has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other. Replication first: RNA world Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can catalyze both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that offspring were likely to have different genomes from their parents, and evolution occurred at the level of genes rather than organisms. RNA would later have been replaced by DNA, which can build longer, more stable genomes, strengthening heritability and expanding the capabilities of individual organisms. Ribozymes remain as the main components of ribosomes, the "protein factories" in modern cells. Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga. Although short self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA. In 2003, it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about and ocean-bottom pressures near hydrothermal vents. Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores. Membranes first: Lipid world It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles," and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than outside. The clay hypothesis RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some clays, notably