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重大的地质和生物事件是地质时序表的特征，其中包括大约46亿年前的地球起源、大约35亿年前的生命起源、大约15亿年前的真核生命形态（细胞中有真正细胞核的生物）起源，以及大约6亿年前的动物起源。

The geologic timescale is marked by significant geologic and biological events, including the origin of Earth about 4.6 billion years ago, the origin of life about 3.5 billion years ago, the origin of eukaryotic life-forms (living things that have cells with true nuclei) about 1.5 billion years ago, and the origin of animals about 0.6 billion years ago.

动物起源这一最近事件标志着寒武纪的开端。

The last event marks the beginning of the Cambrian period.

动物的起源相比于地球的历史来说相对较晚，只占地球历史的十分之一。

Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history.

从地质学角度来看，所有的现代动物群体（包括那些已灭绝的动物）都在这短暂的一亿年的时期内逐渐进化。

During a geologically brief 100-million-year period, all modern animal groups (along with other animals that are now extinct) evolved.

这一快速的动物起源与多样化通常被成为“寒武纪生命大爆发”。

This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.”

一百多年来，科学界对寒武纪生命大爆发提出了不少重要问题。

Scientists have asked important questions about this explosion for more than a century.

相对于地球历史来讲，它为何发生得如此之晚？

Why did it occur so late in the history of Earth?

与生命的起源相比，多细胞生命体的起源似乎只是很简单的一步。

The origin of multicellular forms of life seems a relatively simple step compared to the origin of life itself.

在动物的进化过程中，为何化石记录中没有关于这一系列的进化演变？

Why does the fossil record not document the series of evolutionary changes during the evolution of animals?

为何动物生命进化得如此之快？

为了揭开这些问题的谜底，古生物学家们不断搜寻着相关的化石记录。

Paleontologists continue to search the fossil record for answers to these questions.

关于这段重要时期缺乏化石的一个解释是早期动物都是软体动物，根本无法形成化石。

One interpretation regarding the absence of fossils during this important 100-million-year period is that early animals were soft bodied and simply did not fossilize.

虽然不像硬体动物那样容易形成化石，但软体动物的确能够以化石形式被保留下来。

Fossilization of soft-bodied animals is less likely than fossilization of hard-bodied animals, but it does occur.

沉积物的迅速覆盖为软体动物的石化提供了条件，这一环境可以阻止软体动物的腐烂。

Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages decomposition.

事实上，很多年前人们就发现了含有软体动物的化石层。

In fact, fossil beds containing soft-bodied animals have been known for many years.

包含迄今为止最古老动物化石的伊迪亚卡拉化石地层就全部是由软体动物所构成的。

The Ediacara fossil formation, which contains the oldest known animal fossils, consists exclusively of soft-bodied forms.

虽然以澳大利亚的一个地方所命名，伊迪亚卡拉地层的分布遍布全球且可追溯至前寒武纪时代。

Although named after a site in Australia, the Ediacara formation is worldwide in distribution and dates to Precambrian times.

这一地层距今已有7亿年历史，虽然它对现代动物起源所能提供的线索有限，但古生物学者们认为它代表了一场并不成功的进化尝试。

This 700-million-year-old formation gives few clues to the origins of modern animals, however, because paleontologists believe it represents an evolutionary experiment that failed.

其中并没有发现现代动物群体的祖先。

It contains no ancestors of modern animal groups.

比伊迪亚卡稍晚形成的托莫特阶地层同样包含动物残骸，它是以俄罗斯的一处地名而得名。

A slightly younger fossil formation containing animal remains is the Tommotian formation, named after a locale in Russia.

形成于寒武纪初期的这一地层同样也只含有软体动物。

It dates to the very early Cambrian period, and it also contains only soft-bodied forms.

曾经有种看法认为在这一化石层所保存的动物进化出了不同种类的现代动物群体，但目前大部分古生物学者们认为这些化石所代表的独特生物都出现于寒武纪初期并于该时期消失，并未进化成现代的动物群体。

At one time, the animals present in these fossil beds were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossils represent unique body forms that arose in the early Cambrian period and disappeared before the end of the period, leaving no descendants in modern animal groups.

第三种化石层同时包含软体动物和硬体动物，它为寒武纪生命大爆发提供了佐证。

A third fossil formation containing both soft-bodied and hard-bodied animals provides evidence of the result of the Cambrian explosion.

这个名为波基斯页岩的化石层位于加拿大不列颠哥伦比亚省落基山脉的优鹤国家公园。

This fossil formation, called the Burgess Shale, is in Yoho National Park in the Canadian Rocky Mountains of British Columbia.

寒武纪爆发不久后，泥石流迅速将成千上万的海洋生物所掩埋，为石化提供了有利条件。

Shortly after the Cambrian explosion, mud slides rapidly buried thousands of marine animals under conditions that favored fossilization.

这些化石层所提供的证据不仅包括大约32种现代动物群体，还有大约20种其他动物，这20种动物由于与现代动物在形体上区别过大导致无法归属于任何现代动物的群体。

These fossil beds provide evidence of about 32 modern animal groups, plus about 20 other animal body forms that are so different from any modern animals that they cannot be assigned to any one of the modern groups.

这其中包括大型水生捕食动物奇虾，以及以腐屑和藻类为食的软体动物威瓦西虫。

These unassignable animals include a large swimming predator called Anomalocaris and a soft-bodied animal called Wiwaxia, which ate detritus or algae.

