old was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada, although the evidence is disputed as inconclusive. Origins of life on Earth Biologists reason that all living organisms on Earth must share a single last universal ancestor, because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms. According to a different scenario a single last universal ancestor, e.g. a “first cell“ has never existed. Instead, the early biochemical evolution of life led to diversification through the development of a multiphenotypical population of pre-cells from which the precursor cells (proto-cells) of the three domains of life emerged. Thus the formation of cells was a successive process (see below, section “Metabolism first: Pre-cells, successive cellularisation”). Independent emergence on Earth Life on Earth is based on carbon and water. Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from carbon dioxide. There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; silicon, the element directly below carbon on the periodic table, does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because silicon dioxide is a hard and abrasive solid in contrast to carbon dioxide at temperatures associated with living things, it would be more difficult for organisms to extract. The elements boron and phosphorus have more complex chemistries, but suffer from other limitations relative to carbon. Water is an excellent solvent and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have electrically negative and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as ammonia, are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages. Organisms based on alternative biochemistry may, however, be possible on other planets. Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. Research on abiogenesis still