Not quite fourteen billion years after the Big Bang, a mass of elements congealed into the protoplanet called Earth.

Earth spun inside a wraith of gasses. Meanwhile, the sun spewed energetic waves and subatomic matter—cosmic radiation and cosmic wind—from a dangerous proximity. As Earth condensed into a violent pile of magma, these forces stripped away the first atmosphere as if sandblasting paint off a house.

Volcanoes disgorged the second atmosphere. This span of time is called the Hadean Eon, because Earth was like Hades—hellish—and the air was a toxic stew of nitrogen, argon, water vapor, and carbon dioxide. Oxygen was rare. For reasons unknown, a magnetic field large enough to engulf the planet formed. It protected the second atmosphere from annihilation, though its methods remain mysterious.

The properties and personalities of molecules in the second atmosphere are the subtext to life on Earth. For example, nitrogen is a teenager. It eschews the company of all other molecules except exactly one other nitrogen. Also, each nitrogen has three hands. When nitrogen uses its three hands to grasp another nitrogen’s three hands, the resulting bond is unassailable, and the resulting molecule is named dinitrogen. The only natural processes with the power to break the bonds of dinitrogen are meteorites, lightning, unusually strong cosmic radiation, and, after life evolved, symbiotes living in the roots of legumes.

Carbon is a mother, and it often holds hands with little oxygens. Carbon has four hands. Oxygen—a normal, childlike atom—has only two. A carbon can bond with two oxygens to form carbon dioxide because it has twice as many hands with which to grasp them.

During the formation of the second atmosphere, chlorine is a rare atom cycling through different kinds of bonds. It will become a part of this story later. Chlorine is a wolf.

The Earth cooled and water vapor pooled into oceans.

Some oceanic carbons fashioned their bonds into long chains and rings adorned with smaller axillary molecules. This is called the prebiotic soup. Eventually, some of these carbons formed a coordinated strategy and built a membrane around themselves, becoming the first cell. More cells followed.

The cells ate the rest of the carbon chains in the soup. They began to starve. A new type of cell arose, one that could manufacture personal food molecules from light, water, and carbon dioxide—the mighty photosynthesizer. The original cells and the first photosynthesizers were anaerobic: they did not use oxygen in their chemical transformation of food to energy because there was no free oxygen. All the oxygen atoms were bonded with carbon mothers. But the anaerobic photosynthesizer manufactured paired oxygen, called dioxygen, and excreted it as waste.

The photosynthesizers were still living alongside their progenitors, the original cells, who were adapted to a world of water, dinitrogen, and carbon dioxide. The photosynthesizers excreted so much dioxygen it first poisoned, then nearly exterminated, the original cells.

The poisoning is called The Great Oxygenation Event, in which millennia of quiet photosynthesizing by oceanic bacteria nearly obliterated all other life on Earth. They excreted so much dioxygen that it became the second most common molecule of the atmosphere. Millions of years later, aerobic bacteria appeared. They used dioxygen in the chemical transformation of food to energy. Their descendants would breathe oxygen with lungs.

Dioxygen mixed with the rest of the atmosphere in global air currents, eventually reaching higher than the highest cloud.

There, it was more susceptible to the cosmic fusillade. When cosmic radiation hits a dioxygen, the resulting impact breaks its bond. The two oxygens do not bond together again. Instead, each scrambles to the nearest dioxygen and shoves in—forming a triad. The ring of three oxygens, like three children playing Ring around the Rosie, is called ozone.

Much of the highest layer of oxygen was transformed to ozone. Cosmic radiation does not have the power to blast apart all three children in a ring of ozone. But it often blasts one awry, overpowering the strength of its little hands to grasp its two brothers so it is ripped away, an orphan. The two remaining oxygens keep a pair bond by holding both their hands.

When this happens, the cosmic ray is shattered, its energy released. Its life has ended in the atmosphere; it will not travel to the Earth and blast apart the elemental bonds in the bodies of living creatures. The orphan oxygen is not shattered, but temporarily shipwrecked. It will bond with the first dioxygen it meets, reforging a molecule of ozone. In this cycle, cosmic radiation is transformed to harmless heat, but ozone is never depleted.

