From organic molecules in space to the origins of life and back

Abstract

Delivery of organic molecules much simpler than building blocks of biological structures may have been sufficient to initiate the process of chemical evolution leading to the first forms of life. By defining the simplest protocellular systems, it is possible to deduce what organic molecules were likely to be necessary for this process. Some of these molecules were building blocks of protocellular structures which self-assembed from amphiphilic compounds into vesicles and other structures, such as micelles and multilayers. There must also have been relatively simple mechanisms by which amino acids or their precursors were incorporated into simple peptides. At some point this process became compartmented in vesicles, which would require the emergence of cellular transport and metabolism. Energy required for these processes may have been provided by the coupling of the transmembrane proton gradient to the synthesis of high energy compounds, such as thioesters, or by carbon disproportionation reactions, starting with sugars. If these conjectures are correct, it follows that the first forms of life emerged as self-contained molecular systems, rather than as macromolecules that somehow incorporated the basic properties associated with the living state.