One of the most important events in the history of life on Earth was the appearance of an oxygen-rich atmosphere. Initially, this event was a disaster–free oxygen was a deadly poison and led to a massive die-off. But for one group of single-celled organisms, oxygen opened the door to a pathway that led to life as we know it, and eventually to us.
When the Earth first formed, about 4.5 billion years ago, it was a very different place than it is now. Over millions of years, it had slowly condensed from the same swirling dust cloud that had formed the Sun and the other planets, growing larger as its gravity pulled in pieces of space rock which impacted the surface. The early seas, formed by water that seeped out of the cooling crust (and also from the impact of water-rich comets) were a soup of dissolved minerals like iron and nickel. The atmosphere was thin and was made up of gases like methane, carbon dioxide, ammonia, hydrogen cyanide, and water vapor. Occasionally, a particle of cosmic radiation would have hit one of these gas molecules and broken it into pieces, releasing atoms of free oxygen, but since oxygen is highly reactive, those atoms would have quickly bonded again with other elements. The early atmosphere would have had only tiny traces of free oxygen, and would have been, to any of today’s oxygen-breathing organisms, a deadly toxic and suffocating place.
All life at that time would have been what we call today “anaerobic”, living without oxygen. They were also “autotrophs”: they did not need to eat “food”–instead they manufactured their own food by assembling it from available materials. Some were “methanogen” bacteria: they lived by extracting molecules of hydrogen and carbon dioxide from the atmosphere, combining them to chemically produce food which they broke apart to release energy to power their cellular processes, and then excreting the leftover molecules, mostly methane gas, as waste products. Other bacteria extracted hydrogen and sulfates from their surroundings, and released hydrogen sulfide as waste. Oxygen gas was, to all of them, a lethal poison. And for at least a billion years, they were the dominant form of life on Earth.
Then, over 2.5 billion years ago, something new happened. A tiny branch of organisms appeared which we now call “cyanobacteria”, and they had developed a new method of making food. They pulled carbon dioxide molecules out of the atmosphere, then used the energy from sunlight to break these into carbon and oxygen atoms. The carbon atoms were combined with hydrogen to make simple sugars for food–the leftover oxygen atoms were released as waste. Today, we call this process “photosynthesis”.
At first, this would have produced little effect. Like those free oxygen atoms occasionally liberated by cosmic radiation, the waste oxygen from the cyanobacteria would have quickly bonded with other elements, and the percentage of free oxygen gas in the atmosphere would have remained minimal. But over time, things changed. The process of photosynthesis was more efficient than the earlier methanogens, and the cyanobacteria began to take over. As the population grew, they released more and more oxygen, and over millions of years those little puffs of oxygen waste, released by unimaginable numbers of photosynthesizing bacteria, saturated the atmosphere so much that there were no longer sufficient atoms of other elements in the air to chemically capture then. The oxygen atoms were therefore captured by the iron atoms dissolved in the ancient seas. When iron and oxygen combine, they form the compound iron oxide–which is the chemical name for “rust”. All over the global seas, tiny particles of rust would have formed and slowly sunk to the bottom to form a layer of red sediment. Once most of the oxygen had been extracted out of the atmosphere, the production of rust particles would stop, and the iron oxide sediment would be once again covered with plain clay ocean seafloor–only to have the process repeated as more oxygen was puffed out by bacteria, to once again saturate the atmosphere and lead to another round of iron oxidation and another layer of rust sediments. Over and over, for millions of years, this process would have cycled, producing deep alternating layers on the seafloor of bright red iron oxides and bland grey ocean sediments.
Finally, about 2.3 billion years ago, the number of cyanobacteria was so vast, and the amount of oxygen that they released so massive, that nearly all of the iron in the oceans was removed through oxidation, there was no longer any chemical pathway to remove the free oxygen from the air, and oxygen gas began to accumulate in the atmosphere. The “Oxygen Crisis” produced Earth life’s first global extinction event. Most single-celled life at the time was anaerobic–oxygen was a deadly poison for them. The result was a massive single-celled slaughter, as the oxygen gas wiped out nearly all the anaerobes. The only places where they could survive were out-of-the-way backwaters where the deadly oxygen could not reach–such as deep inside mud sediments. They still survive there today, inside the refuges which protect them from the poisonous atmosphere. (When you poke a stick into the mucky ooze of a tidal mudflat and catch a whiff of the rotten-egg smell of hydrogen sulfide, you are detecting some of the survivors of that long-ago anaerobe slaughter, which may have killed a higher percentage of Earth life than any other event since.)
But for one group of single-celled organisms, the global mass extinction was a blessing. One group of photosynthetic cyanobacteria was oxygen-tolerant, and could survive even in the presence of the deadly gas. And now, with the near-removal of their methanogen competitors, the Earth was theirs. These single-celled photosynthesizers began clumping together into colonies, in which individual cells were connected into strings like beads. From these “algae”, multicellular “plants” would evolve: in some 1.8 billion years, they in turn would invade the land and form the foundation of our entire modern ecology.
And what happened to the distinctive sediment layers of alternating iron oxide and clay that was formed during this transition? Over billions of years, those deposits were covered by countless new layers of sediment, and were then compressed and hardened into rock. Pushed upward by tectonic forces and uncovered by erosion, the layers of striped rocks became known to geologists as “Banded Iron Formations”, and they became the largest and most economically-important sources of iron ore for our Industrial Age.