Section outline

  • What you will learn 
    ::你会学到什么

    • Earth's geologic and climatic conditions in the late Precambrian
      ::已故Precambrian的地球地质和气候条件
    • How life evolved during the late Precambrian
      ::生命如何演化 在晚前的普伦布里安
    • The Precambrian
      ::普罗坎布里亚人

    Fossil of an ammonite embedded in sediment with small pebbles around it.

    What did early eukaryotic life look like?
    ::早期的食堂生活长什么样?

    Some looked like this. This is a of an ammonite. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
    ::有些人看起来是这样的。 这是一个Ammonite。 Ammonites是极好的索引化石, 通常可以将发现它们所在的岩石层与具体的地质时间段联系起来。

    Multicellular Life: Setting the Stage
    ::多细胞生命:确定阶段

    Nearly 80% of Earth’s history passed before multicellular life evolved. Up until then, all organisms existed as single . Why did evolve? What led up to this major step in the ? To put the evolution of multicellularity in context, let’s return to what was happening on planet Earth during this part of its history.
    ::近80%的地球历史在多细胞生命演变之前就已经过去了。直到那时,所有生物都作为单一存在。为什么演变了?为什么演变了?什么导致了这一重大步骤?为了将多细胞进化的背景化,让我们回到地球历史的这一部分。

    The Late Precambrian
    ::已故的普罗坎布里亚人

    The late Precambrian is the time from about 2 billion to half a billion years ago. During this long span of time, Earth experienced many dramatic geologic and climatic changes.
    ::已故的Precambrian是20亿到5亿年前的时间。 在这漫长的时间内,地球经历了许多巨大的地质和气候变化。

    • Continents drifted. They collided to form a gigantic supercontinent and then broke up again and moved apart. Continental drift changed climates worldwide and caused intense volcanic activity.
      ::大陆漂移。它们相撞形成一个巨大的超大陆,然后又再次解体,又四分五裂。 大陆漂移改变了全世界的气候,并引发了强烈的火山活动。
    • Carbon dioxide levels in the atmosphere rose and fell. This was due to volcanic activity and other factors. When the levels were high, they created a . More heat was trapped on Earth’s surface, and the climate became warmer. When the levels were low, less heat was trapped and the planet cooled. Several times, cooling was severe enough to plunge Earth into an ice age. One ice age was so cold that snow and ice completely covered the planet. Earth during this ice age has been called snowball Earth (see Figure ).
      ::大气中的二氧化碳水平上升和下降。 这是因为火山活动和其他因素。 当火山活动水平高时, 它们创造了一个 。 更多的热被困在地球表面, 气候变暖了。 当温度低时, 热量被困, 地球被冷却了。 有几次, 冷却的强度足以将地球推入冰河时代。 一个冰河时代是如此寒冷,以至于冰雪和冰雪完全覆盖了地球。 这个冰河时代的地球被称为雪球地球(见图 ) 。
    Representation of Earth during the late Precambrian ice age, known as Snowball Earth.
    Snowball Earth. During the late Precambrian, Earth grew so cold that it was covered with snow and ice. Earth during this ice age has been called snowball Earth.

    Life During the Late Precambrian
    ::晚前普罗坎布里安的人生

    The dramatic changes of the late Precambrian had a major impact on Earth’s life forms. Living things that could not adapt died out. They were replaced by organisms that evolved new adaptations . These adaptations included , specialization of cells, and multicellularity .
    ::已故Precambrian的巨变对地球的生命形式产生了重大影响。 无法适应的活物已经绝迹。 它们被新适应的有机体所取代。 这些适应包括细胞的专业化和多细胞性。

    • Sexual reproduction created much more variety among offspring. This increased the chances that at least some of them would survive when the environment changed. It also increased the speed at which evolution could occur.
      ::性生殖在后代中创造了更多的多样性,这增加了他们中至少有一部分在环境变化时生存的机会,也加快了进化的速度。
    • Some cells started to live together in colonies. In some colonies, cells started to specialize in doing different jobs. This made the cells more efficient as a colony than as individual cells.
      ::有些细胞开始在聚居区共同生活。在有些聚居区,细胞开始专门从事不同的工作。这使得细胞作为聚居区比个体细胞更有效率。
    • By 1 billion years ago, the first multicellular organisms had evolved. They may have developed from colonies of specialized cells. Their cells were so specialized they could no longer survive independently. However, together they were mighty. They formed an organism that was bigger, more efficient, and able to do much more than any single-celled organism ever could.
      ::10亿年前,第一种多细胞生物已经进化了。它们可能已经从专门细胞的聚居中发展出来。它们的细胞非常专业化,因此不再能够独立生存。然而,它们在一起是强大的。 它们形成了一个比任何一个单细胞生物都大、效率更高、能够做更多事情的有机体。

    The Precambrian Extinction
    ::普罗坎布里亚远征

    At the close of the Precambrian 544 million years ago, a mass extinction occurred. In a mass extinction , many or even most abruptly disappear from Earth. There have been five mass extinctions in Earth’s history. Many scientists think we are currently going through a sixth mass extinction . What caused the Precambrian mass extinction? A combination of climatic and geologic events was probably responsible. No matter what the cause, the extinction paved the way for a burst of new life, called the Cambrian explosion , during the following .
    ::在5亿4千4百万年前的Precambrian 5亿4千万年前,发生了大规模灭绝。在大规模灭绝中,许多甚至最突然地从地球上消失。在地球历史上发生了五次大规模灭绝。许多科学家认为我们正在经历第六次大规模灭绝。是什么导致了Precambrian 大规模灭绝?气候和地质事件可能是原因之一。不管原因是什么,灭绝为新的生命爆发铺平了道路,在接下来的一段时期被称为Cambrian爆炸。

     

     

     

     

    Summary
    ::摘要

    • During the late Precambrian, continents drifted, carbon dioxide levels fluctuated, and climates changed. Many organisms could not survive the changes and died out.
      ::在已故的Precambrian地区,大陆漂移,二氧化碳水平波动,气候变化。 许多生物无法存活下来,死亡也随之消失。
    • Other organisms evolved important new adaptations. These include sexual reproduction, cell specialization, and multicellularity.
      ::其他有机体也发展了重要的新适应,包括性生殖、细胞专业化和多细胞。
    • The Precambrian ended with a mass extinction, which paved the way for the Cambrian explosion.
      ::Precambrian以大规模灭绝结束,为Cambrian爆炸铺平了道路。

    Review
    ::回顾

    1. When was the late Precambrian?
      ::已故的普雷坎布里安是什么时候?
    2. Describe geologic and climatic changes that occurred during the late Precambrian.
      ::描述已故Precambrian期间发生的地质和气候变化。
    3. What is the greenhouse effect?
      ::温室效应是什么?
    4. What three significant evolutionary events occurred during the late Precambrian?
      ::在已故的普雷坎布里安期间,发生了哪些三次重要的演化事件?
    5. What is a mass extinction?
      ::什么是大规模灭绝?