An Event That Changed Billions of Years of Earth’s History
Imagine if you stopped receiving oxygen; within seconds, your brain would stop functioning, and within a few minutes, the brain would be severely damaged, leading to death. Now imagine a world without oxygen; obviously, there would be no complex life, including reptiles, birds, mammals, flowering plants, and humans, and the oceans might appear reddish-brown due to the presence of unoxidized iron; the landscape of Earth might almost look like present-day Mars. The ultraviolet rays from the Sun would directly hit the Earth due to the absence of the ozone layer. In one sentence, there would be no aerobic life; only microorganisms that can survive without oxygen would exist.
Your imagination was real; billions of years ago, Earth used to be like this.
In 1927, a geologist named A. M. MacGregor was studying very old rocks[1]in present-day Zimbabwe. Inside one of these rocks, he found small, smooth, rounded pieces of a golden-colored mineral called pyrite, also known as fool's gold. Today, pyrite reacts with oxygen and water and slowly changes into rust-like minerals.
The smooth, rounded shape of the pyrite pieces showed that they had once been carried by a river before becoming trapped inside the rock. If there had been plenty of oxygen in Earth's atmosphere at that time, the pyrite would have rusted and broken down while it was being carried by the river. But it did not. Instead, it remained unchanged and was preserved inside the rock for billions of years.
This gave MacGregor an important clue. He realized that when these rocks formed about 2.7 billion years ago, there was very little oxygen in Earth's atmosphere. MacGregor did not stop with just one discovery. He also studied younger rocks, including rocks from the Lomagundi Formation, which formed about 2.2 billion years ago. Unlike the older rocks, these younger rocks showed clear signs of oxidation, meaning they had formed in the presence of much more oxygen. From this, he concluded that Earth's atmosphere became rich in oxygen much later.
Many discoveries[2]like this have shown that there was little or no oxygen in Earth's atmosphere during the first half of its history, but these studies also reveal another important thing: oxygen began to appear after the first 2.5 billion years of Earth’s history.
To understand how oxygen first appeared on Earth, scientists studied ancient rocks and microorganisms that still live in the oceans today.
They discovered ancient layered rock structures called stromatolites. These structures were built by microbial mats, which are thick layers of tiny microorganisms. Many of these microorganisms were cyanobacteria. Some stromatolites are more than 3 billion years old, showing that cyanobacteria lived on Earth long before oxygen became common in the atmosphere.
Cyanobacteria were among the first organisms to produce oxygen from water and atmospheric carbon dioxide through photosynthesis. At first, the oxygen they released reacted with rocks and minerals, so very little entered the atmosphere. Over millions of years, more and more oxygen began to build up in the air. This major event in the history of Earth is called the Great Oxidation Event.
The formation of oxygen in the atmosphere is very beneficial to most of the aerobic creatures like us, but the same oxygen became a poison to most of the microorganisms that survive by producing methane. After the emergence of oxygen, many microorganisms that have been living by converting hydrogen, sulphur, and carbon dioxide into methane have disappeared slowly. Thus, this event has resulted in both the destruction of old life as well as the creation of new life.
How Did Oxygen Support Complex Life?
The oxygen produced by cyanobacteria has a high tendency to accept electrons from other substances compared to most of the elements.
The fast and widespread presence of this oxygen in the oceans and atmosphere has increased the redox potential of the environment - that is, the tendency of the environment to promote the transfer of electrons by supporting oxidation (loss of electrons) and reduction (gain of electrons).
Electron transfer is one of the fundamental processes that makes life possible. Living organisms obtain energy through controlled oxidation-reduction (redox) reactions. During these reactions, electrons are transferred from one molecule to another, and the energy released is captured to produce ATP, the molecule that powers growth, reproduction, movement, and nearly every other cellular activity.
Eventually, some single-celled microorganisms started living in other microbes; this grouping has resulted in the emergence of multicellular organisms like sponges, coral reefs, and complex animals, including humans. Countless evolutionary changes have taken place in between to form the world that we are living in today.
Today, more than half of the world’s oxygen comes from the oceans, which is produced by algae and phytoplankton that live in the water at some depth below the surface where they can get light from the sun to perform photosynthesis. A particular type of cyanobacteria called Prochlorococcus alone produces 20% of the global oxygen, which is nearly equal to the combined amount of oxygen produced by all terrestrial trees on Earth. The fortunate thing is that these oxygen-producing microbes can live in freshwater, saltwater, and wastewater as well, and today these cyanobacteria are helping us produce biofuels, treat dangerous diseases like cancer, and support millions of farmers as biofertilizers.
Therefore, a single-celled microbe whose size ranges from 0.5 micrometers to 10 micrometers has changed most ecosystems. It has paved the way to a new world of aerobic creatures, and there is no doubt that we would not have existed without these microbes.
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