Oxygen makes up 21 percent of the volume of Earth’s atmosphere and 30 percent of the mass of Earth as a whole. It turns iron to rust, causes a spark to burst into flame, and is the vital element in every breath we take. Oxygen is at once so omnipresent, essential, and utterly invisible, it is easy to forget how much it provides and hard to imagine an Earth without it.

And yet for nearly the first half of the planet’s 4.5-billion-year history, Earth had no free oxygen that is, no oxygen gas as part of its atmosphere. When free oxygen did begin to appear, sometime between 2.4 billion and 2.2 billion years ago, its effect on the planet was profound. Gradually released into the atmosphere by photosynthetic microbes, it formed two important gases new to fledgling Earth: breathable oxygen gas, or O2, and ozone, or O3. Together, the buildup of these gases enabled life to emerge onto land and evolve into the rich diversity of life-forms that inhabit Earth today."The appearance of oxygen in the atmosphere is extremely pivotal in terms of how the planet developed from there on in, particularly in relation to life," says Grant Young, a professor of geology at the University of Western Ontario and one of the many Earth scientists currently working to pin down when this critical transformation occurred. "Oxygen is one of the things that renders our planet unique in the Solar System, and possibly in the Universe."That free oxygen should exist at all in Earth’s atmosphere is something of a wonder. Oxygen is highly reactive: it tends to quickly bind with other common elements like hydrogen (H), carbon (C), and iron (Fe) to form molecules and compounds such as water (H2O), carbon dioxide (CO 2), and the oxygen-containing mineral goethite, a component of rust. Combustion, or the process of burning, is simply a chemical reaction that occurs when a fuel like wood or charcoal interacts with oxygen to produce water, carbon dioxide, and some other byproducts. Human respiration works fundamentally the same way: We take in oxygen through our lungs and give off carbon dioxide and water. In our case, fortunately, the "fire" that results during respiration is carefully mediated by our cells, which utilize the energy in a manner that sustains us.

As elements go, oxygen is a social climber, associating with itself to form O 2 (oxygen gas) or O3 (ozone) only until a more influential element comes along. Respiration is common among organisms in part because O 2 is so ready to react. (Human blood gains its effectiveness from oxygen’s strong affinity for iron: red blood cells carry hemoglobin, an iron-containing protein, which binds with oxygen and transports it through the bloodstream.) In fact, today’s oxygen-rich atmosphere is nowhere near as fixed and permanent as it appears. Respiration and other ongoing chemical reactions are continuously breaking down O2 and O3, scavenging oxygen from the atmosphere and squirreling it away in other compounds.Earth’s atmosphere would soon be stripped entirely of free oxygen were it not for a critical moderator: photosynthetic life. Microbes and plants pump out free oxygen in tremendous quantity, replenishing the atmosphere as quickly as it drains. Earth is unique among the known planets in no small part because it harbors the life essential to sustaining an oxygenated atmosphere.

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. 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."

Consider the forces that have shaped planet Earth over time.One tends to picture the grand geophysical events: earthquakes and volcanoes, erosion by wind and water, the drift of continental plates, the warming and cooling of the global climate. But there is another crucial force, microscopic in size yet global in its impact: the microbe.

Single-celled microscopic organisms—microbes—are the oldest and most abundant form of life on Earth. The term "microbes" spans a bewildering range of life-forms, from plants to animals to the ambiguously classified fungi. And microbes occupy an astonishing range of habitats, from the familiar (your shower curtain) to the most forbidding (inside volcanoes on the seafloor). In 1683, Antoni van Leeuwenhoek, the first scientist to view living bacteria through a microscope, exclaimed: "There are more animals living in the scum on the teeth in a man'­s mouth than there are men in a whole kingdom."

Since their first emergence on Earth perhaps more than 3.8 billion years ago, microbes have dramatically altered the chemistry of the atmosphere and, with it, the planet'­s surface. Among the countless kinds of microbes that have evolved, none have quite equaled the accomplishments of the first cyanobacteria: photosynthetic organisms that, like plants, drew on the Sun'­s energy to create oxygen, and in doing so helped create an oxygen-rich atmosphere. Earth today is habitable to multicellular creatures like us largely because cyanobacteria made it so. "One cannot separate the study of Earth'­s early atmosphere from the study of the evolution of life on this planet," says Jay Kaufman, a geoscientist at the University of Maryland. "They are intimately linked."Wherever they may live, microbes thrive on basic chemistry. Think of them as molecular scrap-metal workers: They take apart commonplace chemical compounds and reassemble the individual parts, or ions, into altogether different molecules. For example, phytoplankton, microscopic plants that flourish near the ocean surface, convert carbon dioxide (CO2) and water (H2O) into carbohydrates and oxygen (O2). Though small in size, these microbes are so abundant that they generate half the oxygen we breathe.What does a microbe earn for its labors? An infusion of energy, gained through the handling of miniscule, negatively charged particles called electrons. Every atom is surrounded by electrons, which help bind atoms together into molecules. As molecules are broken down and reformed, their electrons are exchanged and redistributed. A microbe, in the course of reshuffling ions and molecules, siphons off an electron or two for itself, to be used later in still other chemical reactions in its pursuit of food and energy. The molecules it generates, meanwhile, can become fodder for all sorts of other microbes. Leeuwenhoek was right: dental plaque is in fact an assembly line involving several species of bacteria, each playing a different role in the conversion of sugars and carbohydrates into cavity-causing acids. Your teeth are the platform for an entire atomic economy that runs on a currency of electrons.Through eons of evolution, microbes have adopted impressive strategies to exploit and metabolize the many kinds of molecules that exist on Earth. The bacterium Pyrococcus furiosus thrives in the hot water that boils from undersea volcanic vents. This heat-loving microbe doesn'­t breathe oxygen; in fact, oxygen is toxic to it. Instead it takes in sulfur and releases hydrogen sulfide, the same gas that makes rotten eggs stink. This hydrogen sulfide is part of a bizarre seafloor food chain that never sees sunlight and includes creatures like albino clams and tubeworms.

