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  • 研究演变的工具

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    Myths About Earth
    ::关于地球的神话

    This interesting image is a 19th century representation of Earth based on an ancient Hindu myth. According to the myth, Earth rests on the backs of elephants, which in turn stand on the back of a giant turtle. Virtually all human cultures and religions have developed myths about Earth and its origins. Until fairly recently, for example, many Westerners thought that Earth was created in a day and that this occurred just a few thousand years ago. However, a diversity of evidence has since convinced the scientific community that Earth actually formed by natural processes from stardust a mind-boggling 4.5 to 4.6 billion years ago. Evidence also suggests that life first appeared on Earth up to four billion years ago... and has been evolving ever since.
    ::这幅有趣的图象代表了19世纪的地球,它基于古老的印度教神话。根据神话,地球位于大象的背部,而大象又站在巨龟的背面。几乎所有人类文化和宗教都对地球及其起源产生了神话。例如直到最近,许多西方人一直认为地球是在一天之内诞生的,而地球就在几千年前才发生。然而,从那时以来,有各种各样的证据使科学界相信地球实际上是由45亿至46亿年前的星尘的自然过程形成的。证据还表明生命最初出现在地球上,到40亿年前...,并且从那时起一直在演变。

    Earth in a Day
    ::大地在一日中

    It can be difficult to wrap your mind around such vast amounts of time as the age of Earth and its early life forms. A useful way for envisioning the relative amounts of time that passed between Earth's origin and important events in biological evolution is to condense the total period of time to a 24-hour day, as shown in the figure . On this scale, Earth would have formed at midnight, and the first life would have appeared at about 3:00 a.m. Humans would have appeared only during the last minute of the day. If we are such newcomers to planet Earth, how do we know about the vast period of time that went before us? How have we learned about the distant past?
    ::很难在地球时代及其早期生命形式等巨大的时间左右你。设想地球起源和生物演变中重要事件之间的相对时间间隔的有用方式是将整个时间缩短到24小时,如图所示。在这个规模上,地球会在午夜形成,而第一个生命将在凌晨3点左右出现。人类只有在当日的最后一分钟才会出现。如果我们是地球的新人,我们如何知道我们面前的漫长时期?我们如何了解遥远的过去?

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    History of Earth in a Day. In this model of Earth’s history, the planet formed at midnight. What time was it when the first prokaryotes evolved?
    ::地球在一天中的历史。在这个地球历史的模型中,地球是在午夜形成的。第一次环形行星进化的时候是什么时候?

    The Fossil Record
    ::化石记录

    Much of what we know about the on Earth is based on the , which  is an extremely important tool in the study of evolution. The fossil record is the record of life that unfolded over four billion years on Earth as reconstructed from the discovery and analysis of fossils.  Fossils are the preserved remains or traces of organisms that lived in the past. The soft parts of organisms almost always decompose quickly after death. On occasion, the hard parts — mainly , teeth, or shells — remain long enough to mineralize and form fossils. An example of a complete skeleton is pictured .
    ::我们所了解的关于地球上的大部分情况都以地球为基础,而地球是研究进化的一个极为重要的工具,化石记录是地球40多亿年的生命记录,这是从发现和分析化石中重建而来的生命记录,化石是保存的残骸或过去生物的痕迹,生物体的软部分几乎总是在死亡后迅速腐烂,有时硬部分——主要是牙齿或贝壳——仍然足够长的时间来进行矿物化和形成化石,一个完整的骨骼的例子被描绘出来。

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    Extinct Lion Fossil. This fossilized skeleton represents an extinct lion species. It is rare for fossils to be as complete and well preserved as this one.
    ::狮子化石灭绝。这个化石骨骼代表了灭绝的狮子物种。化石很少能像这个一样完整和保存好。

