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  • 哺乳动物祖先 -- -- 高级

    lesson content

    Which mammalian trait evolved first? What was the first mammal like? When did the earliest mammal live?
    ::第一个哺乳动物是什么样子?最早的哺乳动物是什么时候存活的?

    Detailed answers to these questions are still in dispute, though it is probably safe to say the earliest mammals were not the mammoth. Obviously, the mammoth had some specialized traits.
    ::这些问题的详细答案仍然有争议,尽管可以说最早的哺乳动物不是长毛象。 显然,长毛象有一些特殊的特征。

    Mammalian Ancestors
    ::Mammalian 祖先传教士

    The evolutionary history and is currently an area of active research. At one time, there had to be the first of mammal. But what could it have been? Was it a rodent-like organism ? Was it a reptilian-like mammal? Possibly. Keep in mind that evolution is a change. As additional data are analyzed, scientists gain more knowledge of extinct mammalian species and a better understanding of how mammals evolved and how living mammalian species are related to one another. This helps them reconstruct mammalian evolutionary history and determine how best to classify mammals.
    ::进化史目前是一个活跃的研究领域。 有一次,必须首先研究哺乳动物。 但是它会是什么呢? 它像鼠类生物吗? 它像哺乳动物吗? 它像爬虫类哺乳动物吗? 可能。 记住进化是一个变化。 随着更多的数据分析,科学家们获得了更多关于灭绝哺乳动物物种的知识,更好地了解哺乳动物的进化方式以及哺乳动物物种之间的相互联系。这有助于他们重建哺乳动物进化历史并确定如何对哺乳动物进行最佳分类。

    Which mammalian traits evolved first? What was the first mammal? When did the first mammal live? Specific answers to these and other questions about mammalian evolution are still in dispute. Nonetheless, researchers generally agree that similar to mammals first began evolving around 200–300 million years ago, during the Permian Period of the , from ancestors called synapsids. The major events of mammalian evolution are summarized in Table . Refer to the table as you read the rest of this concept.
    ::哺乳动物的特征是哪一种?第一种哺乳动物先进的?什么是第一种哺乳动物?第一种哺乳动物是什么时候活下来的?对于这些以及关于哺乳动物进化的其他问题的具体答案仍然争论不休。 尽管如此,研究人员一般都同意,类似哺乳动物的答案在2亿到3亿年前,在波尔米亚时期,从祖先开始演变,叫作突触。哺乳动物进化的主要事件在表格中摘要列出。您读到这个概念的其余部分时请参考表格。

    Table  shows the parts of the that are relevant to mammalian evolution. It includes only those events that relate directly to the evolution of mammals. The events and their timing are subject to revision as more data, such as the discovery of additional , become available.
    ::表显示与哺乳动物进化有关的部分,只包括与哺乳动物进化直接相关的事件,随着更多数据(如发现更多数据)的出现,事件及其时间将予以修订。

    Era Period Epoch Major Events Start (mya)*
    Cenozoic Neogene Holocene Rise of human civilization. Spread and dominance of modern humans. 0.01
    - - Pleistocene Spread and then extinction of many large mammals. Appearance of modern humans. 1.8
    - - Pliocene Appearance of many existing genera of mammals including the genus Homo. 5.3
    - - Miocene Appearance of the remaining modern mammal families. Diversification of horses and mastodons. First apes. 23.0
    - Paleogene Oligocene Rapid evolution and diversification of . 33.9
    - - Eocene Appearance of several modern mammal families. Diversification of primitive whales. 55.8
    - - Paleocene Appearance of the first large mammals. 65.5
    Mesozoic Cretaceous - Emergence of , , and placental mammals. Possibly the first appearance of four clades (superorders) of placental mammals (Afrotheria, Xenarthra, Laurasiatheria, and Supraprimates). 145.5
    - Jurassic - Spread of mammals, which remain small in size. 199.6
    - Triassic - Evolution of cynodonts to become smaller and more mammal-like. Appearance of the first mammals. 251.0
    Paleozoic Permian - Evolution and spread of synapsids ( pelycosaurs and therapsids). 299.0
    - Carboniferous - Appearance of amniotes , the first fully vertebrates . 359.0
    • mya = millions of years ago.
      ::Mya = 数以百万计的年前。

