also help us explain how other dynamic systems change over time. And hopefully this research will not always remain theoretical. Although we cannot recreate the prebiotic Earth, if we ever go to Europa or peak at another Earth maybe we will be able to see the transition from non-life to life first hand.
It is an exciting time to be alive!
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