GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 5BSYT87T7445C_7JK

When Earth formed 4.6 billion years ago, the Sun burned only 70 percent as brightly as it does today. Yet geologic record contains no evidence for widespread glaciation until 2.3 billion years ago. Sagan and Mullen suggested in the 1970s that ammonia, a greenhouse gas, warmed early Earth's atmosphere, but subsequent research showed that the Sun's ultraviolet rays rapidly destroy ammonia in an oxygen-free environment, such as that of early Earth. Many scientists now attribute much of the warming of early Earth to oxygen-intolerant microbes – methanogens – that produce the greenhouse gas methane. The methanogen hypothesis could help to explain the first global ice age: 2.3 billion years ago, Earth's atmosphere began to fill with oxygen produced by other microbes – cyanobacteria – causing methanogens to decline rapidly.