Our planet-wide oxygen-rich atmosphere was created by tiny photosynthesizing bacteria called cyanobacteria. Cyanobacteria are single celled bacteria, sometimes (misleadingly) referred to as blue-green algae. They live alone or in colonies in any environment that has moisture. Over 2 billion years ago, cyanobacteria triggered what’s known as the Great Oxygenation Event. Every breath you take, you owe to cyanobacteria. It’s thanks to them that we exist in the first place.
When the Great Oxygenation Event occurred 2.5 billion years ago, the Earth was over half its current age, populated by various single-celled organisms subsisting in an atmosphere composed of carbon dioxide, water vapor, ammonia and methane.
Approximately 100 billion years earlier, one type of bacteria had evolved the ability to make its own food using sunlight, carbon dioxide, and water. If that sounds like photosynthesis, it is. Cyanobacteria were the first organisms to photosynthesize and are therefore considered the ancestors of all plants.
Oxygen is a waste product of photosynthesis. Over billions of years of cyanobacteria performing photosynthesis and releasing oxygen, small concentrations of oxygen accumulated in environments where cyanobacteria lived — but not enough to be considered any kind of great event.
Another 100 billion years elapsed from the time cyanobacteria became photosynthetic to when the Great Oxygenation Event occurred 2.35 billion years ago. There are multiple theories attempting to explain the 100 billion year lag time. One favored theory posits that, just 150 million years prior to Great Oxygenation Event, cyanobacteria changed in a way that suddenly made them much more successful, pumping out enough oxygen to transform the atmosphere of the planet.
Scientist B.E. Schirrmeister et al have shown through taxonomic analysis that cyanobacteria evolved multicellularity 2.5 billion years ago. Cyanobacteria were not only the first life-forms to photosynthesize, they were also the first to achieve multicellularity. They formed chains of bacteria with one differentiated cell at the head. As primitive as it was, cyanobacteria’s multicellularity gave them profound new advantages.
Most cyanobacteria lived in layered microbial mats called stromatolites, rounded lumps found in tide pools and underwater, home to many species of microbe. Once cyanobacteria became multicellular, they could hold onto the stromatolites better, and not be swept off by waves and tides. They could also position themselves advantageously within the layers of a stromatolite, and even orient vertically to avoid the Sun’s harmful UV rays.
Photo Source: The Wikimedia Foundation
In short, multicellularity enabled cyanobacteria to outcompete single-cell organisms. The resulting growth of their populations increased the overall amount of oxygen released during photosynthesis. In relatively little time, a quick 150 million years, the planet’s atmosphere became oxygen-rich. The Great Oxygenation Event is the most significant climate change in our planet’s history, creating Earth’s atmosphere and a protective shield from the Sun’s harmful rays.
A harsh reality, the Great Oxygenation Event was also Earth’s first mass extinction. Most organisms alive at the time were anaerobic — intolerant of oxygen — and they all died off. Thanks to the activities of cyanobacteria, nearly every living thing was wiped off the face of the Earth. Small quantities of anaerobic life are still found today in oxygen-free environments, for example, microbes living around hydrothermal vents on the ocean floor, or safely ensconced inside another organism’s oxygen-free gut.
Yet ultimately, Oxygen’s transformative effects were as creative as they were destructive. The new presence of oxygen affected chemical interactions between rocks, causing explosive growth in the diversity of Earth’s minerals. And, being a reactive gas, oxygen allowed for the emergence of larger, more complex life forms, like us.
Today, microscopic cyanobacteria are still ubiquitous, and still photosynthesizing. In fact they perform 90% of photosynthesis in the planet’s oceans. Cyanobacteria comprise the most significant and prolific phylum of bacteria, abundant in freshwater and marine environments, in Antarctica, basically anywhere there is moisture, even in the fur of sloths, the latter being a tale for another time.