The complicated history of how the Earth’s atmosphere became breathable

The Great Oxygenation Event, which occurred around 2.4 billion years ago, was one of the biggest transformations of our planet. Before it, there was practically no oxygen in the atmosphere; after, there was.

Conventionally, the rise of oxygen is seen as life triumphantly terraforming a passive planet. But we’re learning now that Earth was an active participant, and it took two more big lifts of oxygen over the succeeding 2 billion years before it reached breathable levels. So which was more responsible for oxygen’s rise on Earth: the evolution of life or the evolution of the planet? Nature or nurture? And does the same answer apply to all of the rises of oxygen in Earth’s past?

It’s a question beyond curiosity about our past, as it also affects how we might interpret signs of life on exoplanets.

Alien Earth

For almost half of our planet's existence—the entire time before the Great Oxygenation Event, or GOE—Earth was effectively an alien planet. Apart from the obvious (the air was unbreathable), the oceans also lacked oxygen and were full of dissolved iron, while land was lethally irradiated by ultraviolet light, as the atmosphere lacked an ozone layer. Even the color palette was alien: Land lacked the reddish hues of dirt and the greens of vegetation, while the sky was pinkish-orange due to high methane levels.

Life began in that alien environment, and at some point between 3.2 and 2.8 billion years ago, cyanobacteria began to use sunlight to split hydrogen from water, discarding oxygen as waste. That was a whopping 400 million to 800 million years before the GOE, roughly the same time that separates the present from the dawn of complex life.

“Life turned on this set of reactions that can produce oxygen, but what we know from the geological record is that didn't immediately result in huge amounts of oxygen in the atmosphere,” said Dr. Benjamin Mills of the University of Leeds.

Clearly, the invention of oxygen-producing photosynthesis wasn’t enough by itself to oxygenate the atmosphere.

Earth as oxygenator

Earth loses about 90 tons of gas—mainly hydrogen and helium—to space every day. That’s tiny compared to the mass of our atmosphere, so there’s no cause for alarm. But before the GOE, hydrogen loss to space was so massive that it left an imbalance between isotopes of hydrogen today because hydrogen escapes more easily than deuterium, its heavier isotope. That imbalance shows Earth lost the equivalent of a quarter of the water that had filled its oceans due to hydrogen loss.

“The mantle initially could have contained more water than it does now, and that water came out of the mantle initially in the form of hydrogen,” explained Professor Rajdeep Dasgupta of Rice University.

Losing hydrogen from H₂O but keeping the oxygen pushed Earth toward an oxidizing environment, the same way it was developed on Mars. Mars has just enough oxygen, which was left behind after the hydrogen from its water leaked into space, to rust its surface red,

“There is a net oxidation of the planet, including the atmosphere, including the crust and the mantle through time,” Dasgupta said about Earth.

Mellowing Earth

On Earth, with its more active geology, there were many additional things for the oxygen to react with. “Oxygen buildup in the atmosphere is not just how you create oxygen; it's also about how you might or might not destroy oxygen,” explained Dasgupta.

Earth’s early atmosphere was stuffed with oxygen-consuming (“reducing”) gases, like hydrogen, carbon monoxide, hydrogen sulfide, sulfur dioxide, and methane. These were continuously emitted by volcanoes, as well as microbes and seawater reacting with lava. Hydrogen from seawater-lava reactions may have consumed more than 70 million tons of oxygen every year. The oceans were also full of dissolved iron that would rust on contact with any dissolved oxygen, consuming it.

Collectively, these gases soaked up the oxygen as soon as it was made. “You don't just need to make enough to fill the atmosphere with oxygen; you need to make enough to fill it with oxygen thousands and thousands of times over to keep it there,” said Mills.

The cooling planet was crucial for making Earth friendlier to oxygen. Once Earth was cool enough, its crust began to move around the globe in rigid plates that collided, sending material plunging into the mantle and helping to cool the planet's interior further.  As a result, Earth transformed from a water world spotted with volcanic islands into a world with continents and mountains on thick continental crust.

Thickening the crust increased the depth where magma was stored before erupting, thereby increasing the pressure on it. That simple change altered the chemistry of molten rock and thus the chemistry of gases released by volcanoes. “In one case, you will get reduced gases when the crust is thin. In another case, you will get more oxidized gases when the crust is thick,” Dasgupta told Ars. So the production of oxygen-eating gases dropped as continents grew.

Death frees oxygen

Before continents, a lack of nutrients like phosphorous in ocean water may have limited the abundance of life to less than 7 percent of living mass today. This kept the population of cyanobacteria low, suppressing the production of oxygen. But as continents grew, erosion delivered more nutrients to the oceans, and as the chemistry of lavas changed in concert with growing continents, those nutrients came from increasingly phosphorous-rich rocks, boosting the amount of life the planet could support.

