The geologic timescale ismarkedbysignificantgeologic and biological events, including theoriginof Earth about 4.6 billion years ago, theoriginof life about 3.5 billion years ago, theoriginof eukaryotic life-forms (living things that have cells with truenuclei) about 1.5 billion years ago, and theoriginof animals about 0.6 billion years ago. The last event marks the beginning of theCambrianperiod. Animals originatedrelativelylate in thehistoryof Earth—in only the last 10 percent of Earth’shistory. During a geologicallybrief100-million-year period, all modern animal groups (alongwith other animals that are nowextinct) evolved. This rapidoriginanddiversificationof animals is often referred to as “theCambrianexplosion.”

Scientists have asked important questions about thisexplosionfor more than acentury. Why did itoccurso late in thehistoryof Earth? Theoriginofmulticellularforms of life seems arelativelysimple step compared to theoriginof life itself. Why does thefossilrecordnot document theseriesofevolutionarychanges during theevolutionof animals? Why did animal lifeevolveso quickly? Paleontologists continue to search thefossilrecordfor answers to these questions.

Oneinterpretationregarding theabsenceof fossils during this important 100-million-year period is that early animals weresoftbodied andsimplydid notfossilize.

Fossilizationof soft-bodied animals is less likely thanfossilizationof hard-bodied animals, but it doesoccur. Conditions thatpromotefossilizationof soft-bodied animals include very rapid covering by sediments that create an environment that discouragesdecomposition. In fact,fossilbeds containing soft-bodied animals have been known for many years.

The Ediacarafossilformation, which contains the oldest known animal fossils, consistsexclusivelyof soft-bodied forms. Although named after a site in Australia, the Ediacaraformationis worldwide indistributionand dates toPrecambriantimes. This 700-million-year-oldformationgives few clues to the origins of modern animals, however, because paleontologists believe it represents anevolutionaryexperiment that failed. It contains no ancestors of modern animal groups.

Aslightlyyoungerfossilformationcontaining animal remains is the Tommotianformation, named after a locale in Russia. It dates to the very earlyCambrianperiod, and it also contains only soft-bodied forms. At one time, the animals present in thesefossilbeds were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossilsrepresentuniquebody forms that arose in the earlyCambrianperiod and disappeared before the end of the period, leaving no descendants in modern animal groups.

A thirdfossilformationcontaining both soft-bodied and hard-bodied animals providesevidenceof the result of theCambrianexplosion. Thisfossilformation, called the Burgess Shale, is in Yoho National Park in the Canadian Rocky Mountains of British Columbia. Shortlyafter theCambrianexplosion, mud slides rapidly buried thousands ofmarineanimals under conditions thatfavoredfossilization. Thesefossilbeds provideevidenceof 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 swimmingpredatorcalled Anomalocaris and a soft-bodied animal called Wiwaxia, which atedetritusoralgae. The Burgess Shaleformationalso has fossils of manyextinctrepresentatives of modern animal groups. For example, a well-known Burgess Shale animal called Sidneyia is arepresentativeof apreviouslyunknown group of arthropods (acategoryof animals that includes insects, spiders, mites, and crabs).

Fossilformations like the Burgess Shale show thatevolutioncannot always be thought of as a slow progression. TheCambrianexplosioninvolved rapidevolutionarydiversification, followed by theextinctionof manyuniqueanimals. Why was thisevolutionso rapid? No one really knows. Many zoologists believe that it was because so manyecologicalniches wereavailablewithvirtuallyno competition from existingspecies. Will zoologists ever know theevolutionarysequences in theCambrianexplosion? Perhaps anotherancientfossilbed of soft-bodied animals from 600-million-year-old seas is awaiting discovery.

