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  • 哺乳动物演变

    A duckbilled platypus swimming underwater among aquatic plants, illustrating mammal evolution.

    Could you argue that the duckbilled platypus has some characteristics of other species?
    ::你能辩解说 鸭蛋白质有其他物种的特征吗?

    Like a or a ? You could. This might suggest that an ancestor of this may have been one of the early mammals to evolve. 
    ::比如说,你可以。这也许表明,这个基因的祖先 可能是早期进化的哺乳动物之一。

    Evolution of Early Mammals
    ::早期哺乳动物的演变

    The earliest mammals evolved from cynodonts . But the evolution of mammals didn’t end there. Mammals continued to evolve. mammals probably split off from other mammals first. They were followed by . probably evolved last.
    ::最早的哺乳动物从细胞院进化而来。 但哺乳动物的进化并没有就此结束。哺乳动物在继续进化。哺乳动物可能首先从其他哺乳动物中分裂出来。随后的哺乳动物可能是最后的进化。

    Evolution of Monotremes
    ::单体演变

    The first monotremes may have evolved about 150 million years ago. Early monotreme have been found in Australia. An example is a genus called Steropodon , shown in Figure . It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca . These traits are still found in modern monotremes.
    ::第一个单体在1.5亿年前可能已经演化了。早期的单体在澳大利亚已经发现。例如,一个叫斯特罗波登(Steropodon)的基因,如图所示。它可能就是白蚁的祖先。早期的单体保留了它们狂野祖先的一些特性。例如,它们产卵并拥有一个木薯。这些特性仍然存在于现代单体中。

    Reconstruction of Steropodon, an early monotreme possibly related to the platypus.
    Probable Monotreme Ancestor: Steropodon. Like the platypus, Steropodon probably had a bill.

    Evolution of Marsupials
    ::Marsupials 的演进

    The first marsupials may have evolved about 130 million years ago. One of the earliest was the extinct genus Sinodelphys .  Sinodelphys was about 15 centimeters (6 inches) long. Its limb structure suggests that it was a climbing . It could escape from predators by climbing into trees. It probably lived on a diet of and worms.
    ::最早的就是灭绝的中华底植物。 中华底植物大约15厘米(6英寸)长。 它的四肢结构表明它是在攀爬。 它可能通过爬入树木从掠食者身上逃脱。 它可能生活在一种饮食和蠕虫的饮食中。

    Evolution of Placental Mammals
    ::胎盘哺乳动物的演变

    The earliest placental mammals may have evolved about 110 million years ago. The ancestor of placental mammals may be the extinct genus Eomaia . Fossils of Eomaia have been found in what is now China. It was only about 10 centimeters (4 inches) long. It was a tree climber and probably ate insects and worms. Eomaia had several traits of placental mammals. Figure shows how Eomaia may have looked.
    ::最早的胎盘哺乳动物可能在大约1亿1千万年前就已经进化了。胎盘哺乳动物的祖先可能是灭绝的奥马亚人。在现在的中国,已经发现了奥马亚的化石。只有大约10厘米长(4英寸)左右。它是一个爬树者,可能吃掉了昆虫和蠕虫。奥马亚有几个胎盘哺乳动物的特征。图显示奥马亚人可能是如何看的。

    Fossilized remains of Eomaia, an early ancestor of placental mammals.
    Probable Ancestor of Placental Mammals: Eomaia. Eomaia lived a little over 100 million years ago.

    The placental mammal descendants of Eomaia were generally more successful than marsupials and monotremes. On most continents, placental mammals became the dominant mammals, while marsupials and monotremes died out. Marsupials remained the most common and diverse mammals in Australia. The reason for their success there is not yet resolved.
    ::爱奥玛亚的胎盘哺乳动物后代通常比雄性哺乳动物和单体动物更成功。 在大多数大陆,胎盘哺乳动物成为主要哺乳动物,而雄性哺乳动物和单体动物则死亡。 火星哺乳动物仍然是澳大利亚最常见的、最多样化的哺乳动物。 它们成功的原因尚未解决。

      

     

     

     

    Evolution of Modern Mammals
    ::现代哺乳动物的演变

    The Cretaceous Period ended with another . This occurred about 65 million years ago. All of the dinosaurs went extinct at that time. Did the extinction of the dinosaurs allow mammals to take over?
    ::白龙时代以另一个时代结束。这发生在大约6500万年前。当时所有的恐龙都灭绝了。恐龙的灭绝让哺乳动物接管了吗?

