Science & Technology

Earth's Biggest Die-Off Happened Long Before the Dinosaurs

When you think of ancient mass extinctions, you probably think of the asteroid that killed the dinosaurs. That's fair: The KT extinction event, as it's called, decimated around 80 percent of all species on Earth about 66 million years ago. You might even think about the extinction event known as the Permian extinction, or "the Great Dying," which took place around 250 million years ago — before dinosaurs existed — and is recognized as the largest mass extinction event in our planet's history. If you measure by the number of species affected, that's true. But if you measure by the sheer quantity of life lost? Well, you'd have to go back a couple billion years. Nothing compares to the Great Oxygenation Event.

The Great Big, Itty-Bitty Apocalypse

To be fair, the Great Oxygenation Event didn't kill off any cool-looking dinosaurs or even any bizarre sea creatures. The lives lost in this extinction were microscopic. You see, for the first half of Earth's history (a little over 2 billion years ago and earlier), the atmosphere was completely devoid of oxygen. Geologists know this by studying the oldest rocks on Earth, which contain certain chemical fingerprints that tell them the atmosphere was made up of mostly carbon dioxide, methane, and nitrogen.

The tiny bit of oxygen that did exist formed when the sun's energy split it off of carbon dioxide molecules — but it would quickly bond with iron in rocks or hydrogen in volcanic eruptions, making it disappear from the atmosphere as quickly as it came.

As you might expect, the life that existed on Earth at that time got along just fine without oxygen. Those single-celled bacteria were anaerobic, which means they metabolized their food without oxygen — just like bacteria that live on the ocean floor today.

But one group called cyanobacteria got tricky: They invented a way to take energy from sunlight and use it to create sugar from water and carbon dioxide. That's right: They invented photosynthesis, the same method all green plants use to make their own food today. And if there's one thing plants are good at, it's making oxygen.

But in fact, plants don't make oxygen out of the goodness of their vacuoles. Oxygen is just a waste product of photosynthesis. It's also really bad for anaerobic bacteria. So while for the first few million years, the new oxygen these cyanobacteria produced kept on disappearing by bonding to other particles in the atmosphere, eventually it became too much. Oxygen began building up in the air and the water around 2.3 billion years ago. This was the start of the Great Oxygenation Event — and the beginning of the end for a huge portion of life on Earth.

Drowning in Oxygen

Things only got worse from there. Volcanoes stopped pumping out as much hydrogen, giving extra oxygen that much less to bond to. The oxygen also forced phosphorus and iron out of exposed rocks and into the sea, where it acted like fertilizer to help the cyanobacteria thrive and create even more oxygen. At that point, scientists estimate that the oxygen levels in the atmosphere were around the same as they are today.

This poison gas killed off heaps of anaerobic bacteria. It also wasn't good for the cyanobacteria: They eventually exhausted their supply of nutrients in the ocean and began to die as well.

The exact scale of this die-off isn't totally clear, but last month, an international team of researchers used rock samples from a site in Northern Canada to give the best estimate yet. By analyzing crystals of barite, a mineral that keeps a record of oxygen levels in the atmosphere, the team determined how much oxygen levels dropped as a result of the die-off. That, in turn, helped them figure out how much life was lost in the process.

The answer? A drop in the amount of carbon production — a proxy for life — by more than five times. By the end of the Great Oxygenation Event, the authors estimate that the drop could have been by as much as 100 times. Although this event isn't counted among the planet's great die-offs, they write, this bacterial apocalypse "eclipses even the largest extinction events in all of Earth's history."

That wasn't the end of the story, of course. A few anaerobic bacteria survived and even thrived on the ocean floor, and cyanobacteria eventually evolved into photosynthetic plants. The level of oxygen in our atmosphere dipped and spiked over the next few billion years, eventually settling around 21 percent today. No one knows how long that will last, but one thing's for sure: Billions of bacteria gave their lives for every breath you take.

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If you think this story is gripping, then you should read Earth's whole biography: "The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet" by Robert M. Hazen. The audiobook is free with a trial of Audible (and won't kill any photosynthetic life in the process). We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer September 25, 2019

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