Questions:
question_stem: The word “significant” in the passage is closest in meaning to
choices: tag: A
text: numerous

tag: B
text: important

tag: C
text: unexplained

tag: D
text: sudden
answer_key: B
explanation: B significant: 显著的，重要的。A 是许多的；B 是重要的；C 是未经解释的；D 是突然的。答案 B 的 important 正确。

question_stem: The word “relatively” in the passage is closest in meaning to
choices: tag: A
text: surprisingly

tag: B
text: collectively

tag: C
text: comparatively

tag: D
text: characteristically
answer_key: C
explanation: relatively: 相对地。A 是令人意外地；B 是共同地；C 是相对地；D 是典型地。所以 comparatively 相对地正确。对应原文：Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history.原句 relatively late 之后有个破折号对这个进行了解释，说地球历史的最后10%，当然是相对靠后的。A 吃惊 B 聚集 D 有特点都不对。

question_stem: The word “diversification” in the passage is closest in meaning to
choices: tag: A
text: emergence of many varieties

tag: B
text: steady decline in number

tag: C
text: gradual increase in body size

tag: D
text: sudden disappearance
answer_key: A
explanation: diversification: 分化。想到 diverse 多样，所以 A 出现了很多变体正确。对应原文：This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.”原文与之并列的是 origin（起源），带有出现的意思，所以 emergence 对得上；B 的 decline 和 D 的 disappear 都说反了；C 的 body size 与原文无关。

question_stem: The period discussed in the passage is referred to as an “explosion” because it
choices: tag: A
text: occurred 0.6 billion years ago, late in Earth’s history

tag: B
text: was characterized by the unusually fast evolution of many new life-forms

tag: C
text: was characterized by widespread animal extinction

tag: D
text: was characterized by violent volcanic eruptions
answer_key: B
explanation: B 以 explosion 做关键词定位至第一段最后一句，注意这道题的相关信息不在第二段而是在第一段。说 rapid origin and diversification 所以叫 explosion，所以 B 正确。A 的时间不能作为 explosion 的依据；C 灭绝说反了；D 火山爆发没说。

question_stem: According to Paragraph2, which of the following is NOT a question that paleontologists asked about the Cambrian explosion?
choices: tag: A
text: Why was the origin of life a simple step in Earth’s history?

tag: B
text: Why did it take so long for multicellular organisms to develop?

tag: C
text: Why did animal life evolve so rapidly?

tag: D
text: Why does the fossil record lack evidence of animal evolution during that time?
answer_key: A
explanation: A EXCEPT 题，排除法。A 的 simple step 做关键词定位至第三句，但这句话不是一个问题，所以 A 错，选；B 选项的 so long 对应原文第二句的 so late，所以正确，不选，注意不要单纯以 multicellular organism 做关键词，会导致定位错误；C 对应倒数第二句，正确，不选；D 的 lack evidence 对应原文倒数第三句的 not document，正确，不选

question_stem: Which of the following best describes the relationship between paragraph 2 and paragraph 3?
choices: tag: A
text: Paragraph 2 puts forward several scientific claims, one of which is rejected in paragraph 3.

tag: B
text: Paragraph 2 poses several questions, and paragraph 3 offers a possible answer to one of them.

tag: C
text: Paragraph 2 presents outdated traditional views, while paragraph 3 presents the current scientific conclusions.

tag: D
text: Paragraph 2 introduces a generalization that is illustrated by specific examples in paragraph 3.
answer_key: B
explanation: B 问整段的题，注意各段的开头句；第二段提出了很多问题，而第三段一开始就给了一个 explanation，说明是第二段问第三段答，所以 B 正确。第三段与第二段没有对比或者反对的关系，所以 A 和 C 都不对；也不是详细阐述，D 错。

question_stem: The word “promote” in the passage is closest in meaning to
choices: tag: A
text: complicate

tag: B
text: prevent

tag: C
text: encourage

tag: D
text: affect
answer_key: C
explanation: promote: 促进，提升。所以 C 的 encourage 正确。A 是使…复杂；B 是防止；D 是影响。对应原文：Conditions that promote fossilization of soft-bodied animals include very rapid covering by sediments that create an environment that discourages decomposition.原文的 condition 应该是对 fossilization 起到正面的作用，B 的 prevent 和 D 的 affect 都是负面作用，所以都不对；complicate 是使变复杂，原文没有给出相关信息，错。

question_stem: Which of the following is NOT mentioned in paragraph 4 as being true of the Ediacara formation?
choices: tag: A
text: It contains fossils that date back to the Precambrian period.

