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Evolution of cells refers to the evolutionary origin and subsequent evolutionary development of cells. Cells first emerged at least 3.8 billion years ago.〔Schopf, JW, Kudryavtsev, AB, Czaja, AD, and Tripathi, AB. (2007). ''Evidence of Archean life: Stromatolites and microfossils.'' Precambrian Research 158:141-155.〕〔Schopf, JW (2006). ''Fossil evidence of Archaean life.'' Philos Trans R Soc Lond B Biol Sci 29;361(1470):869-85.〕 ==The first cells== The origin of cells was the most important step in the evolution of life on Earth. The birth of the cell marked the passage from pre-biotic chemistry to partitioned units resembling modern cells. The final transition to living entities that fulfill all the definitions of modern cells depended on the ability to evolve effectively by natural selection. This transition has been called the Darwinian transition. If life is viewed from the point of view of replicator molecules, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to keep complex molecules stable in a varying and sometimes aggressive environment; the latter is fundamental for the evolution of biocomplexity. If the freely floating molecules that code for enzymes are not enclosed in cells, the enzymes will automatically benefit the neighbouring replicator molecules. The consequences of diffusion in non-partitioned life forms might be viewed as "parasitism by default." Therefore the selection pressure on replicator molecules will be lower, as the 'lucky' molecule that produces the better enzyme has no definitive advantage over its close neighbors. If the molecule is enclosed in a cell membrane, then the enzymes coded will be available only to the replicator molecule itself. That molecule will uniquely benefit from the enzymes it codes for, giving it a better chance to multiply. Partitioning may have begun from cell-like spheroids formed by proteinoids, which are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could have been the first cellular life forms on Earth. Another possibility is that the shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles made mostly of water tend to burst quickly, oily bubbles are much more stable, lending more time to the particular bubble to perform these crucial experiments. The phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids are composed of a hydrophilic head on one end, and a hydrophobic tail on the other. They possess an important characteristic for the construction of cell membranes; they can come together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is not conducive to harbouring water-soluble organic molecules, but a lipid bilayer bubble () can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction may have occurred when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the right compounds were released into the medium, the development of the first prokaryotes, eukaryotes, and multi-cellular organisms could be achieved.〔This theory is expanded upon in ''The Cell: Evolution of the First Organism'' by Joseph Panno〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolution of cells」の詳細全文を読む スポンサード リンク
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