A membrane boundary structure was essential for the advent of cellular life.The membranes of contemporary cells are composed of a mosaic of proteins embeddedin a bimolecular layer of phospholipids, each of which requires a complex enzymaticpathway for its synthesis. The earliest forms of life could not have had such a highlyevolved pathway in place. Amphiphilic monocarboxylic acids are present in carbonaceousmeteorites and can be synthesized under simulated geochemical conditions. Such compounds have physical and chemical properties that allow them to assemble into bilayermembranes and are therefore plausible components of the first cellular membranes.