    Traditional View
    ::传统观点

    Scientists have long assumed that the extinction of the dinosaurs opened up many niches for mammals to exploit. Presumably, this led to an explosion of new species of mammals early in . Few mammalian fossils from the early Cenozoic have been found to support this theory . Even so, it was still widely accepted until recently.
    ::科学家长期以来一直认为恐龙的灭绝为哺乳动物开辟了许多可加以利用的空间。 估计这导致了早期哺乳动物新物种的爆炸。 早期的辛诺佐克(Cenozoic)发现很少有哺乳动物化石支持这一理论。 即便如此,直到最近,它仍然被广泛接受。

    View from the Mammalian Supertree
    ::从 Mammalian 超级树查看

    In 2007, an international team of scientists compared the of almost all known species of living mammals. They used the data to create a supertree of mammalian evolution. The supertree shows that placental mammals started to diversify as early as 95 million years ago.
    ::2007年,一个国际科学家团队对几乎所有已知的哺乳动物物种进行了比较,他们利用这些数据创造了哺乳动物进化的超级树。 超树表明胎盘哺乳动物早在9500万年前就开始多样化。

    What explains the diversification of mammals long before the dinosaurs went extinct? What else was happening at that time? One change was a drop in Earth’s temperature. This may have favored endothermic mammals over ectothermic dinosaurs. Flowering plants were also spreading at that time. They may have provided new and plentiful foods for small mammals or their insect prey .
    ::在恐龙灭绝之前的很长时间里,哺乳动物的多样化是什么解释?当时还发生了什么呢?一个变化是地球温度的下降。这可能有利于异性哺乳动物而不是切热恐龙。当时,花生植物也在扩散。它们可能为小哺乳动物或其昆虫猎物提供了新的和丰富的食物。

    The supertree also shows that another major diversification of mammals occurred about 50 million years ago. Again, worldwide climate change may have been one reason. This time Earth’s temperature rose. The warmer temperature led to a greater diversity of plants. This would have meant more food for mammals or their prey.
    ::超级树还表明哺乳动物的另一种大规模多样化发生在大约5 000万年前。 再一次,世界范围的气候变化可能是原因之一。 此时地球的温度上升。 更暖的温度导致植物的多样化。 这意味着哺乳动物或其猎物会有更多的食物。

     

    Summary
    ::摘要

    • Monotremes evolved about 150 million years ago. Like modern monotremes, they had a cloaca and laid eggs.
      ::一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿五千万年前,一亿亿多年前,一亿亿亿亿多年前,一亿五亿多年前,一亿亿多年前,一亿五亿亿多年前,一亿亿亿多年前,一亿亿亿万亿多年前,一亿万亿亿多年前,一亿万亿多年前,一亿万亿万亿亿多年前,一亿万亿万亿万亿多年前,一亿万亿亿万亿万亿万亿多年前,一亿万亿亿万亿亿亿亿多年前,一亿万亿万亿万亿万亿万亿万亿多年前,一亿万亿多年前,一亿万亿亿亿亿多亿亿亿亿亿万亿万亿万亿万亿万亿万亿万亿万亿万亿多年前,
    • Marsupials evolved about 130 million years ago. They were very small and ate insects and worms.
      ::马苏比亚在大约1.3亿年前就进化了 它们非常小 食用昆虫和蠕虫
    • Placental mammals evolved about 110 million years ago. They were also small and climbed trees.
      ::胎盘哺乳动物在大约1.1亿年前就进化了,它们也是小树和爬行树。
    • Placental mammals became the dominant land mammals. Most marsupials and monotremes died out, except in Australia.
      ::胎盘哺乳动物成为占支配地位的陆地哺乳动物,除澳大利亚外,大多数的哺乳动物和单体动物死亡。
    • The mammalian supertree shows that placental mammals started to diversify as early as 95 million years ago.
      ::哺乳动物超树显示 胎盘哺乳动物早在9500万年前 就开始多样化了

    Review
    ::回顾

    1. Outline the evolution of monotreme, marsupial, and placental mammals.
      ::概括单体、哺乳动物和胎盘哺乳动物的进化过程。
    2. Describe Sinodelphys .
      ::描述西涅菲斯(Sindelphys) 。
    3. Describe Eomaia .
      ::Eomaia描述。
    4. What is the mammalian supertree?
      ::什么是哺乳动物的超级树?
    5. Explain why the extinction of most therapsids may have allowed mammals to evolve.
      ::解释为什么大多数减速动物的灭绝 可能让哺乳动物进化
    6. What explains the diversification of mammals long before the dinosaurs went extinct?
      ::恐龙灭绝之前 哺乳动物的多样化 是什么原因?