    Amniotes
    ::亚氮

    The story of mammalian evolution actually begins with amniotes. Amniotes are animals that lay eggs with internal membranes. The internal membranes keep inside the eggs, so they do not dry out when exposed to air. This allows amniotes to lay their eggs on dry land. Amniotes were the first fully terrestrial, or land-living, tetrapods (four-legged vertebrates). The other tetrapods, the , produce eggs without internal membranes, so, in most species, the eggs must be laid in water to avoid drying out. The need to return to water to lay eggs prevented amphibians from becoming completely terrestrial.
    ::哺乳动物进化的故事实际上是从羊膜开始的。 羊膜是用内膜产卵的动物。 内膜将卵藏在卵中, 这样在接触空气时不会干涸。 这使得哺乳动物可以在干燥的土地上产卵。 羊膜是第一个完全陆生的或陆生的四胞胎( 四条腿脊椎动物 ) 。 其他四胞胎egg 产卵时没有内膜, 所以在大多数物种中, 卵必须放在水中以避免干燥。 需要回到水中产卵防止两栖动物成为完全陆生的。

    As shown in Table   , the first amniotes appeared in the Carboniferous Period of the Paleozoic Era, more than 300 million years ago. They spread across the land and became virtually the only terrestrial vertebrates. Within a few million years, two important amniote lineages, or evolutionary lines, became distinct: the sauropsids and the synapsids. The sauropsids were ancestors of dinosaurs , , and . The synapsids were ancestors of mammals.
    ::如表所示,第一批羊膜动物出现在3亿多年前的Paleozoic 时代的碳化时期,它们分布在陆地上,几乎成为唯一的陆生脊椎动物。 在几百万年的时间里,两条重要的羊膜系,即进化线,变得截然不同:沙罗皮斯和突触。沙罗皮斯是恐龙的祖先,和。突触是哺乳动物的祖先。

    Synapsids
    ::合成合成器

    The early synapsids had a mix of mammalian and reptilian traits, so they are sometimes referred to as “mammal-like reptiles.” However, they were not really reptiles, and they did not evolve into reptiles. Instead, it is believed that the synapsids evolved into mammals. The evolution of mammals from their early synapsid ancestors most likely began during the Permian Period, beginning at least 275 million years ago. It continued over the next 70 million years into the Triassic Period of the . By the middle of the Triassic Period, there were many species that were starting to resemble mammals.
    ::早期突触有哺乳动物和爬行动物的混合特征,因此有时被称为“哺乳动物类爬行动物 ” 。 但是,它们其实不是爬行动物,也没有演变成爬行动物。相反,人们认为突触会演变成哺乳动物。哺乳动物从早期突触祖先的进化最可能始于至少自2.75亿年前开始的佩尔米亚时期。它持续了7000万年,进入了三亚西克时期。到了三亚西克时期的中间,许多物种开始与哺乳动物相似。

    All of the original synapsids are now extinct and are represented only by fossils. As shown in Figure  , fossil skulls of synapsids can be distinguished from those of sauropsids by the number of pairs of holes in the skull behind each eye. These holes, called temporal fenestrae (sing., fenestra), are found in most amniote skulls. They make the skull lighter without sacrificing strength. This saves energy by not requiring as much . Synapsids had one pair of temporal fenestrae placed low on the skull. Sauropsids, in contrast, had two pairs of temporal fenestrae placed one above the other.
    ::正如图所示,突触的化石头骨与硫化物头骨的区别在于每眼后头骨的孔数。这些孔被称为时间性突触(sing. fenestra),在大多数羊膜头骨中都发现了。它们使头骨轻一些而无需牺牲强度。这节省了能量,因为不需要太多。Synapsid在头骨上放置了一对时间性异质。相比之下,Sauropsidsid则将两对时间性异质放在另一头顶。

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    The synapsid skull on the left has a single pair of temporal fenestrae behind the eye holes (labeled A). Temporal fenestrae are holes found in the skulls of most amniotes that make the bones lighter but not weaker. The sauropsid skull on the right has two pairs of temporal fenestrae behind the eye holes (labeled B and C). This difference allows paleontologists to distinguish synapsid fossil skulls from sauropsid fossil skulls.