A scrap-metal business thrives, or doesn'­t, depending on the availability of certain prized metal parts. So too with microbes. Oxygen-breathing organisms—microbes as well as larger, multicellular creatures like us—abound today only because there'­s plenty of atmospheric oxygen to breathe. Before about 2.4 billion years ago, when there was no atmospheric oxygen, different organisms, all of them microbial, dominated Earth. The sulfur-breathing bacterium P. furiosus is a descendant of the oldest branch of life, the Archaea, which some scientists believe may date back to that early oxygen-less era. Today, many Archaean microbes are relegated to murky, oxygen-free corners of the planet, including seafloor volcanoes and the intestines of cows.Whatever a microbe produces—oxygen, methane, hydrogen sulfide—the task does require some effort, and the microbe must derive the initial energy to perform it from somewhere. Many scientists think that the earliest microbes derived their energy indirectly from Earth'­s internal heat, much as P. furiosus does today. Photosynthesis, the ability to convert the Sun'­s energy into microbial labor, developed somewhat later, perhaps as early as 3.5 billion years ago.Photosynthesis was a major evolutionary invention, as it freed organisms from the ocean depths and enabled them to thrive just below the sea surface. But the greatest innovation was yet to come. Photosynthetic microbes, though able to utilize solar energy, were still restricted to the shallows; ultraviolet radiation from the Sun was so strong that nothing could live exposed on land. Then, around 2.7 billion years ago, a class of organisms called cyanobacteria appeared. Unlike their other photosynthetic cousins, these photosynthetic microbes produced oxygen. (To learn about the fossil evidence for cyanobacteria, watch the accompanying video "Early Fossil Life.")The appearance of cyanobacteria signaled the beginning of a global transformation. Free oxygen began to accumulate in the atmosphere, forming two gases new to Earth. One was molecular oxygen (O2), well known to those of us who breathe it. The other was ozone (O3), a gas that forms high in Earth'­s atmosphere and, acting as a sort of global sunscreen, shields Earth'­s surface from the most harmful UV radiation. In the long run, the gradual rise of oxygen had two sensational effects: It permitted life to evolve on dry land, and it permitted the evolution of organisms that could thrive on oxygen. You can breathe easily, thanks to those early cyanobacteria."Imagine an early Earth that had no global sunscreen, no oxygen, hence no ozone," says Kaufman. "The production of oxygen through photosynthesis created that sunscreen. So biology actually made the surface environments habitable for future life by producing the oxygen we breathe today."The role of microbes didn'­t end 2.4 billion years ago. Microscopic organisms are equally prevalent today, although they tend not to draw the same media attention that flashier, multicellular creatures do. And they'­re still churning out free oxygen, replenishing atmospheric O2 as quickly as other animals and chemical processes use it up. Like an earthquake or volcano, the lowly microbe is a planetary force to be reckoned with and respected."The atmosphere we have today is strongly influenced by biological activity," says Kaufman'­s colleague James Farquhar, a geochemist at the University of Maryland. "It'­s influenced by the types of gases that bacteria and other organisms produce. Life is critical in determining atmospheric composition, just as atmospheric composition is critical in controlling the conditions that are required to allow life to exist as it does."

On a chilly October afternoon, Grant Young and Jay Kaufman stand along a busy roadside in northern Ontario, poring over their favorite Earth-history book. Young, a professor of geology at the University of Western Ontario, and Kaufman, a geoscientist from the University of Maryland, are among the leading scientists trying to attach firm dates to the rise of oxygen in Earth'­s early atmosphere — an event that, when it occurred more than 2 billion years ago, dramatically altered the planet'­s development.

