Chapter 9 - Liquid–liquid phase separation at the origins of life

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This chapter addresses the products of liquid–liquid phase separation (LLPS) in research related to the origins of life (OoL). The products of LLPS are identified by several names, starting before the origin of the phrases, “liquid in liquid phase separation” and “membrane-less organelles.” Other names for these membrane-less organelles are MLOs, biomolecular concentrates, aqueous two-phase systems (ATPS), coacervates, and microdroplets. MLOs could form from an origin of life in the ocean or on land. MLOs provide compartmentalization, confinement, crowding, and stability for molecules. Protocells are more complex than simple MLOs but are based on LLPS. Protocells are defined, in this chapter, as being not at equilibrium, like living cells. Lipids are discussed, both as components of some MLOs and as molecules that can form vesicles, in alternate approaches to the OoL. RNA and DNA are biomolecules often used in research on MLOs at the OoL, as are amino acids, peptides, proteins, and intrinsically disordered proteins (IDPs). The inorganic cations of sodium and potassium are important considerations, given that K+ is present at high concentrations in all living cells, but Na+ is the dominant monovalent cation in seawater. Clays and wet-dry cycles are also potentially relevant for LLPS at the OoL.