    Fossils from the Permian Period indicate that animals in one particular synapsid group, called pelycosaurs, were the most common land vertebrates of the time. Pelycosaurs actually first appeared during the Carboniferous Period and had become common by the early Permian. They remained the dominant land animals for about 40 million years. A genus of pelycosaur called Dimetrodon is shown in Figure .   Fossils of Dimetrodon have been found in North America and Europe.
    ::伯尔米亚时期的Fossil表示,动物在一种称为pelycosaur的突触群中,是当时最常见的陆生脊椎动物,在碳纤维时期,Pelycosaur实际上首次出现,并成为早期Permian人常见的动物,在大约4 000万年中一直是主要陆地动物。图中显示的是称为Dimetrodon的Pelycosaur基因。在北美和欧洲发现了Dimetrodon的化石。

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    This pelycosaur, belonging to the genus Dimetrodon, lived during the Permian Period, between about 280 and 265 million years ago. It was up to three meters in length. The “sail” on its back was probably used to help regulate its body temperature. The large surface area of the “sail” could gain or lose heat more quickly than the body as a whole.

    Like other pelycosaurs, Dimetrodon was not very mammal-like in many of its traits. It had a sprawling, bulky body, and it walked like a lizard. It also had relatively small brain . However, pelycosaurs, like Dimetrodon, had started to develop different types of teeth instead of having teeth that were all the same, as in sauropsids and modern reptiles. Dimetrodon, a predatory carnivore , had two types of teeth that could tear and cut meat into smaller pieces for easier ingestion. This may have been the beginning of the evolution of the four types of teeth that characterize modern mammals.
    ::像其他食肉动物一样,Dimetrodon在许多特性上并不象哺乳动物,它有着一种无孔不入的大体,行走的像蜥蜴。它也有相对较小的大脑。然而,像Dimetrodon这样的食肉动物开始开发不同类型的牙齿,而不是像在食草动物和现代爬行动物中那样拥有完全相同的牙齿。Dimetrodon是一种食肉动物,有两种牙齿可以撕裂和切成小块,以便更容易食用。这也许是现代哺乳动物特有的四种牙齿的进化的开始。

    Therapsids
    ::病原体

    Some pelycosaurs were ancestral to a group of synapsids called therapsids, which arose in the middle of the Permian Period, about 260 million years ago. Some therapsids, in turn, may have been ancestral to mammals. These relationships are represented by the phylogenetic tree diagram in Figure .  You can read more about phylogenetic trees and how they are constructed in the : Phylogeny (Advanced) concept.
    ::一些食肉动物曾是一群叫做“草原”的突触动物的祖先,这些突触发生在大约2.6亿年前的珀尔米亚时期中叶,有些虫原可能是哺乳动物的祖先。这些关系在图中以植物基因树图为代表。你可以阅读更多关于植物基因树以及它们是如何在Phylorgeny(高级)概念中构建的。

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    This diagram represents a simplified phylogenetic tree of the evolution of mammals, beginning with a tetrapod ancestor.

    Descended from pelycosaurs, therapsids went through a series of evolutionary stages to become more and more like mammals. When they first appeared in the middle of the Permian Period, they were very much like their pelycosaur ancestors. Then, during their evolution, a number of traits that are found in modern mammals began to evolve. By the end of the Permian Period, some therapsids could easily be mistaken for mammals. For example, therapsids started to develop a bony palate in the roof of their mouths, and their legs were positioned more vertically beneath their bodies than the sprawling legs of pelycosaurs were.
    ::草原从食腐动物消失后, 经历了一系列进化阶段, 越来越像哺乳动物。 当它们首次出现在珀米亚时期的中间时, 它们与它们的食腐动物祖先非常相似。 在它们进化过程中, 在现代哺乳动物身上发现的一些特征开始演变。 在《 珀米亚 时期》 结束时, 一些草原很容易被误认为哺乳动物。 例如, 草原开始在它们的嘴顶上形成一个粘稠的粘合物, 它们的腿在它们的身体下方的垂直位置比龙虾的双腿要高。

    By the end of the Permian Period, therapsids were the dominant, large terrestrial animals. They were by far the most abundant and diverse animals of that time. They had mainly replaced the pelycosaurs, which were becoming rare. Therapsids included both herbivorous (plant-eating) and carnivorous (meat-eating) species. They were also very diverse in size, ranging from less than a kilogram to more than a metric ton in weight.
    ::到佩尔米亚时期结束时,食草动物是主要的大型陆地动物,它们是当时最丰富和最多样化的动物,它们主要取代了正在变得稀有的食草动物,包括食草动物和食食食食食食食食食食食食食食动物,其大小也非常不同,从不足一公斤到超过一公吨体重不等。

