In today’s state of overwhelm, it’s easy to spend more time consumed with the present and the future than contemplating the events of the past. This constant forward motion can, at times, become exhausting and disorienting. We lose our grounding. We miss out on the context and insights that history can provide, and the lessons that may guide us through the tumult of the present. This holds true not just for national history and global history, but also geological history. That is, even events that unfolded millions or billions of years ago can offer insights that remain relevant to our lives and national policies today.
I wrote Strata: Stories from Deep Time to share this geologic lens with readers, and to spotlight the researchers working to untangle some of our planet’s oldest stories. This excerpt explores how scientists first began recognizing that oxygen didn’t billow up into the atmosphere until roughly halfway through Earth‘s existence — and how the arrival of this highly reactive gas fundamentally changed the planet from the seafloor to the stratosphere. By learning how and why oxygen showed up when it did, and how the planet responded to this period of intense global environmental change, we can gain context for the environmental crises unfolding across our planet today — and become better equipped to set ourselves on a more stable path forward.
***
As you read this line, the oxygen you are pulling inside your body makes your body possible. It is allowing you to digest your most recent meal, move your eyes across these words, and think your thoughts. It is the single most important gas to your survival. You share this in common with every other animal on Earth, save for one lone parasite of Chinook salmon that somehow doesn’t need it. Well done, Henneguya salminicola.
Throughout a given day, you fill your lungs with oxygen some 20,000 times. Most of us probably don’t give it much thought. Maybe we assume that this gas has always been here, a given on this highly habitable pale blue dot.
But it turns out that this dot has not always been highly habitable, nor, for that matter, has it always been blue. The early Earth’s young magma surface sat gooey and cloaked in steam, too hot to hold liquid seas. It took a long time for continents to rise up and for ocean basins to fill in, and far longer still for oxygen to pool up in the atmosphere.
“And so,” writes Rachel Carson in The Sea Around Us, “the rough outlines of the continents and the empty ocean basins were sculptured out of the surface of the earth in darkness, in a Stygian world of heated rock and swirling clouds and gloom.”
Even in those earliest of gloomy days, oxygen — the element O — was all over the place, bound up in molecules like water vapor and quartz and carbon dioxide. It’s the third most abundant element in the universe, and it has been present on Earth since the beginning. But free oxygen — two atoms of O bound together by a pair of shared electrons, liberated from any other material but itself — didn’t emerge as a gas until more than halfway through Earth’s existence.
If you reach out your arms and imagine Earth’s 4.54-billion-year history as a timeline that extends from the tip of your right hand to the tip of your left, the arrival of oxygen gas falls around your heart, at about 2.4 billion years ago, give or take a couple hundred million years.
The fashionably late arrival of oxygen may sound like a planetary sigh of relief. Finally, the possibility for life larger than one cell, with lungs and lips and all the rest of it. But scientists familiar with oxygen’s highly reactive habits suggest its arrival was more like a nightmare.
As is true of all elements, an atom of oxygen contains a cloud of negatively charged electrons that spin in an arrangement of “shells” around a positively charged nucleus. The outermost electron shell constantly seeks stability by filling to its capacity. In oxygen’s case, its outermost shell is two electrons short — comparatively fewer than other elements — and the configuration of those electrons contribute to oxygen’s high reactivity. Oxygen’s electron cloud is also relatively thin compared to other elements. Without much of a barrier between it and the outside world, the positive pull of the nucleus easily seeps out and lures in the negative charges of the two electrons it needs to stabilize. Two atoms of oxygen bound together as oxygen gas have a pull similarly as strong as a single atom on its own.
When oxygen first appeared on Earth, it desperately rooted out and bonded with anything willing to share its electrons, fundamentally changing the materials it bonded with. It weaseled into microbial cells and mutilated their machinery. It sulked into currents and eddies and made arsenic more soluble, it spread hydrogen peroxide poisons into DNA. With all the havoc it wreaked, this gas might have initiated one of the worst mass extinctions in all of Earth history — though it’s hard to know this for sure, since the single-celled beings that would have gone extinct were too squishy to leave behind reliable fossils. Even so, some call this geologic moment the Oxygen Catastrophe.
Over time, molecules from the bottom of the ocean to the top of the atmosphere grew to accept oxygen’s reactivity, and living things evolved ways to cope with this new gas. Their cells grew to tolerate it, and then to depend on it. They used it to break down food and generate energy that allowed them to grow larger and more complex, with multiple cells that communicated across newly sophisticated membranes. These oxygen-fueled innovations expanded and cascaded and eventually led to the evolution of eyeballs and brains and lungs and lips and, over billions of years, the possibility of us.
So what, exactly, happened around 2.4 billion years ago? Why did oxygen arrive when it did? And how can we read this in the rock record?
Courtesty of W. W. Norton & Company
THE SEARCH FOR OXYGEN’S origin began with a problem. When Charles Darwin published On the Origin of Species in 1859, he agonized over the seeming absence of fossils in the planet’s oldest rocks. The ages of rocks at this time were known only in a relative sense — as in, what formed first and what followed. The scientific law of superposition, proposed by Danish geologist Nicolas Steno in the seventeenth century, helped clarify that younger strata always sit atop older strata, since that’s how sediments accumulate in lake beds and seafloors and so on.
