One of the greatest puzzles of all time is how did life arise? It has been universally presumed that life arose in a soup rich in carbon compounds, but from where did these organic molecules come? In this article, I will review proposed terrestrial sources of prebiotic organic molecules, such as Miller–Urey synthesis (including how they would depend on the oxidation state of the atmosphere) and hydrothermal vents and also input from space. While the former is perhaps better known and more commonly taught in school, we now know that comet and asteroid dust deliver tons of organics to the Earth every day, therefore this flux of reduced carbon from space probably also played a role in making the Earth habitable. We will compare and contrast the types and abundances of organics from on and off the Earth given standard assumptions. Perhaps each process provided specific compounds (amino acids, sugars, amphiphiles) that were directly related to the origin or early evolution of life. In any case, whether planetary, nebular or interstellar, we will consider how one might attempt to distinguish between abiotic organic molecules from actual signs of life as part of a robotic search for life in the Solar System.
↵1 Defining ‘organic’ and ‘reduced’: when I refer to organic compounds, I mean those composed primarily of carbon, but may also contain nitrogen, oxygen and other elements. These are the kinds of molecules from which we and all living things are made, as opposed to carbon in the form of carbonate rocks, which is considered ‘inorganic’. Technically ‘reduced’ carbon is that bearing hydrogen atoms, such as in methane (CH4). Oxidized carbon is that such as in carbon dioxide (CO2) or carbonate rock, where all of the bonding is satisfied by oxygen atoms. For the purposes of being good prebiotic molecules partially oxidized carbon species (e.g. ketones>C=O) will be of use in making bigger, more complicated, and biologically important compounds. A ‘reduced’ gas is rich in hydrogen, or compounds that bear hydrogen, such as methane (CH4) and ammonia (NH3). Reduced gas mixtures produce more complex organic molecules than do oxidizing ones which is why the oxidation state of the early Earth's atmosphere is so important.
- © 2006 The Royal Society