When phospholipids or simple lipids like fatty acids are placed in water, the molecules spontaneously arrange such that the hydrophobic tails are shielded from the water, resulting in the formation of membrane structures such as bilayers, vesicles, and micelles. In modern cells, vesicles are involved in metabolism, transport, buoyancy control, and enzyme storage. They can also act as natural chemical reaction chambers. A typical vesicle or micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle center. This phase is caused by the packing behavior of single-tail lipids in a bilayer. Although the spontaneous self-assembly process that form lipid monolayer vesicles and micelles in nature resemble the kinds of primordial vesicles or protocells that might have existed at the beginning of evolution, they are not as sophisticated as the bilayer membranes of today's living organisms. However, in a prebiotic context, electrostatic interactions induced by short, positively charged, hydrophobic peptides containing seven amino acids in length or fewer, can attach RNA to a vesicle membrane, the basic cell membrane.
Rather than being made up of phospholipids, early membranes may have formed from monolayers or bilayers of simple fatty acids, which may have formed more readily in a prebiotic environment. Fatty acids have been synthesized in laboratories under a variety of prebiotic conditions and have been found on meteorites, suggesting their natural synthesis in nature. Oleic acid vesicles represent good models of membrane protocells
Cohen et al. (2022) suggest that plausible prebiotic production of fatty acids — leading to the development of early protocell membranes — is enriched on metal-rich mineral surfaces, possibly from impact craters, increasing the prebiotic environmental mass of lipids by 10 times. They evaluate three different possible synthesis pathways of fatty acids in the Hadean, and found that these metal surfaces could produce 10 - 10 kg of 6-18 carbon fatty acids. Of these products, the 8-18C fatty acids are compatible with membrane formation. They also propose that alternative amphiphiles like alcohols are co-synthesized with fatty acid, and can help improve membrane stability. However, despite this production, the authors state that net fatty acid synthesis would not yield sufficient concentrations for spontaneous membrane formation without significant evaporation of Earth's aqueous environments.