70 Formation of Organic Molecules in an Earthly Reducing Atmosphere
70 Formation of Organic Molecules in an Earthly Reducing Atmosphere
A prerequisite to the prebiotic chemical experimentation is a source of organic molecules. Just as life requires energy (to do anything and everything!), converting inorganic molecules into organic molecules requires an input of free energy. As we have seen, most living things today get free energy by oxidizing nutrients or directly from the sun by photosynthesis. Recall that in fact all the chemical energy sustaining life today ultimately comes from the sun. But, before there were cells, how did organic molecules form from inorganic precursors? Oparin and Haldane hypothesized a reducing atmosphere on the prebiotic earth, rich in inorganic molecules with reducing power (like H2, NH3, CH4, and H2S) as well as CO2 to serve as a carbon source. The predicted physical conditions on this prebiotic earth were:
- lots of water (oceans).
- hot (no free O2).
- lots ionizing (e.g., X, 𝛾) radiation from space, (no protective ozone layer).
- frequent ionizing (electrical) storms generated in an unstable atmosphere.
- volcanic and thermal vent activity.
Origins of Organic Molecules and a Primordial Soup
Oparin suggested that abundant sources of free energy fueled the reductive synthesis of the first organic molecules to create what he called a “primeval soup”. No doubt, he called this primeval concoction a “soup” because it would have been rich in chemical (nutrient) free energy. The Oparin/Haldane proposal received strong support from the experiments of Stanley Miller and Harold Urey (Urey had already won the 1934 Nobel Prize in Chemistry for discovering deuterium). Miller and Urey tested the prediction that, under Haldane and Oparin’s prebiotic earth conditions, inorganic molecules could produce the organic molecules in what came known as the primordial soup. Their famous experiment, in which they provided energy to a mixture of inorganic molecules with reducing power, is illustrated below (Figure 1).
Miller’s earliest published data indicated the presence of several organic molecules in their ocean flask, including a few familiar metabolic organic acids (lactate, acetate, several amino acids…) as well as several highly reactive aldehydes and nitriles. The latter can interact in spontaneous chemical reactions to form organic compounds. Later analyses further revealed purines, carbohydrates and fatty acids in the flask. Later still, 50 years after Miller’s experiments (and a few years after his death), some un-analyzed sample collection tubes from those early experiments were discovered.
When the contents of these tubes were analyzed with newer, more sensitive detection techniques, they were shown to contain additional organic molecules not originally reported, including 23 amino acids. To read more, see Surprise Goodies in the Soup!
Clearly, the thermodynamic and chemical conditions proposed by Oparin and Haldane could support the reductive synthesis of organic molecules. At some point, Opar