The book they'­re reading is an ancient geological masterpiece: the Huronian Supergroup, a massive formation of rock laid down gradually between about 2.5 billion and 2.2 billion years ago, precisely the period when oxygen began to accumulate in the atmosphere. The Huronian Supergroup is 10 or 11 kilometers (six or seven miles) thick and extends well below ground. From atop a nearby outcrop, a viewer can survey the landscape for miles around. At the moment, however, Kaufman and Young are at road level, examining a segment of the outcrop that was exposed back when the highway was built. Individual layers of ancient sediment form horizontal stripes on the rock. From a few steps back, the rock wall looks like a cross-section of a giant, stone encyclopedia."When we look at a sequence of rocks, it'­s like the pages of a book," Young says. "The page at the bottom is the oldest and the page at the top is the youngest. We read history by starting at the bottom layer and working our way up. The Huronian Supergroup is particularly exciting and interesting because, by chance, these rocks were laid down at a period when the atmosphere underwent a transition from containing no free oxygen to containing at least some free oxygen."It may seem at first like an odd strategy: studying rocks in order to understand the atmosphere. It'­s one thing to examine fossils, the solid remains of ancient, solid creatures. But what can rocks reveal about something as formless as air, much less air that existed billions of years ago? How does one study the ancient atmosphere when no samples of it are left to collect?

Fortunately, the geological record contains a history of more than just rock. The atmosphere, then as now, constantly interacts with Earth'­s crust. As exposed rock weathers, its composition is altered by compounds in the air. This alteration is apparent even billions of years later and reveals important details about the atmosphere at the time. By studying a shoeprint in the mud, a police detective can determine not only the kind of shoe that made it, but also critical details about its wearer: size, weight, gender, even age, and whether or not he or she walked with a limp. The ancient atmosphere left an equally telling signature in the rock record. By flipping backward through pages of rock, a geologist can begin to form a picture of that atmosphere and how it changed through time."I'­ve often wished that I had a time machine to go back and collect a sample of ancient atmosphere or an ancient bit of seawater," says Kaufman. "But we can'­t. All we can do is collect rocks that were formed under those waters and under that atmosphere."Oxygen is a highly reactive element; it readily combines with other elements to form new compounds. As these compounds form, they become part of the geological record, leaving behind a trail of molecular "crumbs" that point to oxygen'­s whereabouts through history. One clue to the nature of the ancient atmosphere comes from rock formation known as "redbeds," the oldest of which date back about 2.2 billion years. Redbeds are sediments that were deposited on floodplains by water exposed to the atmosphere. They contain a mineral called hematite, a compound of iron and what must have been atmospheric oxygen. After 2.2 billion years ago, redbeds become increasingly common in the geological record.

"It'­s a very simple kind of test," says Young, who has studied redbeds extensively over the course of his career. "But it does give us at least a first-order idea as to whether there was free oxygen and whether there wasn'­t."In recent years Kaufman'­s colleague James Farquhar, a geochemist at the University of Maryland, devised an even more precise method of dating the rise of oxygen. He collected rocks from the Huronian Supergroup and other deposits around the world, ground them to powder in the laboratory, and studied them for traces, not of oxygen, but of an entirely different element: sulfur. Sulfur compounds are emitted in vast quantities by volcanoes, which were especially active during Earth'­s youth. Like other airborne compounds, they undergo reactions in the atmosphere and eventually end up deposited in the geological record.As it happens, there are four different kinds, or isotopes, of sulfur. By far the most common — about 95 percent of all atmospheric sulfur — is sulfur-32, or sulfur with an atomic weight of 32. The other isotopes are sulfur-34 (4.2 percent), sulfur-33 (0.75 percent), and sulfur-36 (0.02 percent). The relative proportion of these four isotopes has tended to remain steady over time. But Farquhar and his colleagues found that in rocks older than about 2.4 billion years, the proportion of sulfur-33 varied widely, whereas rocks younger than about 2.1 billion years showed no significant variation. What accounted for the variation, and for the change?The answer, Farquhar and Kaufman believe, was oxygen. Early in the planet'­s history, before enough free oxygen had accumulated to form a protective layer of ozone (O3), Earth'­s atmosphere was scorched by intense ultraviolet radiation from the Sun. The UV radiation may have reacted with the atmosphere to produce some compounds with a high sulfur-33 to sulfur-32 ratio and other compounds with a low sulfur-33 to sulfur-32 ratio. Later, with the rise of oxygen and the formation of an ozone layer which blocked incoming UV radiation, that photochemical reaction stopped, and the ratio of sulfur-33 to sulfur-32 ceased to vary. Amazingly, these signatures of sulfur isotopes are recorded in the rocks. In old rocks, before the buildup of atmospheric oxygen, the ratio of sulfur-33 to sulfur-32 in rocks is variable; in young rocks it is constant and in the same ratio as today.Farquhar'­s technique, though indirect, is remarkably exact: he has determined that free oxygen began to accumulate in the atmosphere about 2.45 billion years ago and was well established by 2.1 billion years ago. He also has been able, for the first time, to provide a rough measure of how much oxygen there was compared to today. "The sulfur research probably provides the strongest evidence for the buildup of oxygen in the atmosphere," Farquhar says. "The change from a large signature to a much smaller signature is a result of a large change in atmospheric oxygen content, from levels 100,000 times less than present to levels within about 100 times less than the present level.""The most exciting thing to me about this research is that it quantifies amounts of oxygen in the atmosphere," Kaufman adds. "Before, we just had this qualitative sense of, well, it was low here, it must have risen here. But the signatures that we'­re seeing allow us to actually get at numbers — to get at the timing of this rise, so it'­s not just a fairytale. We can actually write some sentences on the pages of the book of atmospheric oxygen."