Earth’s atmosphere has changed through time. Compared to the Sun, whose composition is representative of the raw materials from which Earth and other planets in our solar system formed, Earth contains less of some volatile elements, such as nitrogen, argon, hydrogen, and helium. These elements were lost when the envelope of gases, or primary atmosphere, that surrounded early Earth, was stripped away by the solar wind or by meteorite impacts, or both. Little by little, the planet generated a new, secondary atmosphere by volcanic outgassing of volatile materials from its interior.

Volcanic outgassing continues to be the main process by which volatile materials are released from Earth—although it is now going on at a much slower rate. The main chemical constituent of volcanic gases (as much as 97 percent of volume) is water vapor, with varying amounts of nitrogen, carbon dioxide, and other gases. In fact, the total volume of volcanic gases released over the past 4 billion years or so is believed to account for the present composition of the atmosphere with one important exception: oxygen. Earth had virtually no oxygen in its atmosphere more than 4 billion years ago, but the atmosphere is now approximately 21 percent oxygen.

Traces of oxygen were probably generated in the early atmosphere by the breakdown of water molecules into oxygen and hydrogen by ultraviolet light (a process called photodissociation). Although this is an important process, it cannot begin to account for the present high levels of oxygen in the atmosphere. Almost all of the free oxygen now in the atmosphere originated through photosynthesis, the process whereby plants use light energy to induce carbon dioxide to react with water, producing carbohydrates and oxygen.

Oxygen is a very reactive chemical, so at first most of the free oxygen produced by photosynthesis was combined with iron in ocean water to form iron oxide-bearing minerals. The evidence of the gradual transition from oxygen-poor to oxygen-rich water is preserved in seafloor sediments. The minerals in seafloor sedimentary rocks that are more than about 2.5 billion years old contain reduced (oxygen-poor) iron compounds. In rocks that are less than 1.8 billion years old, oxidized (oxygen-rich) compounds predominate. The sediments that were precipitated during the transition contain alternating bands of red (oxidized iron) and black (reduced iron) minerals. These rocks are called banded-iron formations. Because ocean water is in constant contact with the atmosphere, and the two systems function together in a state of dynamic equilibrium, the transition from an oxygen-poor to an oxygen-rich atmosphere also must have occurred during this period.

Along with the buildup of molecular oxygen (O2) came an eventual increase in ozone (O3) levels in the atmosphere. Because ozone filters out harmful ultraviolet radiation, this made it possible for life to flourish in shallow water and finally on land. This critical state in the evolution of the atmosphere was replaced between 1100 and 542 million years ago. Interestingly, the fossil record shows an explosion of life forms 542 million years ago.

Oxygen has continued to play a key role in the evolution and form of life. Over the last 200 million years, the concentration of oxygen has risen from 10 percent to as much as 25 percent of the atmosphere, before setting (probably not permanently) at its current value of 21 percent. This increase has benefited mammals, which are voracious oxygen consumers. Not only do we require oxygen to fuel our high-energy, warm-blooded metabolism, our unique reproductive system demands even more. An expectant mother’s used (venous) blood must still have enough oxygen in it to diffuse through the placenta into her unborn child’s bloodstream. It would be very difficult for any mammal species to survive in an atmosphere of only 10 percent oxygen.

Geologists cannot yet be certain why the atmospheric oxygen levels increased, but they have a hypothesis. First photosynthesis is only one part of the oxygen cycle. The cycle is completed by decomposition, in which organic carbon combines with oxygen and forms carbon dioxide. But if organic matter is buried as sediment before it fully decomposes, its carbon is no longer available to react with the free oxygen. Thus there will be a net accumulation of carbon in sediments and of oxygen in the atmosphere.

地球的大气层随着时间的推移而改变。太阳的组成元素是有代表性的原材料，这些原材料是形成地球和太阳系中其他行星的原材料。地球含有少量挥发性元素，如氮、氩、氢和氦。当围绕着早期地球的气体或原始大气的表层被太阳风或陨石撞击或两者侵袭时，这些元素便会丢失。渐渐地，地球从火山内部释放出挥发性物质，产生了一种新的次生大气。 火山喷发仍然是地球释放挥发性物质的主要过程，尽管现在正在以更慢的速度进行。火山气体的主要化学成分是水蒸气（占据容积的97%），含有不同数量的氮、二氧化碳和其他气体。事实上，过去40亿年左右释放的火山气体总量被认为构成了目前大气组成的元素，除了一个重要例外：氧气。40亿多年前，地球的大气中几乎没有氧气，但现在大气中大约有21%的氧气。 追溯氧气的来源，它可能是在早期大气中水分子通过紫外光分解成氧和氢（一种称为光解）的过程产生的。虽然这是一个重要的过程，但它不能就此解释目前大气中的高含氧量。现在大气中几乎所有的游离氧都是通过光合作用产生的，光合作用是植物利用光能诱导二氧化碳与水反应，产生碳水化合物和氧气的过程。 氧是一种反应性很强的化学物质，所以一开始光合作用产生的大部分游离氧与海洋中的铁结合形成含氧化铁的矿物。从贫氧到富氧水的逐渐过渡的证据被保存在海底沉积物中。超过25亿年的海底沉积岩中的矿物含有还原性（贫氧）铁化合物。在不到18亿年的岩石中，氧化（富氧）化合物占了主导地位。在过渡过程中沉淀的沉积物含有交替的红色（氧化铁）和黑色（还原铁）矿物带。这些岩石被称为带状铁形成物。由于海水与大气不断接触，这两个系统在动态平衡的状态下共同作用，因此在这段时间内也必然发生从贫氧到富氧大气的过渡。 随着分子氧（O2）的积累，大气中的臭氧（O3）量最终增加。因为臭氧过滤掉有害的紫外线辐射，这使得生命可以在浅水和陆地上繁衍。大气演化的这种临界状态在1100年至5亿4200万年前被取代了。有趣的是，化石记录显示了5亿4200万年前生命形式的爆发。 氧在生命的进化和形成中继续起着关键的作用。在过去的2亿年中，氧在大气中的浓度从10%上升到25%，其当前值为21%（可能不是永久的）。这一增长得益于哺乳动物，它们是贪婪的氧气消耗者。我们不仅需要氧气来为我们的高能量、温血的新陈代谢提供燃料，我们独特的生殖系统也需要更多的氧气。一个怀孕的母亲使用过的（静脉）血液内必须有足够的氧气，通过胎盘扩散到她未出生的孩子的血流中。对于任何哺乳动物来说，在只有10%氧气的环境中生存是非常困难的。 地质学家还不能确定为什么大气中的氧含量会增加，但他们有一个假设。首先光合作用只是氧循环的一部分。这个循环是通过分解来完成的，在分解过程中有机碳和氧气结合形成二氧化碳。但是如果有机物在沉积物完全分解之前被掩埋，它的碳就不再能够与游离氧反应。因此，沉积物中的碳和大气中的氧气都都要有净积累。