    To be preserved as fossils, remains must be covered quickly by sediments, or preserved in some other way. They may be frozen in glaciers or trapped in tree resin, like the frog shown . Sometimes, traces of organisms — such as footprints or burrows — are preserved. Fossilized dinosaur footprints are also shown below. The conditions required for fossils to form rarely occur, so the chance of any given organism being preserved as a fossil is extremely low.
    ::为了作为化石加以保存,遗骸必须迅速被沉积物所覆盖,或以某种其他方式加以保存。它们可能被冻在冰川中,或被困在树脂中,如青蛙所展示的那样。有时,生物的痕迹——如脚印或洞穴——会得到保存。下面还显示化石化化的恐龙脚印。化石形成化石所需的条件很少发生,因此任何特定生物作为化石被保存的可能性极低。

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    The photo on the left shows an ancient frog trapped in hardened tree resin, or amber. The photo on the right shows the fossil footprints of a dinosaur.
    ::左边的照片显示一只被困在硬树脂或琥珀中的古老青蛙。 右边的照片显示了恐龙的化石足迹。

    In order for fossils to “tell” us the story of life, their chronology must be established. Fossils must be dated. Only then can they help scientists reconstruct how life changed over time. Fossils can be dated in two different ways, called relative dating and absolute dating .
    ::为了让化石“告诉我们”生命的故事,必须确定它们的年表。 化石必须日期。 只有这样,它们才能帮助科学家重建生命随时间变化的方式。 化石可以两种不同的方式出现,称为相对约会和绝对约会。

    • Relative dating determines which of two fossils is older or younger than the other, but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in the figure .
      ::相对日期决定了两个化石中哪个比其他化石年长或更小, 而不是年老。 相对日期是根据岩石层的化石位置来决定的。 下层是先前的, 因此假定它们含有老化石。 这一点在图中可以说明 。
    • Absolute dating determines how long ago a fossil organism lived, giving the fossil an age in years. Absolute dating may be based on the amount of carbon-14 or other radioactive element that remains in a fossil. This is illustrated in the figure .
      ::绝对日期决定了化石生物活了多久, 给化石一个年限。 绝对日期可以基于在化石中残留的碳-14或其他放射性元素的数量。 图中说明了这一点 。

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    Relative Dating Using Rock Layers. Relative dating establishes which of two fossils is older than the other. It is based on the rock layers in which the fossils formed.
    ::相对日期 使用岩石层。 相对日期确定两个化石中的哪一个比另一个老。 它以化石形成时的岩石层为基础 。

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    Because carbon-14 decays at a known rate, the amount of carbon-14 remaining in a fossil can be used as a method of absolute dating.
    ::由于碳-14以已知速度衰减,化石中的碳-14残留量可以用作绝对约会的方法。

    Molecular Clocks
    ::分子钟

    are also valuable tools for studying evolution. A  molecular clock uses sequences (or the amino acid sequence of that DNA encodes) to estimate how long it has been since related diverged from a common ancestor . Molecular clocks are based on the assumption that accumulate over  time at a steady rate for a given region of DNA. Species that have accumulated greater differences in their DNA sequences are assumed to have diverged from their common ancestor in the more distant past. Molecular clocks based on different regions of DNA may be used together for more accuracy.
    ::分子钟也是研究进化的宝贵工具。分子钟使用序列(或该DNA编码的氨基酸序列)来估计它与一个共同的祖先相分离的时间长度。分子钟所依据的假设是,随着时间的推移,在一定的DNA区域以稳定的速率积累。在DNA序列中积累了较大差异的物种假定在较远的过去与其共同的祖先不同。基于不同区域DNA的分子钟可以一起使用,以便更加精确。

    Look at the comparisons of DNA in the table . The table shows how similar the DNA of several species is to human DNA. Based on these data, which organism do you think shared the most recent common ancestor with humans?
    ::图表显示了几个物种的DNA与人类DNA的相似性。根据这些数据,你认为哪个生物与人类分享了最新的共同祖先?

