Closing In on Abiogenesis
One of the frequent, albeit misguided, criticisms leveled against evolutionary biology is that it doesn't offer an explanation for how the very first living thing arose. A frequent cry amongst the Creationist crowd is that complex organic molecules like nucleic acids can't arise on their own from inorganic chemicals. In this context, "inorganic" means something very different from its use in chemistry, where any carbon-based compound can be termed organic, even something as simple as methane. The Creationist definition, as nearly as I can figure it, has to do with relative complexity. Still, we would all be in agreement that DNA is a complex and organic molecule.
DNA is composed of four nucleotides bound together with phosphate groups. One of those nucleotides is adenine, and a recent publication by University of Georgia chemists demonstrates a model whereby a simple and abundant organic molecule — hydrogen cyanide (HCN) — can combine to form adenine. The phenomenon has been observed before; it's been reported that a flask of cyanide in ammonia, frozen solid for 25 years, spontaneously gave rise to adenine.
According to a press release, the UGA researchers
arrived at an answer by solving a series of key riddles. They worked out the processes in which five cyanide molecules might combine to form adenine under terrestrial conditions. Their predictions are based on extensive computations of sequences of reaction steps along possible mechanistic routes.The article, entitled Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions, describes a model in great detail that resolves many outstanding questions about one of the origins of nucleotides, and hence those of the nucleic acids and, by extension, gives us a glimpse into how the stage was set for the arisal of life on earth. While the article does not, and cannot, speculate as to the origins of this raw material for all the life we see today, this is a matter of no consequence in terms of the model it proposes. The abstract is reproduced here, with the appropriate citation:
“Finding a viable, thermodynamically feasible, step-by-step mechanism that can account for the formation of adenine was far from straightforward,” the authors said. “Our report provides a more detailed understanding of some of the chemical process involved in chemical evolution, and a partial answer to the fundamental question of molecular biogenesis. Our investigation should trigger similar investigations of the abiotic formation of the remaining nucleic acid bases as well as other biologically relevant molecules.”
Fundamental building blocks of life have been detected extraterrestrially, even in interstellar space, and are known to form nonenzymatically. Thus, the HCN pentamer, adenine (a base present in DNA and RNA), was first isolated in abiogenic experiments from an aqueous solution of ammonia and HCN in 1960. Although many variations of the reaction conditions giving adenine have been reported since then, the mechanistic details remain unexplored. Our predictions are based on extensive computations of sequences of reaction steps along several possible mechanistic routes. H2O- or NH3-catalyzed pathways are more favorable than uncatalyzed neutral or anionic alternatives, and they may well have been the major source of adenine on primitive earth. Our report provides a more detailed understanding of some of the chemical processes involved in chemical evolution, and a partial answer to the fundamental question of molecular biogenesis. Our investigation should trigger similar explorations of the detailed mechanisms of the abiotic formation of the remaining nucleic acid bases and other biologically relevant molecules.Using models such as this, based on fundamental physical and quantum chemistry, it should indeed be possible to clarify precisely how the molecules necessary as precursors for life came about using a set of relatively simple, natural laws. As the reader will find, at no time does a supernatural explanation for the origin of adenine arise in this article.
Roy D., Najafian K. and von Ragué Schleyer, P. 2007. Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions. PNAS.2007; 104: 17272-17277. doi: 10.1073/pnas.0708434104
While abiogenetic events are beyond the scope of evolutionary theory, it is nevertheless a fascinating field of inquiry. As time goes by, science continues to peel away the layers of mystery — ultimately little more than a glyph of ignorance — and provide real, feasible answers to questions like this. While we do not yet understand exactly how life arose, the day is coming soon when enough of the pieces of the puzzle, such as this one, will be joined together to set forth something that makes sense and that contributes to other avenues of inquiry as well.
Many of us will be quite happy with these answers. Others within the human race will be crying out for vengeance against the purported burners of the holy books. It will be very interesting to see what, if any, response to this latest result is forthcoming from those who insist on theological explanations for scientific questions.