As the life in the oceans flourished, it boosted a process called "the carbon pump." Today, the entire plankton population in the surface layer of the world’s oceans is murdered by planktonic grazers and viruses every few days. While much of the carbon in that carnage is recycled into new life, some settles onto the seabed where it gets buried, a process known as the “biological carbon pump.” With the exception of grazers, which didn’t exist yet, something similar was going on in early Earth.

That organic carbon also reacts with oxygen, making CO₂. So for oxygen to build up in the atmosphere, organic carbon must be buried. To put it another way, carbon burial promotes the rise of oxygen.

As continents grew, so did the supply of iron washed into oceans, which bound to organic carbon, protecting it from being recycled by microbes until it was safely buried away, thus enhancing the carbon burial. Larger continents provided more space for sedimentary basins that also buried organic carbon, helping oxygen to rise.

Wobbly transition

With all these factors at work, it’s perhaps unsurprising that the GOE wasn’t a simple off-on switch. The rock record shows occasional “whiffs” of oxygen began hundreds of millions of years before the GOE, building to a climactic switch over to oxygen in the atmosphere, with oxygen levels continuing to wobble for another 200 million years afterward.

“If that flux [of oxygen-eating gases] is decreasing over time, you're approaching a switch point where eventually it flips over,” said Professor Ariel Anbar of Arizona State University. “As you approach that switch point, the system should become less and less stable. What was an overwhelming amount of reductant becomes a ‘just enough’ amount of reductant.”

Flipping the oxygen state of the planet plunged it into crisis. “You're keeping the Earth warm because of the good graces of methane greenhouse, and then along come whiffs of oxygen… and you start to erode that greenhouse,” said Anbar. “So you end up creating glacial episodes.”

Consequently, Earth plunged into a series of planet-wide “Snowball Earth” ice ages right after the GOE and continuing for some 220 million years.

Bored and unfulfilled

The GOE changed the composition of the Earth by creating some 3,000 oxidized minerals that didn’t exist before. Sunlight in the stratosphere converted some oxygen into ozone, forming a layer that shielded land from sterilizing ultraviolet radiation. Methane oxidized to CO₂, turning the sky blue; combined with the CO₂ emitted by volcanoes, the planet had enough greenhouse gas to keep it from freezing. The dissolved iron in the oceans mostly precipitated into iron ore that is mined today, and oxygen reacted with hydrogen to make water, slowing its escape to space and preserving Earth’s oceans. A new kind of cell evolved—eukaryotes, many of which have a metabolism that relies on oxygen—that eventually enabled complex life.

And yet the promise of breathable oxygen remained unfulfilled; it remained a mere 1 percent of present levels.

It wasn’t the supply of oxygen-eating volcanic gases that kept oxygen in check, as geochemical data show that those diminished steadily. If they were controlling oxygen levels, oxygen should have been rising.

“I think it's telling us something about what's really limiting the biosphere,” said Anbar, “and it's not the availability of oxygen, and it's not really the availability of energy. There's plenty of energy at the surface. Life figured out pretty early how to capture it up to limits, which are set by nutrient availability.”

This low-oxygen state lasted a billion and a half years, coinciding with a period of muted geological activity dubbed the “Boring Billion.” Although the causes and consequences of this underwhelming period are elusive, there seems to have been a long-lived supercontinent with limited mountain-building activity. Whatever mountains that once existed were worn to hills, their nutrients weathered out or stranded in a stable landscape, unable to feed sea life. And although the ocean's surface stayed oxygenated, its depths remained anoxic and dissolved iron began to build up again.

“You seem to have sort of a return to more reducing conditions in the oceans than before,” said Anbar. “It's hard to have a lot of iron dissolve in seawater if you have any oxygen around,” he said.

Second verse, kinda the same as the first

Then, a billion and a half years after the GOE, Earth had its second big increase in oxygen levels, called the “Neoproterozoic Oxygenation Event,” or “NOE,” which occurred between about 800 million and 500 million years ago and took oxygen to about half of modern levels.

Although the details are debated, the NOE was spookily similar to the GOE, with big fluctuations in oxygen for some 300 million years. Like the GOE, it coincided with major evolutionary advances in life, as well as a change in the style of plate tectonics; it, too, was followed by Snowball Earth glaciations.

Again, fluctuations in oxygen reflect Earth at another tipping point. “You enter this period of instability, which seems to be a natural thing that happens when the ocean is about to become well-oxygenated,” said Mills. “If you have an ocean that's oxygenated, you suddenly change a whole bunch of stuff. You change what minerals are going to form [and] you pull phosphorus out of the ocean. So you can have quite a dynamic shift when you oxygenate or deoxygenate the oceans.”