tag: B
text: It contains only soft-bodied animal fossils.

tag: C
text: It is located on a single site in Australia.

tag: D
text: It does not contain any fossils of the ancestors of modern animals.
answer_key: C
explanation: C 的相关信息来自第二句，但第二句说的是 worldwide in distribution，不是只在澳大利亚有，所以 C 说反了，正确。 
A 选项对应文中的：the Ediacara formation is worldwide in distribution and dates to Precambrian times. 
B 选项对应文中的：consists exclusively of soft-bodied forms.
D 选项对应文中的：It contains no ancestors of modern animal groups.

question_stem: 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.
choices: tag: A
text: The animals found in the Tommotian fossil bed were once thought to belong to a variety of modern animal groups, but now they are thought to have descended from a single group.

tag: B
text: Animals in the Tommotian fossil beds were initially assigned to modern animal groups but are now thought to belong to groups that emerged and died out during the Cambrian period.

tag: C
text: Though at first they thought otherwise, paleontologists now agree that the animals in the Tommotian have body forms from which modern animals have descended.

tag: D
text: It is unclear whether the Tommotian fossils from the early Cambrian period represent unique body forms or whether they should be assigned to various modern animal groups.
answer_key: B
explanation: B 原句最核心的关系是转折，后半句是作者的真实意思。转折之后说：大部分的古生物学家同意这些化石代表着非常特别的 body forms，在 Cambrian 早期出现，在末期消亡，没有留下子孙后代。D 项没有确切结论（从 whether 看出），原文有，所以 D 错；原文后半句说 T 出现在 C 早期，晚期就没了，也没有后代，A 和 C 都说有后代，错；所以答案 B 正确。

question_stem: Why does the author mention “Anomalocaris” and “Wiwaxia”?
choices: tag: A
text: To contrast predators with animals that eat plants such as algae

tag: B
text: To question the effects of rapid mud slides on fossilization

tag: C
text: To suggest that much is still unknown about animals found in the Burgess Shale

tag: D
text: To provide examples of fossils that cannot be assigned to a modern animal group
answer_key: D
explanation: Anomalocaris 和 Wiwaxia 是两种 unassignable 动物的例子，往前看：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.前句很清楚说有些动物是不能被分在任何一个 group 里，接着就给了这两种作为例子，所以 D 正确，其他都没说。

question_stem: “Sidneyia” is an example of
choices: tag: A
text: a relative of Anomalocaris and Wiwaxia

tag: B
text: a previously unknown Burgess Shale animal

tag: C
text: an extinct member of a currently existing category of animals

tag: D
text: an animal that cannot be assigned to any modern animal group
answer_key: C
explanation: C 也是一道功能目的题，前句说 BS 有很多已经灭绝的动物，它们是现代动物 group 的代表；然后就=给出了 S 这个例子，所以 C 正确；不要受到 B 的干扰，原文说 S 是一种以前不知道的 anthropod 的代表，没说是以前不知道的 BS 的代表，所以错。

question_stem: What can be inferred from paragraph 7 about why the Cambrian explosion is so unusual?
choices: tag: A
text: It generated new ecological niches through the extinction of many unique animals.

tag: B
text: It was a period of rapid evolution, and evolution is often thought of as a slow process.

tag: C
text: It is a period whose evolutionary sequences are clearly marked.

tag: D
text: It generated a very large number of ancient fossil beds containing soft-bodied animals.
answer_key: B
explanation: B 以 Cambrian Explosion 做关键词定位至第二句，注意倒数第二句也有一个 Cambrian Explosion，但因为与问题无关，所以不读。本句说寒武纪大爆发包含了很多动物的快速出现然后快速灭亡，接着提出问题为什么这么快并给出解答。所以问题中的 unusual 就是说快的不正常，所以答案 B 正确。A 的 new ecological niches 原文没有相关信息；C 原文只是一个问题，C 变成了答案，不对；D 错，原文最后一句说一个 bed，不是很多 beds，而且是 awaiting discovery，不是已经发现了。