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寒武纪生命大爆炸是一个独特的地球历史时期,基本上所有的动物类群第一次出现化石记录。然而,到目前为止，在试图从化石记录中破译信息的过程中,重要的是要首先理解关键性的原因,了解得寒武纪爆发是发生的一个重大事件。570年到505亿年前,古生代被认为是历史上的主要转折点,当动物生活越来越多元化，越来越差异性的时候,物种之间的形态差异更加突出(马歇尔,2006)。必须强调，在此之前，动物仍然存。 寒武纪辐射,第一个跟踪到的寒武纪化石非常丰富,延长了约6500万年(马歇尔,2006)。在寒武纪时期，几乎没有化石明确地体现基底干细胞群的出现和共同祖先的出现(马歇尔,2006)。 The Cambrian explosion is a unique period in Earth history, when essentially all animal phyla first appear in the fossil record. However, before trying to decipher the information gathered thus far from fossil records, it is important to first understand the key principle that makes Cambrian explosion such a significant event. The Cambrian period occurred between 570 and 505 million years ago, during the Paleozoic era, and is thought to be the primary point in history when animal life diversified and disparities, the morphological differences between species, became more prominent (Marshall, 2006). It must be emphasised that animal life still existed before this period, but with limited morphological body plans. The Cambrian radiation, from the first abundant trace fossils through to the end of the Cambrian, extended for approximately 65 million years (Marshall, 2006). There are virtually no fossils that are unequivocally assignable to the basal stem groups of the phyla that arose during the Cambrian, the last common ancestor of each phylum (Marshall, 2006). The time of onset is constrained by the evolution of the environment, whereas its duration appears to be controlled primarily by rates of developmental innovation. (Marshall, 2006). There are three possible primary explanations that are believed to have led to the Cambrian explosion: changes in the abiotic environment, changes in the genetic or developmental capacity of the taxa involved, and finally, changes in the biotic environment, i.e., in ecology. understanding the interactions between organisms, their environment (both the biotic and abiotic), and their genetic potential for change. (Marshall, 2006) It has been argued that late Neoproterozoic global deep freezes may have played a critical role in the Cambrian explosion (Marshall, 2006). However, this event occurred too early to solely account for the radiation. Animal fossil records beginning after the last of the great glaciations suggests that perhaps the link to animal evolution is through the effects the Snowball Earths had on global oxygen levels (Marshall, 2006). Before higher order taxa could be sustained, the environment had to have been conducive to complex animal life. The accumulation of enough oxygen to sustain large animals is perhaps the most accepted explanation associated with the Cambrian explosion (Marshall, 2006). Testing this hypothesis has proved difficult, as minimum oxygen requirements of the first animals are unknown. Unidentified precise values for Proterozoic oxygen levels through the Cambrian interval also provide constraints on testing the “atmospheric oxygen level” hypothesis (Marshall, 2006). Many Ediacaran organisms had large body sizes, suggesting that there was enough oxygen to support large animals by the late Neoproterozoic (Marshall, 2006). Therefore, oxygen levels had to increase beyond Ediacaran levels to support the Cambrian radiation. It is proposed that higher oxygen levels were required to support the morphologies and tissues needed for predation (Marshall, 2006). carbon isotopic anomaly The presence of an unusually large negative carbon isotopic anomaly, of between 7 to 9 per mil, at the Neoproterozoic/Cambrian boundary (Figure 1) points to an environmental disturbance at a critical time in Earth history, so it is perhaps natural to assume it played a role in the Cambrian “explosion” (Marshall, 2006) it is has been assumed that it represents a sufficiently large environmental disturbance that it caused a mass extinction of the Ediacaran biota, and in doing so laid the foundation for Cambrian radiation(Marshall, 2006) how a simple (even if large) environmental disturbance can lead to an increase in disparity. In fact, one wonders whether the carbon isotopic anomaly might be independent of the unfolding Cambrian “explosion,” or even a consequence of the radiation. (Marshall, 2006) True polar wander rapid movement of the continents caused huge methane “burps”, which in turn triggered the Cambrian “explosion.” They argue that the increased temperatures induced by the methane releases drove the observed increase in diversity. No explanation is offered as to why an increase in diversity, per se, should have led to new levels of disparity. (Marshall, 2006) why an environment permissive of complex animal life should necessarily lead to the evolution of complex animal life, and especially why we should see a shift from diploblastic-grade organisms to complex triploblasts. (Marshall, 2006) Changes in the way the genes are wired is a major, if not the major, source of morphological innovation(Marshall, 2006)a relatively small increase in underlying genomic complexity can lead to rich morphogenetic potential. (Marshall, 2006) the uniqueness problem by invoking a theoretical niche space. They argue that if large morphological changes are less likely to occur than small changes, and if the probability of a new major innovation being successful depends on there being limited competition at the time the large morphological change occurred (so that the nascent higher taxon has a chance to adjust to its new ecological niche), then at the initiation of the radiation many higher taxa should appear, but as the ecosystem's niches fill up, it will be progressively harder for new body plans to become established; the world's niche space becomes saturated. The reason, then, why the end-Permian mass extinction did not lead to new phyla is that the relatively few Triassic survivors still had most of the niche space occupied despite the decimation, so that new body plans could not get an ecological foothold. (Marshall, 2006) It is clear that the environment must be permissive of animals before they could have evolved. It is also clear that the genetic machinery for making animals must have been in place, at least in a rudimentary way, before they could have evolved. And finally, organisms must be able to leave viable offspring to survive and evolve, so ecology had to be important too. (Marshall, 2006) all the processes associated with variation (point mutation, recombination, hybridization, gene conversion, insertion and deletion, post-transcriptional changes in mRNA processing, etc.) are able to effectively explore fitness landscapes on geological timescales; evolution is able solve the np-hard problem of exploring the rich combinatorial potential embedded in the genome in the order of 10–20 million years. (Marshall, 2006) Early Cambrian fossil assemblages are distinctly different. Although at least some Ediacarans survived into the Cambrian (17), the Cambrian fauna consists predominantly of stem and crown group members of extant bilaterian phyla, along with diverse sponges and rarer cnidarians and ctenophores (18). Mineralized skeletons are widely distributed, if formed by a minority of Cambrian species. Trace fossils independently record a notable expansion of behavioral complexity within bilaterian animals(Knoll, & Carroll, 1999) any pre-Ediacaran metazoans must have been rare, small, or gossamer forms unlikely to be preserved. (Knoll, & Carroll, 1999) Earliest Cambrian assemblages contain few taxa, and the diversity of trace and body fossils grew only over a protracted interval (3, 26, 31, 32). Hyoliths and halkierids (extinct forms thought to be related to mollusks), true conchiferan mollusks and, perhaps, chaetognaths enter the record during the first 10 to 12 million years of the Cambrian, but crown-group fossils of most other bilaterian phyla appear later: the earliest body fossils of brachiopods, arthropods, chordates, and echinoderms all post-date the beginning of the period by 10 to 25 million years (Fig. 2) (32). Trace fossils suggest earlier appearances for some groups, notably arthropods(Knoll, & Carroll, 1999) It does not represent the origin of life or of the animal clade. Nor, given the the presence of mollusks near the beginning of the Cambrian and probable stem bilaterians in Ediacaran rocks, does it appear to represent the initial divergence of major animal clades. Rather, the Cambrian explosion records the radiation of bilaterian animals (and sponges) with modern body plans (33)—the diversification of crown groups within clades that diverged earlier(Knoll, & Carroll, 1999) With most of the 35 or so modern metazoan phyla represented or inferred by the end of the Cambrian(Knoll, & Carroll, 1999) Arthropods are the most abundant Cambrian fossils, and by the time of the Burgess Shale, they had evolved a number of different body plan designs and many variations upon these themes(Knoll, & Carroll, 1999) biophysical considerations relate maximum body size to oxygen availability in animals that obtain oxygen by diffusion(Knoll, & Carroll, 1999) Oxygen would not in and of itself have caused animals to evolve. Rather, it would have removed an environmental barrier to the evolution of large, metabolically active animals. (Knoll, & Carroll, 1999) Suggests that regardless of genetic potential, large bilaterian animals could not have diversified before the late Proterozoic Eon. (Knoll, & Carroll, 1999) once rising oxygen levels removed physical barriers to the evolution of large size, no other extrinsic drivers were required; morphological innovations such as complex nerve nets or sensory, locomotory, and feeding appendages made the Cambrian explosion inevitable. (Knoll, & Carroll, 1999) Globally distributed sedimentary successions document a strong (7 to 9 per mil) but short-lived negative excursion in the carbon-isotopic composition of surface seawater at the stratigraphic breakpoint between(Knoll, & Carroll, 1999) Ediacaran-rich fossil assemblages and those that document the beginning of true Cambrian diversification(Knoll, & Carroll, 1999) comparable events(Knoll, & Carroll, 1999)the Permo-Triassic crisis, when some 90% of marine species disappeared, is marked by an excursion similar to but smaller than the Proterozoic-Cambrian boundary event(Knoll, & Carroll, 1999) It is biogeochemistry that lends substance to the hypothesis that Ediacaran and Cambrian faunas are separated by mass extinction. (Knoll, & Carroll, 1999) moderate diversity of mammalian stem groups lived for millions of years in dinosaurdominated ecosystems. Only with the demis e of the dinosaurs did mammals radiate to produce the crown group diversity seen in Tertiary and modern faunas. Perhaps Ediacaran animals are the “dinosaurs” of the terminal Proterozoic oceans, simple but successful organisms that placed ecological constraints on(Knoll, & Carroll, 1999) Cambrian diversification be seen as an ecosystem- wide phenomenon that affected protists, sponges and bilaterians alike. (Knoll, & Carroll, 1999) The Ediacaran biota first appears shortly after the Gaskiers glacial and all but become extinct at, or shortly after, the Precambrian/Cambrian boundary(Marshall, 2006) In the last five million years of the Ediacaran, the first lightly skeletonized forms appear (Figure 1), including the relatively simple multicone-shaped Cloudina and the rather more complex, but still small, Namacalathus and its relatives(Marshall, 2006) most agree that most phyla have at least some Precambrian history. (Marshall, 2006) Whether disparity is measured by the number of phyla, or by their constituent classes (Figure 1), the rate of accumulation of disparity during the (Early) Cambrian “explosion” is unprecedented and unique. (Marshall, 2006) Along with the dramatic increase in disparity comes a dramatic increase in the number of species (Figure 1), although the increase in diversity persists longer than the increase in disparity. (Marshall, 2006) the onset of the Cambrian “explosion” is in the Ediacaran when there is evidence of stem group bilaterian fossils, when the trace fossil record suggests bilaterians large enough to burrow appeared, and when large animals, the enigmatic Ediacaran biota, occur with some abundance. Nonetheless, the first threedimensional trace fossils and the first appearance of numerous heavily skeletonised taxa occur in the Cambrian, so designating the base of the Cambrian as the time of onset of the radiation is appropriate(Marshall, 2006) Never before or since has there been such as dramatic emergence of animal disparity as seen in the Cambrian “explosion.” Even after the largest mass extinction of the Phanerozoic, the end-Permian mass extinction, no new phyla appeared. (Marshall, 2006) It was not until the subsequent Ordovician radiation that typical Phanerozoic diversities were reached. It was also not until the Ordovician that typical Phanerozoic ecosystems emerged. So, the Ordovician radiation was also a time of great significance(Marshall, 2006) animals did not invade the terrestrial realm until later in the Paleozoic, and they did not master the aerial realm until the later Paleozoic (arthropods) and in the Mesozoic (pterosaurs and birds) and Cenozoic (bats); there are many other major events in the history of animal life other than the Cambrian “explosion.” (Marshall, 2006) Perhaps some of the Ediacarans are in fact the missing bilaterians stem groups. Indeed, a number of taxa have been variously assigned as stem groups of existing phyla, for example, Arkarua as basal echinoderm (Gehling 1987); Kimberella as basal mollusk (Fedonkin and Waggoner 1997); and perhaps taxa like Dickinsonia and Spriggina are basal ecdysozoans, the relatively recently recognized grouping of phyla initially recognized on the basis of 18S rRNA data (for review see Halanych 2004) that include Annelida and Arthropoda, among others. (Marshall, 2006) Cambrian faunas were preceded by the so-called Ediacaran biota, preserved as impressions, casts, and molds in uppermost Proterozoic rocks around the world(Knoll, & Carroll, 1999) while paleontological discoveries shed increasing light on biological diversity just before the Cambrian explosion, new fossils have not extended bilaterian crown group morphologies deeply below the Proterozoic-Cambrian boundary. They do, however, allow us to reject the episodically popular view that the lack of recognizable crown group bilaterians in Proterozoic rocks can be attributed to a dearth of suitable rocks or preservational opportunities (5). Latest Proterozoic sedimentary rocks are widely distributed and, commonly, well preserved and exposed. They are full of body and trace fossils that reflect the complete spectrum of fossilization mechanisms responsible for the succeeding Cambrian record. Crown group bilaterians may yet be demonstrated in terminal Proterozoic rocks, but the diversification of crown groups was principally a Cambrian event. Cambrian diversification thus reflects an evolutionary milestone regardless of the length or character of earlier animal history. (Knoll, & Carroll, 1999) Independent of any molecular clock considerations, fossils of red algae and other protists indicate that the major radiation of eukaryotic life—a divergence that includes the animals (29)—began no later than 1200 to 1000 Ma (30). Multicellularity and clade divergence came early to several algal groups and may have done so in the animal clade as well, but at present the fossil record is silent on these issues. (Knoll, & Carroll, 1999) Paleontology indicates that these ancestors lived before the Cambrian—at least 560 Ma and possibly more than 600 Ma. Molecular clock estimates are controversial and yield disparate results (5, 67, 68), but even those estimates most nearly concordant with paleontological data suggest a prostome-deuterostome split at least 100 m.y. before bilaterians appear in the fossil record (67). Collectively, then, available data suggest that two substantial periods of bilaterian evolution preceded the Cambrian: the evolution of the bilaterian stem lineage leading to Urbilateria and the subsequent diversification of the three major bilaterian clades. (Knoll, & Carroll, 1999) moderate diversity of mammalian stem groups lived for millions of years in dinosaurdominated ecosystems. Only with the demis e of the dinosaurs did mammals radiate to produce the crown group diversity seen in Tertiary and modern faunas. Perhaps Ediacaran animals are the “dinosaurs” of the terminal Proterozoic oceans, simple but successful organisms that placed ecological constraints on(Knoll, & Carroll, 1999) The uniqueness of the event is either due to ensuing developmental limitation, to ecological saturation, or simply to the exhaustion of ecologically viable morphologies that could be produced by the nascent bilaterian developmental system. (Marshall, 2006) the increases in disparity and diversity that characterize the Cambrian “explosion” are largely over by the end of the Early Cambrian, many crown group higher taxa are first found in the Middle and Late Cambrian and beyond(Marshall, 2006) Never before or since has there been such as dramatic emergence of animal disparity as seen in the Cambrian “explosion.” Even after the largest mass extinction of the Phanerozoic, the end-Permian mass extinction, no new phyla appeared. (Marshall, 2006) uniqueness of the Cambrian “explosion” may simply represent the exhaustion of ecologically viable alternatives that can be generated by the bilaterian developmental system (Table 1), rather than being due to ensuing developmental limitation (developmental entrenchment) or ecological saturation, which simply prevented new body plans for gaining a foothold. (Marshall, 2006) if the environmental conditions necessary for the evolution of large metazoans had occurred earlier, when the animal developmental system was perhaps in a simpler state, then perhaps the body plans seen would have been different; while the environment, per se, probably exerts very little control on patterns of observed disparity and diversity, perhaps the time the environment became conducive to the evolution of animals played a major role in the morphologies that evolved. (Marshall, 2006) Extrinsic events also helped to shape early animal evolution altering environments in ways that doomed some clades and created opportunity for others. (Knoll, & Carroll, 1999) Marshall, C. (2006). Explaining the Cambrian “Explosion” of animals. Annual Review of Earth and Planetary Science, 34, 355-384. (Marshall, 2006) Knoll, A., & Carroll, S. (1999). Early animal evolution: Emerging views from comparative biology and geology. Science, 284, 2129 – 2137. (Knoll, & Carroll, 1999) The beginning of the 20th century, with the transformation of Chinese society from tradition to modern, from the traditional culture, developed the traditional literature, already cannot satisfy and adapt to new social people in mental, emotional and sensory requirements, going to decline.Time request new and change of Chinese literature.Under this big background, all kinds of modern western ideological trend of relying on the power of its economy, politics, science and technology, the big push into China, Chinese traditional literature in the ideological trend of the rapid disintegration of fragile, under the impact of a new generation of writers in husserl, heidegger, Nietzsche, Schopenhauer, kierkegaard, under the influence of Freud and others thought, set off a huge new literature movement.The beginning of the 20th century Chinese literature as a starting point, from closed to open, by local facing the world.Writers in this period, all under the influence of modern western ideological trend, lu xun's "cry" the basic spirit of realism and technique, to absorb the symbolism and romanticism, and other methods, established the basic character of modern Chinese fiction;Guo moruo's "goddess" by tagore, Goethe, Whitman, Shelley, etc multiple effect, formed the romanticism style;Bing xin, zhou zuoren's prose, but also under the influence of western prose writers formed their own style...In the 30 s, by Roy's doctrine and the influence of western modernism and postmodernism, China also appeared such as liu sonar gulls, MuShiYing represented by the new feeling novel.During this period, however, western modern ideological trend of Chinese literature thought is limited, its main role in China's literary form, without thorough marrow of Chinese literature, not really affect the spirit of Chinese literature.Chinese writer of modern western ideological trend of the study, only stay on the surface, also can't into the depths of the ideological trend of western culture. |
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