Of course, that’s not all that happens on our planet. All but a hundredth of a percent of this photosynthetic stuff is consumed by creatures like us (and many quite unlike us) in the exact reverse process. That is, we use up that same waste oxygen in order to burn that photosynthetic organic matter, whether by munching leaves, or flesh reconstituted from leaves, to steal that chemically captured sunlight for ourselves. We then release the original CO 2 and H 2 O back to the environment as exhaust.

This process of respiration, this metabolic burn, represents a perfect and complete reversal of photosynthesis. That is, after a tree spends a lifetime producing oxygen and tree stuff, that oxygen is used up in its own undoing—by decomposers such as fungi, or by bugs and animals that eat its leaves, and by bacteria that respire dead stuff in the earth. In the ocean, this unraveling is carried out by creatures that skim algal muck from the surface of the sea, and big things that eat one another, and bacteria that feed on the snowfall of tiny carcasses sinking through ocean depths, or larger ones resting on the seafloor. Organic carbon stuff does not like to sit around for long.

In the long run, and from the perspective of oxygen, it’s a wash. As much is consumed as is created—and not only by life. Free oxygen likes to react with almost everything on the planet, whether that’s rocks at Earth’s surface, or sulfur in volcanic gases, or iron in ocean crust. Left to its own devices, oxygen will disappear all by itself.

On their own, then, trees—and even entire forests and seas of plankton—are not enough to fill the atmosphere with a surplus of oxygen. If 99.99 percent of the vast reservoir of oxygen created by the living world is consumed by the living world, that gets you an atmosphere with 0.01 percent oxygen, not our modern 20.9 percent. Photosynthesis is a necessary but not sufficient condition for a world that is hospitable to white-hot oxygen-burning furnaces like us.

“The notion that we owe the breath we breathe to the rain forest, or the [phytoplankton] off the rain forests’ coasts, is just a little bit misinformed on the long timescale,” says Peters.

You don’t get to 20.9 percent, or an atmosphere that can host animal life, without geologic time, and without the fossil record. The tiny remainder of photosynthetic stuff that isn’t consumed and respired again by life—that 0.01 percent of plants and phytoplankton that manages to escape from this cycle of creation and destruction—is responsible for the existence of complex life on Earth. It’s the organic carbon that, once created, doesn’t get consumed again. Somehow this rounding error of plant stuff gets shuttled away after it dies, and is shielded from decomposition before it can be undone by the oxygen it produced in life. By not getting destroyed by oxygen, this conserved plant stuff gifts a tiny surplus of the unused gas to the atmosphere above. On the time scale of tens of millions of years, such meager gifts can accumulate—apparently to 20.9 percent.