    Triassic Takeover
    ::三assic 接任

    Between the Permian Period of the Paleozoic Era and the Triassic Period of the Mesozoic Era, a catastrophic occurred. The causes of this mass extinction are not known for certain, although large asteroids striking Earth may have played a role. Regardless of the causes, the majority of organisms, including most vertebrates, appear to have gone extinct in a relatively short period of time (also see the concepts). Only about a quarter of all animal species survived.
    ::在Paleozoic时代的Permian时期和Mesozoic时代的Triassic时期之间发生了一场灾难性的灾难,尽管撞击地球的大型小行星可能起到了一定的作用,但这种大规模灭绝的原因并不为人所知。 不管原因如何,大多数生物,包括大多数脊椎动物,似乎在相对较短的时间内灭绝了(也见概念 ) 。 只有大约四分之一的动物物种存活下来。

    Like other animals, most therapsids apparently went extinct at the end of the Permian Period. Their niches were taken over by sauropsids, which had been relatively unimportant until then. Recall that sauropsids included ancestors of dinosaurs, reptiles, and birds. This change in dominance, with sauropsids becoming more important than synapsids, is often called the Triassic takeover . By the middle of the Triassic period, sauropsids had evolved into dinosaurs. Dinosaurs became increasing important throughout the Mesozoic Era and came to dominate most terrestrial niches. This is why the Mesozoic Era is called the “Age of Dinosaurs” (see the Reptiles and Birds concepts).
    ::与其它动物一样,大多数草原显然在佩尔米亚时期结束时灭绝了,它们的位置被相对无关紧要的沙罗普西德人占据,直到那时为止,它们一直相对不重要。大家记得,沙罗普西德人包括恐龙、爬行动物和鸟类的祖先。这种主导地位的变化,随着沙罗普西德人变得比突触更加重要,常常被称为三亚西克人接管。在三亚西克时期的中间,沙罗普西德人已经演变成恐龙。 恐龙在整个中变得日益重要,并占据了大多数陆地的草原。 这就是为什么中摩索伊尔人被称作“迪诺萨人时代”(见爬虫人和鸟类概念 ) 的原因。

    Cynodonts
    ::阴道

    Ironically, the Triassic takeover may have helped the few remaining therapsids evolve into mammals. The surviving therapsids were relatively small animals including therapsids called cynodonts ( Figure ). The cynodonts first appeared at the end of the Permian Period, and they flourished worldwide throughout the first half of the Triassic Period. Some cynodonts were carnivores, probably eating mostly . The carnivorous forms became progressively smaller as the Triassic Period progressed. The cynodonts were most likely nocturnal , meaning that they were active during the night.
    ::具有讽刺意味的是,Triassic的接管也许帮助了少数剩下的食草动物演变成哺乳动物。幸存的食草动物是相对较小的动物,包括叫作cynodonts(Figore)的食草动物。在Permian时期的末期首次出现,它们在整个三亚希克时期的上半段期间在全世界繁荣发展。有些食草动物是食肉动物,可能大部分是食食肉动物。随着三亚希克时期的演变,食肉动物的形式逐渐变小。细胞动物很可能是夜行的,这意味着它们在夜间很活跃。

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    This small mammal-like creature was a cynodont, a type of therapsid that may have been ancestral to mammals. It was about the size of a rat.

    Being nocturnal may have saved the cynodonts from extinction at the end of the Permian Period, when most other therapsids went extinct. A nocturnal niche was one of the few niches that the increasingly common and successful dinosaurs did not come to dominate. Becoming better adapted to a nocturnal niche may have led to the evolution of several of the mammalian traits of cynodonts:
    ::夜行也许拯救了在珀尔米亚时期末期绝种的细胞圈,当时,其他大多数的幼虫都灭绝了。 夜行是日益常见和成功的恐龙无法主宰的少数位置之一。 更好地适应夜间的幼虫,也许已经导致一些幼虫哺乳动物特征的演化:

    • The ability to regulate body temperature might have been selected for because it allows nocturnal animals to be active in the cool of the night.
      ::之所以选择调节体温的能力,可能是因为它允许夜间动物在夜凉中活动。
    • A good sense of hearing might have been selected for because it is more useful than good when hunting in the dark.
      ::可能选择了一种良好的听力感,因为它在黑暗中打猎时比好更有用处。

