Oxygen makes up about one-fifth the volume of Earth's atmosphere today, and is a central element of life as we know it.
But that wasn't always the case. Oxygen, although always present in compounds in Earth's interior, atmosphere, and oceans, did not begin to accumulate in the atmosphere as oxygen gas (O2) until well into the planet's history. What the atmosphere was like prior to oxygen's rise is a puzzle that Earth scientists have only begun to piece together.
Earth coalesced a little more than 4.5 billion years ago from bits of cosmic debris. Liquid oceans existed on the planet almost from the beginning, although in all likelihood they were repeatedly vaporized by the massive meteorites that regularly clobbered the planet during its first 700 million years of existence. Things had settled down by 3.8 billion years ago, when the first rocks that formed under water appear in the geologic record. (They exist in what is now southwest Greenland.)
If Earth had water, it must have had an atmosphere, and if it had an atmosphere, it must have had a climate. What was Earth's early atmosphere made of? Nitrogen (N2), certainly. Nitrogen makes up the bulk of today's atmosphere and likely has been around since the beginning. Water vapor (H2O), probably from volcanic emissions. Carbon dioxide (CO2), also emitted by volcanic eruptions, which were plentiful at that time. And methane (CH4), generated inside the Earth and possibly also by methane-producing microbes that thrived on and in the seafloor, as they do today.
Carbon dioxide, water vapor, and methane played an important role in Earth's subsequent development. Four billion years ago, the Sun was 30 percent dimmer, and therefore colder, than it is today. Under such conditions, Earth's water should have been frozen, yet clearly it wasn't. The water vapor, carbon dioxide, and methane acted as greenhouse gases, trapping heat and insulating the early Earth during a critical period in its development.
Of oxygen, meanwhile, the early atmosphere held barely a trace. What did exist likely formed when solar radiation split airborne molecules of water (H2O) into hydrogen (H2) and oxygen (O2). Hydrogen, a lightweight gas, would have risen above the atmosphere and slowly been lost to space. The heavier oxygen gas, left behind, would have quickly reacted with atmospheric gases such as methane or with minerals on Earth's surface and been drawn out of the atmosphere and back into the crust and mantle. Oxygen could only begin to accumulate in the atmosphere if it was being produced faster than it was being removed'—in other words, if something else was also producing it.
That something was life. Although the fossil evidence is sketchy, methane-producing microbes may have inhabited Earth as long ago as 3.8 billion years. By 2.7 billion years ago, a new kind of life had established itself: photosynthetic microbes called cyanobacteria, which were capable of using the Sun's energy to convert carbon dioxide and water into food with oxygen gas as a waste product. They lived in shallow seas, protected from full exposure to the Sun's harmful radiation. (To learn more about these organisms and the fossil evidence for them, watch the accompanying video "Early Fossil Life.")
