An Ode to O

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.

To learn more about the role of life in the creation of Earth’s atmosphere, read the accompanying essay "Life Makes a Mark."