    Toward the end of the Triassic Period, cynodonts included tiny shrew-like animals that were quite mammal-like. For example, their teeth were fully differentiated, they had relatively large brains, and many of them walked in an upright manner. They also had the complete bony palate of mammals. Their middle ears contained the three tiny bones that are found in the modern mammalian ear. The cynodonts had ribcages similar to those of modern mammals, so they may have had diaphragms for . Although it is not known for certain, they were probably endothermic and may have had the ability to lactate.
    ::在三思西克时期末期结束时,细胞激素包括了像哺乳动物那样的细小的细毛动物。例如,它们的牙齿完全有区别,它们的大脑相对较大,其中许多人以正直的方式行走。它们还有完全的哺乳动物的发热。它们中间的耳朵里有现代哺乳动物耳中发现的三块小骨头。细胞激素有类似于现代哺乳动物的肋骨,因此它们可能有隔膜。虽然它们并不肯定,但它们可能是最后的炎热,并且可能有能力哺乳。

    The presence of hair is a defining trait of modern mammals. The earliest unambiguous fossil evidence of hair comes from an extinct cynodont that lived about 165 million years ago, during the Jurassic Period. Although it does not represent a true mammal, this fossil is important because it establishes the presence of hair at a relatively early date in mammalian evolution.
    ::发型的存在是现代哺乳动物的决定性特征。 最早明确的发型化石证据来自一个在侏罗纪时期(侏罗纪时期 ) 1.65亿年前存活下来的绝迹细胞。 尽管它并不代表真正的哺乳动物,但这个化石很重要,因为它在哺乳动物进化过程中早期就确定了头发的存在。

    Cynodonts are probably the ancestors of all mammals. Although the cynodonts themselves appeared to have died out during the Jurassic or Cretaceous Period of the Mesozoic Era, early mammals continued to evolve. In fact, with early mammals may have led to the cynodont extinction.
    ::幼哺乳动物可能是所有哺乳动物的祖先。 尽管幼哺乳动物本身似乎在中索索契时代的侏罗纪或白鲸时期死亡,早期哺乳动物却在不断演变。 事实上,早期哺乳动物可能导致幼哺乳动物灭绝。

    Summary
    ::摘要

    • The evolutionary history and classification of mammals is currently an area of active research.
      ::哺乳动物的进化历史和分类目前是一个积极研究的领域。
    • Researchers generally agree that animals similar to mammals first began evolving around 200–300 million years ago, during the Permian Period of the Paleozoic Era, from ancestors called synapsids.
      ::研究人员普遍认为,与哺乳动物相似的动物最初在2亿到3亿年前,即Paleozoic时代的Permian时期,从被称为突触的祖先开始演变。
    • The early synapsids had a mix of mammalian and reptilian traits, so they are sometimes referred to as “mammal-like reptiles.”
      ::早期的突触具有哺乳动物和爬行动物的混合特征,因此有时被称为“像哺乳动物一样的爬行动物”。
    • Fossils from the Permian Period indicate that animals in one particular synapsid group, called pelycosaurs, were the most common land vertebrates of the time.
      ::伯尔米亚时期的化石表明,动物在一种称为“粉色龙”的突触群中是当时最常见的陆地脊椎动物。
    • Some pelycosaurs were ancestral to a group of synapsids called therapsids, which arose in the middle of the Permian Period, about 260 million years ago. Some therapsids, in turn, may have been ancestral to mammals.
      ::某些食肉动物是一群叫做“狂妄”的突触动物的祖先,这些突触发生在大约2.6亿年前的珀尔米亚时期中叶。 一些食肉动物可能是哺乳动物的祖先。
    • After the Permian mass extinction, the surviving therapsids were relatively small animals including therapsids called cynodonts.
      ::在伯尔米亚大规模灭绝之后 存活下来的草原 是相对较小的动物 包括叫做细胞激素的草原
    • Cynodonts are probably the ancestors of all mammals.
      ::幼崽可能是所有哺乳动物的祖先

    Review
    ::回顾

    1. When did the evolution of mammals begin?
      ::哺乳动物的进化是什么时候开始的?
    2. Synapsids are sometimes referred to as "mammal-like reptiles." Why is this term misleading?
      ::合成物有时被称为“哺乳类爬行动物”。为什么这个术语误导人?
    3. What role do the temporal fenestrae play? What is the difference between synapsid skulls and sauropsid skulls?
      ::时间性骨骼起什么作用? 突触头骨和被割裂头骨有什么区别?
    4. What important adaptation occurred when pelycosaurs evolved?
      ::当食糖动物进化时,有哪些重要的适应变化?
    5. What traits did therapsids share with modern mammals?
      ::和现代哺乳动物有什么区别?
    6. How did the Triassic takeover contribute to the evolution of mammals?
      ::Triassic的接管对哺乳动物的进化有何贡献?
    7. What mammalian characteristics were exhibited in cynodonts?
      ::细胞激素中有哪些哺乳动物特征?