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Evolution

50 51 aPPendix i Prokaryotes The image opposite shows the family tree of life on earth. There are essentially two fundamental domains of life, firstly the subject of this page, singlecelled microorganisms with no nucleus, called prokaryotes, which come in two important kinds, bacteria archaea, and secondly those with multiple cells containing nucleii and bacteria often mitochondria, which are collectively known as eukaryotes. Although physically tiny, varieties of prokaryotes massively outnumber eukaryotes, as evolution has provided many more ecological niches for them to fill. Most of these free living microorganisms or microbes cohabit environments alongside other organisms or inside them and have a fast life cycle often dividing every 20 minutes, enabling them to diversify rapidly. Others particularly archaea survive in the more extreme habitats that were once ubiquitous on earth some are known to have survived in salt crystals for around 250 million years. The structural simplicity of bacteria and archaea means that they are neither plant nor animal. Having no nucleus, their DNA is loose within their cell wall, and they can swap it freely with others, meaning that most bacteria can, in fact, be regarded as cells in a single global superorganism. Bacteria are incredibly diverse. Some create spores or filaments, others glow in the dark, one or two turn milk to yogurt. Their classification is not complete, but here are some of the dozen or so phyla kingdoms Aquificae, Xenobacteria, Cyanobacteria, Proteobacteria 1650 different species, Firmicutes 2500 species, Spirochetes, Bacteroids, Flavobacteria, Fusobacteria, Thermomicrobia, Chlorobia, Sphingobacteria ... Even more simple than prokaryotes are viruses and prions, which are regarded as non living, despite their status as organic entities. Viruses are bundles of nucleic acid within a shell and cannot grow or reproduce outside of host cells. Most parasitise the cells of eukaryotic organisms. retroviruses transfer their DNA into the chromsomes of their hosts, others viruses invade bacteria and are known as bacteriophages bacteriaeaters. Prions lack nucleic acid and shells. Being little more than particles of protein, they duplicate themselves either inside or outside the cells of the host organism.
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