2016年11月12日托福阅读真题解析（新东方）_第2页

　　2016年11月12日托福阅读真题第二篇

　　题材划分： 考古类

　　主要内容： 解释mass extinction的假说。最开始提出行星撞击地球的理论，科学家不相信，因为他们觉得地球上的事和extraterrestrial无关，而且学校里也不教这方面的事情。但在K-T clay中含有大量的iridium,这一事实支撑了该理论。第一次提出支持行星碰撞理论的是在海底有iridium,科学家显示反对，认为是海洋生物产生的，后来被反驳，因为在陆地发现了同样含有大量iridium的岩石。有人提出了火山的猜想，认为iridium一开始在地心，由于火山爆发被带到地表，然后受到风的作用，吹到四处。该猜想备反驳，因为在夏威夷K地，一次爆发仅含有少量iridium,要达到K-T clay那么多的iridium需要成百上千次，所以不可能。

　　相似TPO练习推荐：

　　TPO15- Mass Extinctions

　　相关知识背景：

　　An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Because the majority of diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.

　　Extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.

　　The Great Oxygenation Event was probably the first major extinction event. Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate. The most recent and debatably best-known, the CretaceousPaleogene extinction event, which occurred approximately 66 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In addition to the five major mass extinctions, there are numerous minor ones as well, and the ongoing mass-extinction caused by human activity is sometimes called the sixth extinction. Mass extinctions seem to be a mainly Phanerozoic phenomenon, with extinction rates low before large complex organisms arose.

　　Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.