It's hard to keep oxygen molecules around, despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff? The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day. For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms—and their "extremophile" descendants today—thrived in the absence of oxygen, relying on sulfate for their energy needs. But roughly 2.45 billion years ago, the isotopic ratio of sulfur transformed, indicating that for the first time oxygen was becoming a significant component of Earth's atmosphere, according to a 2000 paper in Science. At roughly the same time (and for eons thereafter), oxidized iron began to appear in ancient soils and bands of iron were deposited on the seafloor, a product of reactions with oxygen in the seawater. "What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago. It took up residence in atmosphere around 2.45 billion years ago," says geochemist Dick Holland, a visiting scholar at the University of Pennsylvania. "It looks as if there's a significant time interval between the appearance of oxygen-producing organisms and the actual oxygenation of the atmosphere."So a date and a culprit can be fixed for what scientists refer to as the Great Oxidation Event, but mysteries remain. What occurred 2.45 billion years ago that enabled cyanobacteria to take over? What were oxygen levels at that time? Why did it take another one billion years—dubbed the "boring billion" by scientists—for oxygen levels to rise high enough to enable the evolution of animals? Most important, how did the amount of atmospheric oxygen reach its present level? "It's not that easy why it should balance at 21 percent rather than 10 or 40 percent," notes geoscientist James Kasting of Pennsylvania State University. "We don't understand the modern oxygen control system that well." Climate, volcanism, plate tectonics all played a key role in regulating the oxygen level during various time periods. Yet no one has come up with a rock-solid test to determine the precise oxygen content of the atmosphere at any given time from the geologic record. But one thing is clear—the origins of oxygen in Earth's atmosphere derive from one thing: life. The atmosphere is the layer of gases that surrounds the Earth. It provides gases that are essential to life. It has evolved over time and human activity is having an impact on the atmosphere. No one knows of any other planet where you can do this simple thing. Other planets and moons in our solar system have atmospheres, but none of them could support life as we know it. They are either too dense (as on Venus) or not dense enough (as on Mars), and none of them have much oxygen, the precious gas that we Earth animals need every minute. So how did our atmosphere get to be so special? Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today.
Now we have Earth’s “third atmosphere,” the one we all know and love—an atmosphere containing enough oxygen for animals, including ourselves, to evolve. So plants and some bacteria use carbon dioxide and give off oxygen, and animals use oxygen and give off carbon-dioxide—how convenient! The atmosphere upon which life depends was created by life itself.
The atmosphere life depends on was created by life itself? Woah! Note that this pie does not account for any water in the air. As you can see, air is mostly nitrogen (78%), with oxygen a distant runner up (21%). Argon and some other gases make up another small amount (about 1%). And carbon dioxide is only a very tiny slice (.0385% or only about 385 parts per million parts of air). But these proportions were quite likely different in Earth’s much younger days. The planet is generally thought of as having three distinct atmospheres over the course of its lifetime. When earth was first formed, its atmosphere was likely composed of hydrogen, helium, and other gases that contained hydrogen. Yet this atmosphere didn't last for very long because the solar wind from the sun blew it away. Solar wind is stream of charged particles such as electrons, protons, and alpha particles. We now have a magnetic field surrounding earth that shields us from solar wind. The second atmosphere formed a little after 4.5 million years ago (mya) and was produced due to volcanic outgassing. Outgassing is the release of gas that was trapped in some other material. In this case, volcanic outgassing released hot gases trapped deep within the interior of the planet. Water vapor, carbon dioxide, methane, ammonia, and other gases similar to the ones produced by volcanoes today were expelled. Over a vast amount of time, millions of years, the earth gradually cooled. When the temperature dropped enough, water vapor condensed and went from a gas to liquid form. This created clouds. From these clouds, the oceans formed and the oceans absorbed a lot of the carbon dioxide in the atmosphere. A small amount of oxygen was produced by the photolysis of carbon dioxide and water vapor by ultraviolet radiation. Lastly, we have the third atmosphere. Around 2.5 mya, the amount of oxygen available in the atmosphere started to rise due to the evolution of photosynthetic organisms that produced oxygen. These organisms were oceanic cyanobacteria. Over time, aerobic organisms evolved and consumed some of the oxygen produced. Read more about how oxygen became a major part of our atmosphere here. This is the composition of the atmosphere we have today: |