As hard as paleontologists of that time looked, they couldn’t find any remnants of ancient life in the oldest, bottom-most strata that they examined. Then bits and bobs appeared in what looked like an explosion of living things in strata above a certain age. This troubled Darwin deeply. Any such explosion of life undermined his theory of natural selection, a process of elimination that he argued should inherently take a very long time to unfold. By his estimations, it could never have taken place as instantaneously as those earliest fossils suggested.
Halfway through On the Origin of Species, he gravely acknowledged the implications of this predicament. “The case at present must remain inexplicable[,]” he wrote, “and may be truly urged as a valid argument against the views here entertained.”
But here we are, still entertaining Darwin’s views more than 150 years later. And that is thanks largely to rocks discovered not long after World War II.
At the end of the war, a wave of mineral exploration arose across the world to meet the needs of rapidly expanding economies. Federal agencies hired geologists to scour continents for oil, gas, and coal to fuel those economies, along with metals like iron and uranium to build up arsenals of defense. This was of national interest, not just private economic interest.
As geologists marched around the globe and sketched up their maps of these resources, they noticed other curious details about the planet’s history. That is, in their search for the materials that humans desired, they found inklings of how we got to be here desiring anything in the first place.
In the summer of 1953, Wisconsin geologist Stanley Tyler was studying iron-rich rocks on the north shore of Lake Superior in Canada when he took a Sunday off to rent a boat and go fishing. While his lure bobbed in the water, he absently noted the shapes and colors along the shore, as any geologist might. One outcrop caught his eye, so he motored over to take a closer look.
Tyler recognized the deposit as an extension of the Gunflint Chert, a rock formation with the texture of tightly packed brown sugar and the contents of ancient seafloor sediments. Cherts can take on a whole range of colors depending on the conditions they form within, from beige to red to green to other hues in between. Most of the chert that Tyler had found on that trip had been maroon, but this outcrop caught his eye for its striking shade of jet black. He knew that the color black in rocks was sometimes indicative of organic material, remnants of ancient life.
He lopped off a chunk, stashed it in his boat, and motored on.
Back at his lab in Madison he placed a sliver of that black chert under a microscope, and found shapes that did not speak the language of minerals. The rods, spheres, and squiggles he found did, as he suspected, look more lifelike than lithic.
Based on geologic maps of the region, he knew these rocks had formed during the allegedly fossil-free epoch that had so troubled Darwin. Tyler’s gut told him he may have just found some of the earliest evidence of life ever discovered, but he was a mineralogist more than a paleontologist and so he needed a second opinion.
That fall, he took photographs of his findings to a geology conference in Boston and shared them with a couple of colleagues. One among them, a Harvard paleobotanist named Elso Barghoorn, agreed that the samples looked rather lifelike, and the two published a short paper describing what they had found.
This publication quadrupled the length of the fossil record. It was groundbreaking, but was brief and preliminary. They needed more time to study the fossils to do justice to the scope of their findings.
For years, they didn’t make progress on a follow-up paper. A decade went by and, in 1963, Tyler passed away at the age of 57 from heart complications, without the satisfaction of sharing his discoveries more completely with the world. By 1965, an impatient colleague named Preston Cloud — a bantamweight boxing champion turned acclaimed Earth historian — threatened to beat Barghoorn to the punch with his own paper on the fossils. That was enough to push Barghoorn into gear. He rushed to complete a manuscript and published it in the journal Science a couple months before Cloud published his.
“For all of time it will probably stand as the most important article ever written in the field . . . ,” writes William Schopf, a graduate student who helped Barghoorn pen that manuscript, but who humbly declined authorship himself because he didn’t feel he had contributed enough.
Spurred by this new paper on the Gunflint Chert, geologists went searching for evidence of ancient life in black cherts around the world. Papers flooded out, claiming to have solved Darwin’s dilemma and showing how fossils had been in those seemingly lifeless rocks all along — they had simply been microscopic. The theory of natural selection persevered, and the lengthy record of our ancient roots began to fill out.
But while those microscopic rods and squiggles resolved one nagging dilemma, they opened up a slew of other questions. What, exactly, were those fossils? What kind of world did they evolve into? And what kind of world did they create with their growth?
Around the same time that these questions began bubbling up, another set of observations from the rock record thickened the plot of the squiggles. Geologists were compiling evidence that, before those lifeforms lived, the planet’s atmosphere had no oxygen gas in it at all. Minerals that disintegrate in the presence of oxygen were found locked up in ancient riverbeds older than a certain age. Then, around the time they believed those squiggles showed up on the scene, those riverbed minerals disappeared and the very first, rusty red fingerprints of oxygen began appearing in strata around the world.
Perhaps, some reasoned, those squiggles were responsible for painting the world’s soil and seafloor sediments red, by ushering in the very first poofs of oxygen. And perhaps, in their delivery of this gas, they catapulted Earth out of its original barrenness and into the tangle of complex life we know today.
Excerpted from Strata: Stories from Deep Time. Copyright © 2025 by Laura Poppick. Used with permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.