Before the reign of photosynthesizers, there was no ozone in the atmosphere. As the colonies of photosynthetic bacteria grew from mere pips to expansive swarms, more oxygen floated high above the earth; as more oxygen was transformed to ozone, a layer of ozone accumulated to shield the earth from cosmic radiation. 

Before the formation of the ozone layer, cosmic radiation obliterated any cell venturing on land. By this time, the oceans supported such advanced life forms as mollusks and arthropods, but the land was sterile. As the ozone layer grew in might, oceanic cells evolved into ooze, and the ooze oozed onto land.

The ooze proliferated and diversified until the surface of Earth was woolly with life. Humans blinked into existence. First, they made novel tools, then they made novel molecules. One such molecule is called chlorofluorocarbon. In this molecule, carbon, with her four hands, grasps a fluorine and three chlorines. Each chlorine is a wolf.

Humans added chlorofluorocarbons to refrigerators, air conditioners, industrial solvents, foam insulation, and spray cans, where they are harmless. Within a few decades, air currents had transported trillions of chlorofluorocarbons to the ozone layer, where they are not harmless.

When cosmic radiation pummels a chlorofluorocarbon, it relinquishes a chlorine. The chlorine attacks ozone like a wolf attacks unsuspecting children. The wolf destroys an ozone by ripping one little oxygen from its brothers and holding it in its jaws like a prize.

The remaining oxygen pair will not ungrasp hands to rescue their brother from the maw of the wolf. But orphan oxygens are wandering in the carnage. An orphan oxygen will unmew the kidnapped oxygen from the chlorine’s jaws and form dioxygen. Where once there was an ozone, an orphan oxygen, and a hungry chlorine, there now are two dioxygens and a hungry chlorine.

The chlorine will attack a nearby ozone, and the cycle will continue.

In this way, the chlorine will never be satiated. It will be a wolf in a field of children, and it will tear apart billions of their alliances before being absorbed into a more powerful molecule. When cosmic radiation attacks ozone, the ozone regenerates. When chlorine attacks ozone, the ozone is destroyed.

A small number of chlorofluorocarbons can tear a hole in the ozone layer. Working in tandem, the wolves and radiation reduce rings of ozone to pairs of oxygen and pairs of oxygen to wandering orphans. Only ozone can shield life on Earth against cosmic radiation.

Dioxygen is the second most common molecule of the atmosphere, but dinitrogen is the first. It became so by way of its unbreakable bond; by being unbreakable, it resisted attrition into cells and geological cycles. Nitrogen is a coveted element. Living bodies need it to build protein and DNA, but most of Earth’s nitrogen wafts in the atmosphere, triple bonded so tightly to another nitrogen that it remains unavailable to the creatures of the biosphere. Europeans encountered a nitrogen shortage shortly before the First World War. A paucity of nitrogen in the soil reduced crop production as infant mortality plummeted.

Two Germans named Fritz Haber and Carl Bosch devised a way to avert mass starvation. The process they invented siphons dinitrogen from the local air into containers where its triple bond is blasted apart. Each divorced nitrogen is attached to three hydrogens, forming two molecules of ammonia. The ammonia is mixed with water and applied to soil as fertilizer. The Haber-Bosch process, as it is called, feeds half the world’s current population, is powered by fossil fuels, and will cease to exist in their absence. The two men won the Nobel Prize.

A few years later, when Haber invented a process that married two chlorine atoms, he became the father of chemical warfare. The paired chlorines created a heavy, horrific, yellow gas, which, once pumped into barrels and released from the upwind side of a sunny battlefield, could cross it at a strolling pace and sink into the trenches and eye sockets of the enemy. Chlorine is a wolf.

Once inside a body, paired chlorines release each other to attack water. Water is an oxygen that holds two hydrogens—one in each hand—as if each were a treasured toy. The first chlorine steals a hydrogen, and the second chlorine traps the oxygen along with its remaining hydrogen. Both chlorine compounds are now acids, and acids destroy flesh. The sunny battlefield then becomes a quilt of bodies.

The many hearts of the many bodies stop beating, and so the blood of the bodies stops coursing. But the cells of the bodies continue to metabolize. No messenger arrives to tell them they are dead. Metabolism creates carbon dioxide as a byproduct, but the blood is not coursing because the hearts are not pumping, and so the carbon dioxide is not exhaled.