    Organism Similarity with Human DNA (percent)
    Chimpanzee 98
    Mouse 85
    Chicken 60
    Fruit Fly 44

    Geologic Time Scale
    ::地质时标

    Yet another tool for understanding the history of Earth and its life is the , shown in the figure . The geologic time scale divides Earth’s history into smaller units of time (such as eons, eras, and periods) that are based on major changes in geology, climate , and the . It organizes Earth’s history and biological evolution on the basis of important events, instead of on the basis of elapsed time alone. This helps tell the story of Earth's past and puts more focus on recent events, about which we know the most.
    ::理解地球及其生命历史的另一个工具是图中显示的。地质时间尺度将地球历史分为小的时间单位(如太阳、时代和周期 ) , 以地质、气候和时间的重大变化为基础。 它组织地球历史和生物进化的基础是重要事件,而不是仅仅根据时间的流逝。 它有助于讲述地球过去的故事,并更加关注我们最了解的近期事件。

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    Geologic Time Scale. The geologic time scale divides Earth’s history into units that reflect major changes in Earth and its life forms. During which eon did Earth form? What is the present era? 
    ::地质时间尺度。 地质时间尺度将地球历史分为反映地球及其生命形式重大变化的单位。 地球在哪个时期形成? 当今时代是什么?

    Feature: Myth vs. Reality
    ::特征:神话对现实

    Myth: Gaps in the fossil record disprove evolution.
    ::传说:化石记录中的差距与进化不相符。

    Reality: Gaps in the fossil record — where transitional fossils between ancestral and descend a nt groups  have not been found — are to be expected. The chances of organisms being fossilized is low. Some organisms do not preserve well, and conditions needed for fossilization are only rarely present. If evolution is occurring rapidly, the chances of transitional fossils forming is even lower. Even if fossils of transitional organisms do form, they must be discovered by researchers to be added to the fossil record. The vast majority of fossils have not been found. Researchers are studying the fossilization process to shed light on how much of the fossil record has not yet been discovered.
    ::现实:在化石记录中——没有发现祖传和后代群体之间的过渡化石——的差距是预期的;生物化石化化的可能性很低;一些生物体保存不好,化石化所需的条件很少;如果演变迅速,转化化石形成的可能性甚至更小;即使过渡生物体的化石确实形成,研究人员也必须发现它们加入化石记录;绝大多数的化石尚未被发现;研究人员正在研究化石化过程,以揭示化石记录中有多少尚未被发现。

    Fortunately, like fingerprints at a murder scene, the fossil record is just one type of evidence for evolution. In addition to fossils, molecular sequences and other types of evidence are all used together to reveal how life on Earth evolved.
    ::幸运的是,就像谋杀现场的指纹一样,化石记录只是进化的一种证据。 除了化石、分子序列和其他类型的证据,它们都一起被用来揭示地球上的生命是如何演变的。

    Summary
    ::摘要

    • Considering Earth's 4.5-billion-year history as a 24-hour day helps to put such unimaginable time spans into human perspective.
      ::将地球45亿年的历史视为24小时的一天,有助于将这种难以想象的时间跨度纳入人类的视角。
    • The fossil record is the record of life that unfolded over four billion years on Earth, as reconstructed from the discovery and analysis of fossils. It is one of the most important tools in the study of evolution, but it is incomplete because fossilization is rare. To be added to the fossil record, fossils must be dated using relative or absolute dating methods.
      ::化石记录是地球上40多亿年生命的纪录,这是从发现和分析化石中重建出来的。 它是研究进化的最重要工具之一,但因为化石化是罕见的,因此是不完全的。 要加入化石记录,化石必须使用相对或绝对的约会方法注明日期。
    • Molecular clocks are additional tools for reconstructing how life on Earth evolved. Molecular clocks use DNA or protein sequences to estimate how much time has passed since related species diverged from a common ancestor. 
      ::分子钟是重建地球上生命如何演变的又一个工具。 分子钟使用DNA或蛋白质序列来估计相关物种与共同祖先分离后已经过了多少时间。
    • The geologic time scale is a timeline of Earth's history. It divides Earth's chronology into smaller units of time, such as eons and eras. These units are based on major changes in geology, climate, and living things.
      ::地质时间尺度是地球历史的一个时间轴。 它将地球的年表分为较小的时间单位, 如太阳和纪元。 这些单位基于地质、气候和生物的重大变化。

    Review
    ::回顾

    1. If you look at the Earth's history in terms of a 24-hour clock , at what time did mammals evolve? How much of Earth's past had already taken place by that time?
    ::1. 如果从24小时的角度来看待地球历史,哺乳动物在什么时间演变?到那时,地球过去已经发生了多少?

