Official 45 Passage 1


The Beringia Landscape


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Nevertheless, large animals managed to survive in Beringia.


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  • During the peak of the last ice age, northeast Asia (Siberia) and Alaska were connected by a broad land mass called the Bering Land Bridge. This land bridge existed because so much of Earth's water was frozen in the great ice sheets that sea levels were over 100 meters lower than they are today. Between 25,000 and 10,000 years ago, Siberia, the Bering Land Bridge, and Alaska shared many environmental characteristics. These included a common mammalian fauna of large mammals, a common flora composed of broad grasslands as well as wind-swept dunes and tundra, and a common climate with cold, dry winters and somewhat warmer summers. The recognition that many aspects of the modern flora and fauna were present on both sides of the Bering Sea as remnants of the ice-age landscape led to this region being named Beringia.

    It is through Beringia that small groups of large mammal hunters, slowly expanding their hunting territories, eventually colonized North and South America. On this archaeologists generally agree, but that is where the agreement stops. One broad area of disagreement in explaining the peopling of the Americas is the domain of paleoecologists, but it is critical to understanding human history: what was Beringia like?

    The Beringian landscape was very different from what it is today. Broad, windswept valleys; glaciated mountains; sparse vegetation; and less moisture created a rather forbidding land mass. This land mass supported herds of now-extinct species of mammoth, bison, and horse and somewhat modern versions of caribou, musk ox, elk, and saiga antelope. These grazers supported in turn a number of impressive carnivores, including the giant short-faced bear, the saber-tooth cat, and a large species of lion.

    The presence of mammal species that require grassland vegetation has led Arctic biologist Dale Guthrie to argue that while cold and dry, there must have been broad areas of dense vegetation to support herds of mammoth, horse, and bison. Further, nearly all of the ice-age fauna had teeth that indicate an adaptation to grasses and sedges; they could not have been supported by a modern flora of mosses and lichens. Guthrie has also demonstrated that the landscape must have been subject to intense and continuous winds, especially in winter. He makes this argument based on the anatomy of horse and bison, which do not have the ability to search for food through deep snow cover. They need landscapes with strong winds that remove the winter snows, exposing the dry grasses beneath. Guthrie applied the term "mammoth steppe" to characterize this landscape.

    In contrast, Paul Colinvaux has offered a counterargument based on the analysis of pollen in lake sediments dating to the last ice age. He found that the amount of pollen recovered in these sediments is so low that the Beringian landscape during the peak of the last glaciation was more likely to have been what he termed a "polar desert," with little or only sparse vegetation.In no way was it possible that this region could have supported large herds of mammals and thus, human hunters. Guthrie has argued against this view by pointing out that radiocarbon analysis of mammoth, horse, and bison bones from Beringian deposits revealed that the bones date to the period of most intense glaciation.

    The argument seemed to be at a standstill until a number of recent studies resulted in a spectacular suite of new finds. The first was the discovery of a 1,000-square-kilometer preserved patch of Beringian vegetation dating to just over 17,000 years ago-the peak of the last ice age. The plants were preserved under a thick ash fall from a volcanic eruption. Investigations of the plants found grasses, sedges, mosses, and many other varieties in a nearly continuous cover, as was predicted by Guthrie. But this vegetation had a thin root mat with no soil formation, demonstrating that there was little long-term stability in plant cover, a finding supporting some of the arguments of Colinvaux. A mixture of continuous but thin vegetation supporting herds of large mammals is one that seems plausible and realistic with the available data.

