Where to Next? Unsolved Questions

The first ten chapters of this book are a kind of snapshot that captures the current state of knowledge and proposes a scenario for life’s beginning that is based on the properties of RNA described by Harry Noller in the epigraph (Noller, 2012). Despite this progress, there are still enormous gaps in our understanding that remain to be filled. The purpose of this chapter is to make those gaps explicit for future investigators who might be attracted to the question of how life can begin. Because life is an interacting system of immense complexity, each component of which is essential to cellular function as a living system, the gaps have little in common. They can be presented as a set of questions related to sources and properties of organic compounds, mechanisms for capturing energy, polymerization and replication of nucleic acids, the origin of ribosomes, and the transmission of genetic information. For each question, I will discuss one or more papers that could provide clues to an answer and then add some ideas that might serve as guides to future research. A source of mononucleotides is a problem not just for the hypothesis being presented in this book but for any proposed pathway to the origin of life. Unlike amino acids, there is no obvious source of mononucleotides, for the following reasons: Three different molecular species must be present in an aqueous solution at concentrations sufficient for a reaction to occur. They must somehow, even in this mixture of organic solutes, form a specific ester bond between a phosphate and a ribose, then must form an N-glycoside bond between the ribose and a nucleobase (Fig. 11.1), and these reactions must occur spontaneously in a hydrothermal environment. Furthermore, it is not enough for one base to occur in the solution, all four (adenine, uracil, guanine, and cytosine) must be present.