Could new lifeforms have been the driver of the NOE?

There’s a remarkable diversification of life then, and “sterane,” a biomarker for eukaryotic cells, shows they became more abundant at the time. The earliest animals evolved around then as well. Professor Tim Lenton of the University of Exeter has suggested that evolution led to more efficient carbon burial as the new lifeforms were larger, so they sank to the seabed faster, preserving more carbon from being recycled. Although that idea is controversial, there’s also evidence that algae may have begun colonizing land at that time. If so, the organic acids the algae must have used to extract nutrients from rock would have enhanced the supply of nutrients to the sea.

Carbon isotopes show a big increase in organic carbon burial at this time, which would have encouraged oxygen to rise. The algal rock weathering and extra carbon burial would also have cooled the climate, possibly triggering the snowball glaciations.

Cold slabs

But life may not be the entire story, as Earth was transforming itself at that time of the NOE.

Plate tectonics ended the “Boring Billion” by rifting the “Rodinia” supercontinent into several smaller continents scattered across the tropics. Mountain building was back in fashion, and volcanoes erupted with renewed vigor as continents bulldozed over oceanic plates.

Earth was beginning a new style of plate tectonics, with colder plates producing a new suite of high-pressure rocks called “blueschists” as they plunged, or “subducted,” into the mantle. Earlier in Earth’s history, when the planet was hotter, the majority of plates melted once they entered the mantle, but by the NOE, the planet had cooled enough for most downward-moving plates—dubbed “slabs”—not to melt.

“If you go to cold subduction zones, slab melting is precluded. You are only looking at the dehydration of the slab,” said Dasgupta. “In one case, it's hot fluid, and in another case, it's hydrous magma.”

This new style of plate tectonics resulted in steadier, more sustained plunging of plates into the mantle, which increased the amount of continental crust and carbon sent deep into Earth's interior. Crucially, this thickened mountain belts. The resulting erosion supplied more nutrients and iron to the oceans, which boosted biological activity, carbon burial, and oxygen rise.

Once again, Earth careened on a roller coaster of climate and nutrient extremes. Two “Snowball Earth” glaciations ensued, with ice covering most of the planet for tens of millions of years, each followed by hot “super-greenhouse” conditions that flushed torrents of ice-pulverized rock nutrients into the oceans.

Oxygen continued to fluctuate long afterward, with one low-oxygen episode triggering the oldest documented mass extinction of early animals about 550 million years ago. Despite that, life continued to evolve more energy-demanding lifestyles favored by higher oxygen levels, with organisms building larger bodies, burrowing into the seabed, and moving around under their own power.

“The production of oxygen allows the type of life to change radically,” said Anbar. “Because suddenly… you have a lot more energy available, you can evolve aerobic metabolism, which is a much more energy-rich metabolism. You can then start doing more complicated things… evolution can figure out more complicated tricks.”

Third time’s a charm

Earth’s final oxygen rise, the “Paleozoic Oxygenation Event” or “POE,” began about 470 million years ago. It has a much clearer cause: the evolution of land plants. “Land plants certainly increased the rate of oxygen production, and we're pretty convinced now that it was their evolution that bumps up oxygen levels to a level that we could breathe,” said Mills.

The reason, again, comes down to the burial of organic carbon.

“Plants have to build their bodies differently to marine algae and bacteria because they’ve got to stay upright,” explained Mills. That requires more carbon-rich bodies. “They can just start to bury more carbon. You've got a tenfold increase in the carbon-richness of the material that you're burying,” said Mills.

In a faint echo of earlier oxygenation events, the POE brought another severe glaciation. Although it was far shorter and milder than a “Snowball” glaciation, sea levels dropped drastically, and large parts of the oceans lost their oxygen again, causing a major mass extinction. But the glaciation was comparatively brief, and soon, modernish levels of oxygen supported energetic life like fish and land animals.

But higher oxygen levels also brought fire, and fire limits oxygen.

The oldest fragments of charcoal have been found in rocks that formed about 430 million years ago. Since fire can’t happen if oxygen is below 16 percent of the atmosphere, oxygen must have been higher by then. Conversely, the absence of charcoal, or “charcoal gaps,” imply that oxygen crashed below that level a few times since. That happened around 390 million years ago and again right after land plants were devastated by the end-Permian mass extinction 252 million years ago.

“Oxygen levels have most likely been falling since the Cretaceous, and part of that is due to the change in makeup of the terrestrial biosphere, which just means it's much more prone to fire than it used to be,” said Mills.