ONE CLEAR LESSON of the sky’s long history: little remains itself for long. Carbon dioxide will bounce against rock and react with water and, in so doing, bind itself to the surface of Earth. Water vapor will turn to rain, which runs into the seas, carrying carbon-rich dirt. This carbon will settle on the ocean floor, and as tectonic plates shift and jostle, some of it will be tucked into the mantle once more. This is how slowly, slowly, the overabundance of greenhouse gases dwindled into a more manageable sort of air.
Life helped too. Carbon’s atoms are easily chained together, and these chains can be expanded or truncated, and ornamented with other elements, yielding an infinite set of designs. Carbon, in other words, is a perfect building block, a fundamental component of all forms of life that we know. And the carbon in our bodies—in all bodies—is sourced from the sky: photosynthesis creates sugars from free-floating carbon dioxide, which become the source of carbon for fats and amino acids, for all organic compounds. This process helped scrub carbon from the atmosphere. Once carbon entered the world’s first bodies, just single cells, some of these, too, were carried by the flowing water and deposited on ocean floors, dragging even more carbon into the bowels of Earth.
So life subtracted carbon from the atmosphere, but it also added something new: oxygen, spit out as waste in the process of photosynthesis. Oxygen changed everything. Life before oxygen was hard and slow, as the world’s chemical fuels were not very powerful. Oxygen, however, is incredibly willing to give up its electrons, which makes it a kind of nuclear cellular fuel, capable of powering great evolutionary leaps. Cells began to coordinate, eventually merging into single creatures. Species developed into complementary pairs, allowing for sexual reproduction.
The key innovation was respiration, which uses oxygen’s energy to break apart nutrients, such as sugar. You can’t have an ecosystem—a food chain—without respiration. It is what allows some species to be consumers, living off the nutrients made by photosynthesizers or, eventually, off other, smaller consumers. The sky-sourced carbon is passed from body to body to body.
There is one problem with oxygen, though: its great potential makes it hard to keep around. As soon as oxygen appears, some life-form wants its energy. Photosynthesis adds oxygen to the atmosphere. Respiration takes it away. Photosynthesis swallows up one bit of carbon dioxide and turns it into the carbon in our bodies. Respiration releases one bit of that carbon back out into the air. The two processes compose an endless cycle, almost perfectly balanced, a balance only tipped by death—or, really, by burial. Cells that are carried to the ocean will eventually be covered in mud, if they aren’t eaten first. The carbon in those cells, which would have been released into the air through respiration—balancing the oxygen released through photosynthesis—is instead stored in the earth. Dust to dust, we sometimes say at burials; but in the case of carbon, it is air to dust.
Twice in Earth’s history, oxygen levels have spiked upward: First, around 2.5 billion years ago, there was a quick leap that did not last. Then, after another 2 billion years, came another flare of oxygen, which stuck around. The cause of these spikes is mysterious. Perhaps new mountains, thrust upward by tectonic collisions, sent phosphorous tumbling downriver into oceans, sparking a great burst of oxygen-expelling algae. Perhaps plants climbed out of the ocean onto land, and as roots split apart the soil, their dead cells, filled with carbon, went tumbling through rivers into oceans and were buried in the sea, never releasing carbon dioxide, allowing more and more wisps of oxygen to remain in the sky.
There’s no way to know, really; all we know is that the process of life and then death and then life and then death tipped a bit, past some turning point that yielded a whole new kind of air—the “third atmosphere,” as scientists call it. It is what sets this world apart: ours is the only sky we know that holds any oxygen at all.