  • 上一次冰期高峰,东北亚(西伯利亚)和阿拉斯加被一名为白令陆桥的广阔大陆块连接起来。 这座大陆桥的出现是因为,那时候地球上大量的水被冻结成巨大的冰盖,所以海平面比现在低100米。 在10000年到25000年之前,西伯利亚、白令大陆桥和阿拉斯加有许多共同的环境特征。 其中包括,这三个地方都有常见的由大型哺乳动物组成的哺乳动物群和由广袤的草地、寒风凌厉的沙丘和冻原组成的植物群,以及冬季寒冷干燥、夏季较暖和的气候。 人们认为现在白令海两岸的植物群和动物群在许多方面都可以被认为是冰河时代的残存者,所以这个地区被命名为白令陆桥。

    正是通过白令陆桥,一些捕捉大型哺乳动物的猎人小团体慢慢地扩大了他们的狩猎领地,最终占领了北美洲和南美洲。 在这一点上考古学家普遍表示认同,但是在其他方面大家就产生了分歧。 在解释美洲印第安人的居住情况时产生的一个比较大的分歧在于古生物学的范畴,但这关键是要理解人类历史:那时候的白令陆桥是什么样的?

    彼时白令陆桥的景观与如今大不相同。 那时山谷宽广,海风吹拂,山脉被冰覆盖,植被稀疏;降水稀少,令人望而生畏。 这个陆块中生存着成群的现今已灭绝的猛犸象、野牛、马,以及现代版的驯鹿、麝牛、麋鹿和赛加羚羊。 以这些食草动物为食的是大型食肉动物,包括巨型短面熊、剑齿猫、和大型狮子。

    哺乳动物的存在需要草地植被,这使得北极生物学家戴尔•古思莱认为虽然白令陆桥地区寒冷干燥,但是应该有大面积的茂密植被来养活庞大的猛犸群、马群和野牛群。 此外,几乎所有冰河时代动物的牙齿都表明它们适应了禾草和莎草;他们不可能只吃现代植物苔藓和地衣。 古思莱还表明,陆桥地区应该时常持续刮强风,特别是在冬季。 对野马和野牛的解剖可以发现它们没有穿过深雪寻找食物的能力,这就证明了他的论点。 野马和野牛需要大风吹走积雪,使得下面的干草显露出来。 古思莱用了“猛犸草原”一词来描述陆桥地区。

    与这个论点相反,保罗.柯林沃斯基于对湖泊沉积物中发现的可追溯至冰河时期的花粉的研究,提出了反对论据。 他发现,在这些沉积物中的花粉是如此之少,以至于他认为末次冰河时代高峰期的白令陆桥很可能是“极地荒漠”,由于植被稀疏,所以这个地区不可能养活那么大群的大型哺乳动物,因此也不会有那么多猎人。 古思莱表示反对,他指出,对白令陆桥地区猛犸象、马、草原野牛的骨头沉积物进行放射性碳定年分析发现这些骨头可以追溯到冰川作用最强烈的时期。

    两种观点一直以来处于僵持状态,直到最近才有了一些重大的新发现。 首先是发现了一个保存下来的1000平方公里的白令陆桥植被区,该植被区可以追溯到17000多年前,也就是上一次冰期高峰。 植被区被火山喷发出来的厚厚的灰烬覆盖,故而得以保存。 对其调查发现,正如古思莱预测的那样,在这几乎不断的灰层覆盖之下,这个地区生长着包括禾草、莎草、苔藓在内的多种植物。 不过这个植被区有一层细细的根系,但是并没有土壤形成,说明本地区的植被不具备长期稳定性,这与柯林沃斯的一些观点吻合。 以现有的数据来看,目前看似合理和现实的解释是,大型哺乳动物群是以这些连续生长的、薄薄的植被层为食的。
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    选项分析:第一空前一句交代了Beringian landscape过去和现在完全不一样;空一后句子对过去的landscape 的地貌进行描述。显然,两个句子之间不需要转折关系;第二个空后的句子开头this land,前面应该需要有指代事物,同时句子讲述land mass生存的动物类型,显然,空二处可以插入一个转折关系联系两个句子:空前讲的是生存环境,空后句子(land 生存动物列举)刚好是对待插入句子中的large animals的具体例证。空三之后的句子具体说明了大型动物是如何生存的因此第二个适合插入句子。