I presented a literature review that contended that a carbohydrate polymer world most likely existed before modulating into the RNA world, which eventually then gave way to the DNA world we know today. Personally, I really enjoyed reading about the chemistry of the origin of life. Although I don’t know nearly enough about chemistry to have fully understood all that this article was saying, just considering how the complex array of chemical life seen today came to existence from simple building blocks amazes me. We had all heard of the Miller-Urey experiment which attempted to recreated those early Earth conditions; Methane, ammonia, and hydrogen were circulated over boiling water, electrodes were introduced to mimic lightning, and then after a certain period of time, the composition of the mixture was analyzed and a number of amino acids were detected. But this article questions: did that really recreate the conditions of early Earth?
This paper includes some interesting conditions that hadn’t really occurred to me before. It’s pretty widely accepted that the reducing conditions of early Earth didn’t include significant amounts of oxygen. No oxygen means no ozone. No ozone means no protection from high energy UV light. And UV light perhaps provided a significant amount of energy that was needed to drive these early reactions.
Also, the paper presents the idea that as more and more complex molecules were created, the concentration still remained considerably small, even if they were all dissolved in the early oceans. Thus, for these molecules to randomly collide to form polymers would be highly unlikely. The authors of this paper thus contended that maybe these early molecules would adhere to mineral-rich clay particles in water. The clay would enable the reactants to congregate on a common surface and the minerals (aluminum, iron, magnesium, calcium, sodium, and potassium to name a few) would serve as catalysts for these early reactions. Eventually more and more complex molecules would be synthesized, and the authors hypothesized that saccharides would have been among the first complex molecules to be formed. Furthermore, the authors presented the claim that since only the simplest molecules were present on early Earth, it was likely that formaldehyde was one of them present in non-trivial amounts. They speculated that maybe formaldehyde self-polymerizes on these clay surfaces to form small saccharides, which eventually undergo aldol condensation to form more advanced hexoses and pentoses.
With this theory about the origin of chemical life, it isn’t at all difficult to then see how these carbohydrate polymers gave way to an RNA-based world. The authors suspect that carbohydrates polymerized along with abundant phosphates to form longer chain polymers. Several polymers are capable of forming with this method, one of them resembling RNA without the base pairs. So essentially, just a series of ribofuranose units linked by phosphate groups (structure sown below). The highly oxidized purine and pyrimidine bases probably only came about a little later, and when they did arrive, only a simple substitution was need to create the first strands of RNA (also shown below).
With the theories presented in this review, the RNA world hypothesis gains some much-needed support after past failures to experimentally recreate the formation of RNA. Certain bonds are simply unable to bond to sugars to form a complete RNA molecule under early Earth conditions. Now, all that is left to do is to show how these nucleotides produced life in a primitive RNA world…
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