2016年11月12日托福閱讀真題第二篇

　　題材劃分： 考古類

　　主要內容： 解釋mass extinction的假說。最開始提出行星撞擊地球的理論，科學家不相信，因為他們覺得地球上的事和extraterrestrial無關，而且學校里也不教這方面的事情。但在K-T clay中含有大量的iridium,這一事實支撐了該理論。第一次提出支持行星碰撞理論的是在海底有iridium,科學家顯示反對，認為是海洋生物產(chǎn)生的，后來被反駁，因為在陸地發(fā)現(xiàn)了同樣含有大量iridium的巖石。有人提出了火山的猜想，認為iridium一開始在地心，由于火山爆發(fā)被帶到地表，然后受到風的作用，吹到四處。該猜想備反駁，因為在夏威夷K地，一次爆發(fā)僅含有少量iridium,要達到K-T clay那么多的iridium需要成百上千次，所以不可能。

　　相似TPO練習推薦：

　　TPO15- Mass Extinctions

　　相關知識背景：

　　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.