When carbon dioxide reacts with the water of the cell, it creates an acid, and the acid destroys the membranes of all the organelles within the cell. The organelles spill their contents like crushed fruit. One type of organelle, the lysosome, contains toxic molecules meant to digest and recycle an assortment of materials. After the destruction of their membranes, the lysosomes surrender their hold on the toxic molecules, and so the first digesters of the bodies are the bodies themselves.

Legions of bacteria are the subsequent digesters. First among them are the bacteria of the guts, which are anaerobic—exactly like the first, quiet cells of the ocean so many millions of years ago. The legions come in waves, both anaerobic and aerobic. The waves erode the quilt of bodies back into their constituent molecules, all of which, at least for a time, eventually return to the atmosphere.

List of Sources

1. Aguado, Edward, and James E. Burt. Understanding Weather and Climate. New Jersey: Pearson Education Inc., 2004.
2. Braakman, Rogier. “Evolution of Cellular Metabolism and the Rise of a Globally Productive Biosphere.” Free Radical Biology and Medicine 140 (August 2019): 172–87. https://doi.org/10.1016/j.freeradbiomed.2019.05.004.
3. Brightling, John. “Ammonia and the Fertiliser Industry: The Development of Ammonia at Billingham.” Johnson Matthey Technology Review 62, no. 1 (January 1, 2018): 32–47. https://doi.org/10.1595/205651318X696341.
4. Brooker, Robert J., Eric P. Widmaier, Linda E. Graham, and Peter D. Stiling. Biology. New York: McGraw-Hill Higher Education, 2008.
5. Canfield, Donald E., Alexander N. Glazer, and Paul G. Falkowski. “The Evolution and Future of Earth’s Nitrogen Cycle.” Science 330, no. 6001 (October 8, 2010): 192–96. https://doi.org/10.1126/science.1186120.
6. Fowler, David, Mhairi Coyle, Ute Skiba, Mark A. Sutton, J. Neil Cape, Stefan Reis, Lucy J. Sheppard, et al. “The Global Nitrogen Cycle in the Twenty-First Century.” Philosophical Transactions of the Royal Society B: Biological Sciences 368, no. 1621 (July 5, 2013): 20130164. https://doi.org/10.1098/rstb.2013.0164.
7. Gebauer, Stefanie, John Lee Grenfell, Helmut Lammer, Jean-Pierre Paul de Vera, Laurenz Sproß, Vladimir S. Airapetian, Miriam Sinnhuber, and Heike Rauer. “Atmospheric Nitrogen When Life Evolved on Earth.” Astrobiology 20, no. 12 (December 1, 2020): 1413–26. https://doi.org/10.1089/ast.2019.2212.
8. Kasting, James F., and M. Tazewell Howard. “Atmospheric Composition and Climate on the Early Earth.” Philosophical Transactions of the Royal Society B: Biological Sciences 361, no. 1474 (October 29, 2006): 1733–42. https://doi.org/10.1098/rstb.2006.1902.
9. Lickley, Megan, Susan Solomon, Sarah Fletcher, Guus J. M. Velders, John Daniel, Matthew Rigby, Stephen A. Montzka, Lambert J. M. Kuijpers, and Kane Stone. “Quantifying Contributions of Chlorofluorocarbon Banks to Emissions and Impacts on the Ozone Layer and Climate.” Nature Communications 11, no. 1 (December 2020): 1380. https://doi.org/10.1038/s41467-020-15162-7.
10. Ritchie, Grant. Atmospheric Chemistry: From the Surface to the Stratosphere. London: World Scientific Publishing, 2017.
11. Sleep, Norman H., and Dennis K Bird. “Evolutionary Ecology during the Rise of Dioxygen in the Earth’s Atmosphere.” Philosophical Transactions of the Royal Society B: Biological Sciences 363, no. 1504 (August 27, 2008): 2651–64. https://doi.org/10.1098/rstb.2008.0018.
12. Smil, Vaclav. “Detonator of the Population Explosion.” Nature 400, no. 6743 (July 1999): 415. https://doi.org/10.1038/22672.