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.")

These organisms became so abundant that by 2.4 billion years ago the free oxygen they produced began to accumulate in the atmosphere. The effect was profound. High in the atmosphere, the oxygen formed a shielding layer of ozone (O3), which screened out damaging ultraviolet radiation from the Sun and made Earth'­s surface habitable. Nearer the ground, the presence of breathable oxygen (O2) opened a door to the evolution of whole new forms of life. One of the enduring marvels of life on Earth is that, by producing oxygen, the earliest organisms created conditions that enabled subsequent, more complex forms of life to thrive. (To learn more about this subject, read the accompanying essay "Life Makes a Mark.")

The rise of oxygen occurred slowly, over hundreds of millions of years, and not without hiccups. Jay Kaufman, a geoscientist at the University of Maryland, points to a series of ice ages'—at least three of them'—that occurred between 2.4 billion and 2.2 billion years ago, when the era of oxygen began. Life, Kaufman and others suspect, may have been partly responsible for these periods of cooling. Even as microbes were busy generating oxygen, they drew carbon dioxide from the atmosphere, perhaps thinning Earth'­s blanket of warmth; the oxygen they produced reacted with methane, reducing another greenhouse gas. The resulting ice age may in turn have reduced microbial activity, allowing carbon dioxide emitted by volcanoes to again build up and the planet to again warm. This cycle may have occurred at least three times, each time resulting in a slightly higher level of atmospheric oxygen. But, as Kaufman emphasizes, much remains unknown about these periods of glaciation, and the work of many researchers will be required to shed further light on this era.

"I would hypothesize that the relationship of the ice ages with atmospheric chemistry is biological," Kaufman says. "We do know that biology can affect the atmosphere. And if biology drew greenhouse gases out of the atmosphere, it could result in those ice ages and the rise of oxygen gas."

Why the rise of oxygen occurred precisely when it did is difficult to say. Instead, scientists have worked to narrow down the exact timing of the transformation. "In today'­s atmosphere, we have a lot of oxygen," says Kaufman. "At some point back in Earth history, the amount of oxygen was much less. At what point was that? How much less was it than it is today?" Answering those questions is one of the many challenges facing Earth scientists. How does a researcher go about studying an atmosphere that no longer exists?

"It'­s a very time-consuming process," says Kaufman. "So we take baby steps, as I think all scientists should, and build up a story. Whether we ever come to a conclusion'—whether our hypotheses ever become theories'—we don'­t know. We just want to slowly build the story based on good empirical evidence."

To learn more about how Earth scientists study the ancient atmosphere, read the accompanying essay "Footprints of the Air."