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The Cambrian explosion, or less commonly Cambrian radiation, was the relatively short evolutionary event, beginning around in the Cambrian Period, during which most major animal phyla appeared, as indicated by the fossil record. Lasting for about the next 20–25〔Kouchinsky, A., Bengtson, S., Runnegar, B. N., Skovsted, C. B., Steiner, M. & Vendrasco, M. J. 2012. Chronology of early Cambrian biomineralization. Geological Magazine, 149, 221–251.〕 million years, it resulted in the divergence of most modern metazoan phyla. Additionally, the event was accompanied by major diversification of other organisms. Prior to the Cambrian explosion,〔At 610 mya, ''Aspidella'' disks appeared, but it is not clear that these represented complex life forms.〕 most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 to 80 million years, the rate of diversification accelerated by an order of magnitude and the diversity of life began to resemble that of today. Many of the present phyla appeared during this period,〔 with the exception of Bryozoa, which made its earliest known appearance in the Lower Ordovician. The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against the theory of evolution by natural selection. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin of animal life. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks. Phylogenetic analysis has been used to support the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. ==History and significance== (詳細はtrilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698. Although their evolutionary importance was not known, on the basis of their old age, William Buckland (1784–1856) realised that a dramatic step-change in the fossil record occurred around the base of what we now call the Cambrian.〔 Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata, specifically for establishing the Cambrian and Silurian periods. By 1859, leading geologists including Roderick Murchison, were convinced that what was then called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell, differed. In ''On the Origin of Species'', Charles Darwin considered this sudden appearance of solitary group of trilobites with no apparent antecedents, and absence of other fossils to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection. He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found due to the imperfections of the fossil record.〔 In the sixth edition of his book, he stressed his problem further as: American paleontologist Charles Walcott, who discovered the Burgess Shale, proposed that an interval of time, the “Lipalian”, was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time. Earlier fossil evidence has since been found. The earliest claim is that the history of life on earth goes back : Rocks of that age at Warrawoona, Australia, were claimed to contain fossils of stromatolites, stubby pillars formed by colonies of microorganisms. However, since Heinrich's paper in 1997, Brasier convincingly debunked this discovery of stromatolites by 2006, being merely "secondary artifacts formed from amorphous granite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass", adding "there is no support for primary biological morphology". Still, Fossils (''Grypania'') of more complex eukaryotic cells, from which all animals, plants, and fungi are built, have been found in rocks from , in China and Montana. Rocks dating from contain fossils of the Ediacara biota, organisms so large that they are likely multicelled, but very unlike any modern organism.〔 In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late Cambrian arose.〔 The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were complex, but different from any living animals.〔Whittington, H. B. (1979). Early arthropods, their appendages and relationships. In M. R. House (Ed.), The origin of major invertebrate groups (pp. 253–268). The Systematics Association Special Volume, 12. London: Academic Press.〕 The most common organism, ''Marrella'', was clearly an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed ''Opabinia'' and spiny slug-like ''Wiwaxia'' were so different from anything else known that Whittington's team assumed they must represent different phyla, seemingly unrelated to anything known today. Stephen Jay Gould’s popular 1989 account of this work, ''Wonderful Life'', brought the matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal phyla had appeared almost simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. Other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian.〔 〕〔 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「Cambrian explosion」の詳細全文を読む スポンサード リンク
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