The formation of cells, called cellularization (USA, or cellularisation, GB), remains a fundamental field of research. There are different hypotheses dealing with the origin and development of the first cells on this planet (main article: History of life). By contrast, the formation of cells in extant organisms (Bacteria, Archaea, Eucarya) has been subject to direct investigations (see below). Formation of the first cells Cellularization occurs in several stages. It begins with the formation of primitive lipids (e.g. fatty acids or isoprenoid acids) in the surface metabolism. These lipids accumulate on or in the mineral base. This lipophilizes the outer or inner surfaces of the mineral base, which promotes condensation reactions over hydrolytic reactions by lowering the activity of water and protons. RNA world hypothesis Under the RNA world hypothesis (“genetics first” scenario), a hypothetical lipid-based bubble-like structure protected RNA and was the ancestral precursor cell (last universal common ancestor – LUCA) of the three domains of life. Iron-sulfur world hypothesis Under the Iron-Sulfur world hypothesis (“metabolism first” scenario), according to Wächtershäuser and Kandler's pre-cell theory), there was no ancestral “first cell“ nor a first individual precursor cell. Instead, cell formation was a successive process. In this scenario the (bio)chemical origin, metabolism and evolution of life led to diversification through the development of a multiphenotypical population of pre-cells, from which the founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life emerged successively. Kandler’s pre-cell theory is supported by Wächtershäuser. A scheme of the pre-cell scenario is presented in the adjacent figure, where essential evolutionary improvements are indicated by numbers. Kandler’s concept may explain 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 of life), as well as the close relationship between the Archaea and the Eucarya. The protection of fragile primordial life forms from their environment by the development of envelopes (i.e. membranes, walls) was an essential improvement. For instance, the development of rigid cell walls by the invention and elaboration of peptidoglycan in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of the geosphere and hydrosphere. A coevolution of the biosphere and the geosphere is suggested: “The evolving life