2021年托福试题阅读理解模拟题考点分析（卷十）.doc

2021年托福试题阅读理解模拟题考点分析（卷十）
Title： The Cambrian Explosion
The earliest fossil evidence for eukaryotes complex organisms wthere been another period of similarly explosive diversification since then We can identify at least four factors that might have contributed to the Cambrian explosion. First, the oxygen level in our atmosphere may have remained well below its present level until about the time of the Cambrian explosion. Thus, the rapid diversification in animal life may have occurred at least in part because oxygen reached a critical level for the survival of larger and more energy-intensive life forms.
A second factor that may have been important was the evolution of genetic eukaryotes evolved, they developed more and more genetic variation in their DNA. Some scientists believe that the Cambrian explosion marks the point at which organisms developed certain kinds of genes (homeobox genes) that control body form and that could be combined in different ways, allowing the evolution of a great diversity of forms over time
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A third factor may have been climate change. Geological evidence points to a series of episodes in which Earth froze over before the Cambrian began. The extreme climate conditions of these episodes eliminated many species, leaving a wide array of ecological niches available into which new species could rapidly evolve when climate conditions e