波基斯页岩同样含有很多已经灭绝了的现代动物群体的代表。

The Burgess Shale formation also has fossils of many extinct representatives of modern animal groups.

例如，著名的西德尼虫就是此前所不为人知的节肢动物群体的一个代表（这类动物包括昆虫、蜘蛛、螨类以及蟹类）。

For example, a well-known Burgess Shale animal called Sidneyia is a representative of a previously unknown group of arthropods (a category of animals that includes insects, spiders, mites, and crabs).

类似波基斯页岩这样的化石层表明动物进化不总是一个漫长的过程。

Fossil formations like the Burgess Shale show that evolution cannot always be thought of as a slow progression.

寒武纪生命大爆发不仅包含快速的进化多样性，同时也伴随着很多独特动物的灭绝。

The Cambrian explosion involved rapid evolutionary diversification, followed by the extinction of many unique animals.

不少动物学家认为其原因在于很多生态环境实际上缺乏来自现有物种的竞争。

Many zoologists believe that it was because so many ecological niches were available with virtually no competition from existing species.

寒武纪生命大爆发的演化序列动物学家们是否知道呢？

Will zoologists ever know the evolutionary sequences in the Cambrian explosion?

或许谜底就位于海底另一处拥有6亿年历史的远古化石层之中，正等待人们的发掘。

Perhaps another ancient fossil bed of soft-bodied animals from 600-million-year-old seas is awaiting discovery.

The word “significant” in the passage is closest in meaning to

The word “relatively” in the passage is closest in meaning to

The word “diversification” in the passage is closest in meaning to

The period discussed in the passage is referred to as an “explosion” because it

According to paragraph 2, which of the following is NOT a question that paleontologists asked about the Cambrian explosion?

Which of the following best describes the relationship between paragraph 2 and paragraph 3?

The word “promote” in the passage is closest in meaning to

Which of the following is NOT mentioned in paragraph 4 as being true of the Ediacara formation?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

Why does the author mention “Anomalocaris” and “Wiwaxia”?

What can be inferred from paragraph 7 about why the Cambrian explosion is so unusual?

Look at the four squares  that indicate where the following sentence could be added to the passage. It is relatively rare because the fossilization of soft-bodied animals requires a special environment.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

The Cambrian Explosion The geologic timescale is marked by significant geologic and biological events, including the origin of Earth about 4.6 billion years ago, the origin of life about 3.5 billion years ago, the origin of eukaryotic life-forms (living things that have cells with true nuclei) about 1.5 billion years ago, and the origin of animals about 0.6 billion years ago. The last event marks the beginning of the Cambrian period. Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history. During a geologically brief 100-million-year period, all modern animal groups (along with other animals that are now extinct) evolved. This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.” Scientists have asked important questions about this explosion for more than a century. Why did it occur so late in the history of Earth? The origin of multicellular forms of life seems a relatively simple step compared to the origin of life itself. Why does the fossil record not document the series of evolutionary changes during the evolution of animals? Why did animal life evolve so quickly? Paleontologists continue to search the fossil record for answers to these questions. One interpretation regarding the absence of fossils during this important 100-million-year period is that early animals were soft bodied and simply did not fossilize.  Fossilization of soft-bodied animals is less likely than fossilization of hard-bodied animals, but it does occur.  Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages decomposition.  In fact, fossil beds containing soft-bodied animals have been known for many years.  The Ediacara fossil formation, which contains the oldest known animal fossils, consists exclusively of soft-bodied forms. Although named after a site in Australia, the Ediacara formation is worldwide in distribution and dates to Precambrian times. This 700-million-year-old formation gives few clues to the origins of modern animals, however, because paleontologists believe it represents an evolutionary experiment that failed. It contains no ancestors of modern animal groups. A slightly younger fossil formation containing animal remains is the Tommotian formation, named after a locale in Russia. It dates to the very early Cambrian period, and it also contains only soft-bodied forms. At one time, the animals present in these fossil beds were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossils represent unique body forms that arose in the early Cambrian period and disappeared before the end of the period, leaving no descendants in modern animal groups. A third fossil formation containing both soft-bodied and hard-bodied animals provides evidence of the result of the Cambrian explosion. This fossil formation, called the Burgess Shale, is in Yoho National Park in the Canadian Rocky Mountains of British Columbia. Shortly after the Cambrian explosion, mud slides rapidly buried thousands of marine animals under conditions that favored fossilization. These fossil beds provide evidence of about 32 modern animal groups, plus about 20 other animal body forms that are so different from any modern animals that they cannot be assigned to any one of the modern groups. These unassignable animals include a large swimming predator called Anomalocaris and a soft-bodied animal called Wiwaxia, which ate detritus or algae. The Burgess Shale formation also has fossils of many extinct representatives of modern animal groups. For example, a well-known Burgess Shale animal called Sidneyia is a representative of a previously unknown group of arthropods (a category of animals that includes insects, spiders, mites, and crabs). Fossil formations like the Burgess Shale show that evolution cannot always be thought of as a slow progression. The Cambrian explosion involved rapid evolutionary diversification, followed by the extinction of many unique animals. Why was this evolution so rapid? No one really knows. Many zoologists believe that it was because so many ecological niches were available with virtually no competition from existing species. Will zoologists ever know the evolutionary sequences in the Cambrian explosion? Perhaps another ancient fossil bed of soft-bodied animals from 600-million-year-old seas is awaiting discovery.