5 - Origin(s) of Life on Earth

Summary

This chapter is devoted to a foundational question in astrobiology: how and where did life originate? The narrative commences with a brief description of the four major categories of biomolecules (proteins, nucleic acids, carbohydrates, and lipids) on Earth and their associated functions. Partly based on this knowledge, biophysical and biochemical constraints on the minimum size of a viable cell are derived. The various origin(s)-of-life hypotheses are discussed next – like the replication-first (e.g., RNA world) and metabolism-first paradigms – along with their attendant strengths and weaknesses. The pathways by which the building blocks of life (e.g., amino acids) could be synthesised through non-biological avenues, such as the famous Miller experiments, are elucidated. Subsequently, the abiotic channels that may facilitate the polymerisation of these molecules to yield biomolecules are delineated. The focus of the chapter is then shifted to the specialised environments that might have enabled the origin(s) of life to readily occur. Two candidates are reviewed in detail (submarine hydrothermal vents and hydrothermal fields), with others mentioned in passing. Finally, the concept of entropy and its subtle connections with living systems are sketched.