    2. What is the fossil record?
    ::2. 化石记录如何?

    3. Why is the fossil record incomplete?
    ::3. 为什么化石记录不完整?

    4. Compare and contrast relative and absolute dating of fossils.
    ::4. 比较和对比化石的相对和绝对日期。

    5. Explain what molecular clocks can reveal about the evolution of life.
    ::5. 解释分子钟能够揭示什么是生命进化过程。

    6. Divisions of the geologic time scale are inconsistent in length. For example, the Paleozoic Era covers about 300 million years, whereas the Mesozoic Era covers only about 180 million years. Explain why units of equal length are not used in the geologic time scale.
    ::6. 地质时间尺度的界限不一,例如,Paleozoic Eraa的长度大约为3亿年,而Mesozoic Era则只涵盖大约1.8亿年,解释为什么在地质时间尺度中没有使用相同长度的单位。

    7. Why is it important for the study of evolution to know a fossil’s relative age compared to another fossil?
    ::7. 为什么对进化的研究必须了解一个化石相对于另一个化石的相对年龄?

    8. If fossil A is located above fossil B and fossil B is located above fossil C in different rock layers, arrange the three fossils in order of their likely age, from oldest to youngest.
    ::8. 如果化石A位于化石B之上,而化石B位于化石C之上,位于不同的岩石层,则按其可能的年龄,从最老到最小,排列三个化石。

    9. Life first appeared on Earth  ________  years ago.
    ::9. 多年前,生命首次出现在地球上。

    a. four billion
    ::a. 40亿

    b. 4.5 billion
    ::b. 45亿

    c. four million
    ::c. 400万

    d. 544 million
    ::d. 5.44亿

     

    10. Which tool could you use to study the evolutionary relationships between species that are still alive?
    ::10. 你能使用何种工具来研究仍然活着的物种之间的进化关系?

    a. carbon-14 dating
    ::a. 碳-14日期

    b. molecular clocks
    ::b. 分子钟

    c. relative position in the fossil record
    ::c. 化石记录相对位置

    d. none of the above
    ::d. 以上无任何内容

     

    11. Use the History of Earth in a Day model above to answer the following questions:
    ::11. 以上文的 " 地球日 " 模式使用《地球历史》,回答下列问题:

    a. Which came first: free oxygen on Earth or the evolution of animals?
    ::a. 最先出现的是:地球上的免费氧气还是动物的进化?

    b. During which geologic period did multicellular life evolve?
    ::b. 多细胞生命在哪个地质时期演变?

    c. About how much of Earth’s history had elapsed before eukaryotes evolved?
    ::c. 关于在Eukaryotes进化之前地球历史的过去多少?

    d. What is the name of our current era?
    ::d. 我们当今时代的名称是什么?

     

    12. True or False:  Fossils are always composed of actual tissue from extinct organisms.
    ::12. 真实或假:化石总是由灭绝生物的实际组织组成。

     

    13. True or False:  Absolute dating of fossils is usually done using a molecular clock.
    ::13. 真实或假:化石的绝对约会通常使用分子钟进行。

    Explore More  
    ::探索更多

    What would it look like if we took Earth's 4.5 billion year history and stuffed it into a normal day's 24 hour time-frame? Check it out here:
    ::如果我们把地球45亿年的历史 塞进正常的24小时时间范围

     

     

     

     

     

     

     

     

     

    Carbon dating allows us to estimate the age of organic material. Learn more here:
    ::碳约会让我们可以估计有机材料的年龄。