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Bacteria are single celled micro-organisms of astonishing diversity and abundance that first appeared more than 3.5 billion years ago, in the Archean period. The pattern of bacterial diversification into distinct groups is not certain, but molecular evidence suggests that the common ancestor may have been thermophilic, living in hot, hydrothermal ecosystems. The gut bacterium E. coli is cultured for use in many laboratory experiments, and others such as Shigella and Salmonella are feared as pathogens. Early in the history of life some lineages of nucleated cells (eukaryotes) acquired organelles for respiration and photosynthesis by capturing and integrating, through endosymbiosis, certain forms of bacteria. For example, α-proteobacteria were incorporated to become the respiring mitochondria of eukaryotes, and cyanobacteria were engulfed (apparently on several independent occasions) to form photosynthetic chloroplasts.
Bacteria are a fertile source of information on evolutionary convergence, with a prime example being experiments with E. coli carried out to investigate the consequences of "re-running the tape of life". Ancestral populations can be kept in stasis and compared with a range of descendants after thousands of generations (due to the very fast nature of bacteria replication), an approach which so far has shown that evolutionary trajectories repeatedly take the same course even when run many times independently.
Bacteria from four distantly related groups (e.g. gram-positive actinobacteria and cyanobacteria such as Anabaena and Nostoc) have been independently recruited for nitrogen fixation in the root nodules of leguminous plants and other plants including hornworts and the water fern Azolla (which both recruit different species of Anabaena for N fixation). All of these bacteria use nitrogenase enzymes to reduce atmospheric nitrogen (N2), fixing it as ammonia (NH3) for use in synthesis of essential compounds such as DNA and amino acids.
Among bacteria adapted to extremes of temperature, salinity, acidity or alkalinity (termed 'extremophiles'), notable convergences with certain Archaea (or 'archebacteria', unicellular micro-organisms unlike bacteria or eukaryotes) occur. One such example concerns the similarity between the extremely saline tolerant ('hyper-halophilic') bacterium Salinibacter and the archeal Halobacterium salinarium that also inhabits brines. Heat-tolerance, or 'thermophily' appears to have evolved in several bacterial groups, including Thermotogales (e.g. Thermotoga maritime), Aquificales (e.g. Aquifex), Deinococcus-Thermus (Deinococcus, Thermus) and Chloroflexi (Chloroflexus aurantiacus). Magnetotactic bacteria synthesize crystals of iron minerals (typically magnetite or iron sulphide) to orientate themselves in the ambient magnetic field and find the optimum position in an area of redox boundaries. This remarkable ability evolved at least twice, namely in the α- and δ-proteobacteria.
The propulsive flagellum of bacteria is a complex molecular motor that appears to have evolved convergently (by co-option of pre-existing proteins) within the bacteria and also in the Archea. Certain cyanobacteria have light sensitive pigments (e.g. opsin in Anabaena) and extraordinarily the cyanobacterium Leptolyngbya has an 'eye-spot' for phototaxis. Finally, bacteria appear able to 'communicate' using quorum sensing, as social insects do, to enhance their co-operative activity when forming structures (e.g. biofilms), attacking prey or commencing dispersal.