montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analogue of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters. Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles," and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids. Metabolism first: Iron–sulfur world A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts. Most of the steps required temperatures of about and moderate pressures, although one stage required and a pressure equivalent to that found under of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents. Metabolism first: Pre–cells (successive cellularisation) In this scenario, the biochemical evolution of life led to diversification through the development of a multiphenotypical population of pre-cells, i.e. evolving entities of primordial life with different characteristics and wide-spread horizontal gene transfer. From this pre-cell population the precursor cells (proto-cells) of the three domains of life arose in the founder groups A, B, C, successively leading first to the domain Bacteria, then to the domain Archea and finally to the domain Eucarya. For the development of more stable cells (cellularisation), the pre-cells had to be protected from their surroundings by envelopes (i.e. membranes, walls). For instance, the development of rigid cell walls by the invention of peptidoglycan in bacteria (domain Bacteria) may have been a prerequisite for their success: their survival, radiation and colonisation of virtually all habitats of the geosphere and hydrosphere. This scenario offers explanations for the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity