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2018-11-01 01:20:23

•   The History of Animal Evolution
  For many people animals are perhaps the most familiar, and most interesting, of living things。
     This may be because we are animals ourselves。 As such, we have a number of features in common with all the organisms placed in the animal kingdom, and these common features indicate that we have a shared evolutionary history。
    
  All animals and plants are classified as multicellular eukaryotes多细胞性的真核: their bodies are made up of large numbers of cells, and microscopic inspection of these cells reveals that they contain a nucleus and a number of other organelles细胞器。
     Compared to prokaryotic organisms such as bacteria, plants and animals have a relatively recent evolutionary origin。 DNA evidence suggests that the first eukaryotes evolved from prokaryotes, between 2500 and 1000 million years ago。
     That is, eukaryotes as a taxon date from the Proterozoic Era, the final Era of the Precambrian。 Fossils of both simple unicellular and more complex multicellular多细胞性organisms are found in abundance in rocks from this period of time。
     In fact, the name "Proterozoic" means "early life"。
  Plants and animals both owe their origins to endosymbiosis, a process where one cell ingests another, but for some reason then fails to digest it。
     The evidence for this lies in the way their cells function。 Both plant and animal rely on structures called mitochondria线粒体to release energy in their cells, using aerobic respiration有氧呼吸作用to produce the energy-carrying molecule ATP。
     There is considerable evidence that mitochondria evolved from free-living aerobic bacteria: they are the size of bacterial cells; they divide independently of the cell by binary fission二进制分裂; they have their own genome染色体in the form of a single circular DNA molecule; their ribosomes核糖体are more similar to those of bacteria than to the ribosomes核糖体found in the eukaryote cell's cytoplasm; and like chloroplasts they are enclosed by a double membrane as would be expected if they derived from bacterial cells engulfed by another cell。
     
  Like the plants, animals evolved in the sea。 And that is where they remained for at least 600 million years。 This is because, in the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation。
     Once photosynthesis had raised atmospheric oxygen levels high enough, the ozone layer formed, meaning that it was then possible for living things to venture onto the land。
    
  象植物, 动物演变了在海。并且那是他们保留了至少600 百万年的地方。这是因为, 在没有防护臭氧层时, 土地沐浴了在紫外辐射的致死的水平。一旦光合作用上升了大气氧气成水平足够高, 臭氧层被形成, 意味, 它是然后可能为生存事冒险土地。
    
  The oldest fossil evidence of multicellular animals, or metazoans, is burrows that appear to have been made by smooth, wormlike organisms。
     Such trace fossils have been found in rocks from China, Canada, and India, but they tell us little about the animals that made them apart from their basic shape。
    
  多细胞性的动物的最旧的化石看上去由光滑, wormlike 有机体做了的证据, 或metazoans, 是挖洞。这样踪影化石被发现了在岩石从中国、加拿大,和印度, 但他们告诉我们一点关于做他们除他们基本的形状之外的动物。
    

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  2018-11-01 01:20:23

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