Despite these challenges, astrobiologists have managed to be clever enough to identify some of the key steps leading from nonlife to life. In particular, it is well recognized that a critical stage in the origin of life was the emergence of the first functional biopolymers – things that may have been akin to very primitive RNA or peptides (peptides are short sequences of amino acids much less complex than modern proteins). This early stage in evolution is relatively uncharacterized given that we know very little about the first biopolymers, the nature of which has obscured by literally billions of years of ensuing evolutionary history. It therefore presents a “missing era” of prebiotic evolution, wedged right in between prebiotic synthesis and the latter stages of evolution based on the evolutionary refinement of already established molecular assemblies. At the Center for Chemical Evolution based at Georgia Tech, this key stage is a major focus of research. So Martha Grover, Nick Hud, and I therefore set out to develop a model to attempt to tackle characterizing how functional evolution may have been initiated during this period of history deep within our biosphere’s past.
The idea we started with is pretty simple – use basic physical and chemical conditions, likely to occur on prebiotic Earth, and follow the dynamics of random populations of polymer sequences over time. We choose dehydration-hydration cycling to drive periods of polymer formation and degradation, since in the absence of the sophisticated enzymes of modern life, prelife would have had to rely on the environment for at least some of its tricks. Because we don’t know the exact nature of the first biopolymers, we focused on exploring the dynamics under a variety of physical and chemical conditions (i.e. kinetic rate constants and diffusivities) and some interesting things popped out. For one, stringent maintenance of sequence information doesn’t appear to be of critical importance at this stage like it is in modern life, which for example must preserve genomic information from generation to generation. Very early on, the search for functional sequences may be the more crucial issue, and highly dynamic chemistries do that best. Studying these kinds of systems therefore gives us a window into what chemical systems might be best to study in the lab. Perhaps most interestingly, evolution in such systems appears to be dramatically different than that governed by strictly Darwinian processes, which dominant most of the biosphere today. In particular, these early stages may have been dominated by collective evolution of system-wide polymer aggregates as shown in the figure.
The results are not exactly intuitive when extrapolating backward in time from modern evolutionary biology, but one can start to envision how evolution of these simple chemical systems may have led to the modern biosphere we observe billions of years later. It is interesting to think that the processes dominating this early stage of prelife might not look much like modern biology at all! But perhaps we could identify prelife if we did find it. Working toward better characterizing this “missing era” in prebiotic evolution may allow us to lay down one more piece in the puzzle as we work to finally see the full picture of how life got started on our planet. As is the case with most of the natural world, I am sure this picture will be a masterpiece.
You can learn more by checking out our recently published paper at PLoSOne