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在地质维度上短时间内许多物种灭绝的情况被称为大灭绝。

Cases in which many species become extinct within a geologically short interval of time are called mass extinctions.

在白垩纪末期（大约7,000万年前）就发生了一次这样的事件（大灭绝）。

There was one such event at the end of the Cretaceous period (around 70 million years ago).

在二叠纪末期（大约2.5亿年前），还有另一个范围更大的大灭绝。

There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago).

二叠纪（灭绝）事件引起的关注要比其他大规模灭绝事件（引起的关注）少得多，因为当时灭绝的物种大多是人们不熟悉的。

The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.

化石记录显示出，至少有五次包含许多海洋生物家族灭亡的物种大灭绝。

The fossil record shows at least five mass extinctions in which many families of marine organisms died out.

今天物种灭绝的速度与当年这些大灭绝期间物种灭绝的速度一样高。

The rates of extinction happening today are as great as the rates during these mass extinctions.

因此，许多科学家得出结论，第六次大灭绝正在发生。

Many scientists have therefore concluded that a sixth great mass extinction is currently in progress.

是什么导致了如此高的灭绝速度呢？

What could cause such high rates of extinction?

有几种假说，包括地球变暖或变冷，季节性波动或洋流的变化以及各大陆的位置变化。

There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents.

生物假说包括昆虫与开花植物之间的合作方式的进化或海洋中以底层食物为食的捕食者的进化所带来的生态变化。

Biological hypotheses include ecological changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators in the oceans.

某些提出的机制需要很短的一段时间内所有物种都突然灭绝；其他机制更可能会在更长的时期内或在不同的大陆上的不同时间内逐渐地发生。

Some of the proposed mechanisms required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents.

一些假设无法解释陆地和海洋物种同时灭绝的情况。

Some hypotheses fail to account for simultaneous extinctions on land and in the seas.

Each mass extinction may have had a different cause.

有证据表明，人类的狩猎和对动物栖息地的破坏是造成当前大规模灭绝的可能原因。

Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction.

研究过许多化石物种的灭绝率的美国古生物学家戴维·劳普（David Raup）和约翰·塞普斯基（John Sepkoski）指出，自白垩纪中期以来，大约每2600万年就有一次灭绝增多的事件反复发生。

American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period.

白垩纪晚期恐龙和菊石类物种的灭绝只是一系列此类反复灭绝事件中更为严重的一种。

The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes.

大规模灭绝可能定期发生的可能性带来了这样的假设，例如，伴随着一个长周期轨道的伴星使其他物体偏离其正常轨道，使其中一些作为流星坠落到地球上，并在撞击时造成广泛的破坏。

The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.

试图解释白垩纪晚期灭绝的各种假说中，近年来引起人们最多关注的假说是路易斯（Luis）和沃尔特·阿尔瓦雷斯（Walter Alvarez）首先提出的小行星影响假说。

Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez.

根据这个假设，地球与一颗直径估计为10公里的小行星或几颗小行星相撞，这些小行星的总质量与直径为10公里的小行星相当。

According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable.

碰撞的力将大量碎屑喷入大气，使天空在更细的颗粒沉降之前变暗了好几年。

The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled.

光合作用水平的降低导致各种植物的大量减少，这首先导致了草食动物忍饥挨饿，接着是食肉动物的受饿。

The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores.

在这种假设下，大规模灭绝将会突然发生。

The mass extinction would have occurred very suddenly under this hypothesis.

阿尔瓦雷斯（Alvarez）假设的一个有意思的检测是基于稀土元素铱Ir的存在。

One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir).

地壳中几乎不含这种元素，但大多数小行星的铱元素的含量要比地球高的多。

Earth’s crust contains very little of this element, but most asteroids contain a lot more.

通过小行星碰撞而被扔到大气中的碎片可能包含大量的铱，而大气流将把该元素带到世界各地。

Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe.

对跨越白垩纪和第三纪之间边界的沉积物的搜索表明，在该边界处，铱的丰度有了明显的急剧增加。

A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary.

即使还没有小行星被找到，这种铱元素的异常也为阿尔瓦雷斯假说提供了有力的支持。

This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.

即使小行星本身因撞击而崩解，这种大小的小行星也可能留下巨大的陨石坑。

An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact.

强烈的冲击力会在许多类型的岩石中产生热震石英。

The intense heat of the impact would produce heat-shocked quartz in many types of rock.

同样，大块岩石被撞击抛在一边会形成围绕主坑的次级撞击坑。

Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater.

迄今为止，在墨西哥的尤卡坦半岛附近已经发现了几个这样的次级撞击坑，并且在墨西哥和海地都发现了热震石英。

To date, several such secondary craters have been found along Mexico’s Yucatán Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti.

尤卡坦海岸沿岸一个名为希克苏鲁伯的地点被认为是陨石撞击的主要地点。

A location called Chicxulub, along the Yucatán coast, has been suggested as the primary impact site.

Paragraph 1 supports which of the following statements about mass extinctions?

According to paragraph 2, scientists base their belief that a mass extinction is going on at present on which of the following?

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

According to paragraph 3, each of the following has been proposed as a possible cause of mass extinctions EXCEPT

Paragraph 3 supports which of the following ideas about mass extinctions?

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.

According to paragraph 4, what aspect of extinction episodes does the companion-star hypothesis supposedly clarify?

The phrase “account for” in the passage is closest in meaning to

According to paragraph 6, what made iridium a useful test of the Alvarez hypothesis?

In stating that “no asteroid itself has ever been recovered,” the author emphasizes which of the following?

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

What is the purpose of paragraph 7 in the passage?

Look at the four squares  that indicate where the following sentence could be added to the passage. In general, it is believed that these two extinctions resulted from drastic environmental changes that followed meteorite impacts or massive volcanic eruptions.  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.

Mass Extinctions   Cases in which many species become extinct within a geologically short interval of time are called mass extinctions.  There was one such event at the end of the Cretaceous period (around 70 million years ago).  There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago).  The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time. The fossil record shows at least five mass extinctions in which many families of marine organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress. What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal fluctuations or ocean currents, and changing positions of the continents. Biological hypotheses include ecological changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators in the oceans. Some of the proposed mechanisms required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents. Some hypotheses fail to account for simultaneous extinctions on land and in the seas. Each mass extinction may have had a different cause. Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction. American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact. Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable. The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis. One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth’s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered. An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact. The intense heat of the impact would produce heat-shocked quartz in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater. To date, several such secondary craters have been found along Mexico’s Yucatán Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatán coast, has been suggested as the primary impact site.