托福阅读真题再现：
大气中最初是没有氧气的，因为光合作用和火山爆发而慢慢变化，水蒸气产生大量氧气，然后开始出现了生物。通过和太阳比较，现在空气中比以前少了些元素，氧气的产生过程中，水分解放出氧，植物进行转化。研究发现氧先和铁结合进入海洋，因为两个时间点的石头构成不同，生物爆发正好与后一个时间点重合，氧气含量还与decompose有关。
TPO相似文章练习推荐：
TPO 10：Variations in The Climate
相关背景：
Oxygen is an importantpart of the atmosphere, and is necessary to sustain most terrestrial life as itis used in respiration. However, it is too chemically reactive to remain a freeelement in Earth's atmosphere without being continuously replenishedby the photosynthetic action of living organisms, which usethe energy of sunlight to produce elemental oxygen from water. Another form(allotrope) of oxygen, ozone (O3),strongly absorbs UVB radiation and consequently thehigh-altitude ozone layer helps protect the biosphere from ultraviolet radiation, but is apollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes, atomic oxygen is asignificant presence and a cause of erosionfor spacecraft. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air,electrolysis of water and othermeans. Uses of elemental oxygen include the production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy and lifesupport systems in aircraft, submarines, spaceflight and diving.
Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteriaevolved, probably about 3.5 billion years ago. Free oxygen first appeared insignificant quantities during the Paleoproterozoic eon (between 3.0 and 2.3 billion yearsago). For the first billion years, any free oxygen produced by theseorganisms combined with dissolved iron in the oceans to form banded iron formations. When suchoxygen sinks became saturated, free oxygen began to outgas from the oceans3–2.7 billion years ago, reaching 10% of its present level around1.7 billion years ago. The presence of large amounts of dissolved and freeoxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the Great Oxygenation Event (oxygen catastrophe) about 2.4billion years ago. However, cellularrespiration using O2 enables aerobic organisms to produce much moreATP than anaerobic organisms, helping the former to dominate Earth's biosphere. Cellular respiration of O2 occurs in all eukaryotes, including all complexmulticellular organisms such as plants and animals. Since the beginning of the Cambrian period 540 million years ago, O2 levels have fluctuated between 15% and30% by volume. Towards the end of the Carboniferous period (about 300 million yearsago) atmospheric O2 levelsreached a maximum of 35% by volume, which may have contributed tothe large size of insects and amphibians at this time. Human activities, including theburning of 7 billion tonnes of fossilfuels each year have had verylittle effect on the amount of free oxygen in the atmosphere. At the current rate of photosynthesisit would take about 2,000 years to regenerate the entire O2 in the present atmosphere.
更多托福备考信息尽在网托福频道!