Cyanobacteria originated pivotal evolutionary developments which transformed our planet. These microscopic bacteria live alone or in colonies, in any environment that has moisture. Over 2 billion years ago, cyanobacteria triggered the Great Oxygenation Event. Every breath we take, we owe to cyanobacteria. It is thanks to them that complex life forms evolved in the first place.
When the Great Oxygenation Event occurred 2.5 billion years ago, the Earth was 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 million years earlier, cyanobacteria had evolved the ability to make their own food using sunlight, carbon dioxide, and water — photosynthesis. They were the first organisms to photosynthesize.
Of course, oxygen gas is a waste product of photosynthesis. Over millions of years of cyanobacteria performing photosynthesis, small concentrations of oxygen accumulated in the environments where cyanobacteria lived.
Fast-forward 100 million years, and the Earth’s atmosphere was full of oxygen. Multiple theories attempt to explain the 100 million year lag between the time cyanobacteria became photosynthetic to when the Great Oxygenation Event occurred, 2.35 billion years ago. A favored theory posits that a mere 150 million years prior to Great Oxygenation Event, cyanobacteria changed in a way that suddenly made them more successful, enabling them to release enough oxygen to transform the atmosphere of the planet.
So what happened? Through taxonomic analysis, scientist B.E. Schirrmeister et al have shown that cyanobacteria evolved multicellularity 2.5 billion years ago. Therefore, 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 over single-celled species.
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 found ways to position themselves advantageously within the layers of a stromatolite, even orienting vertically to avoid the sun’s harmful ultraviolet rays.
Photo Source: The Wikimedia Foundation
Basically, multicellularity enabled cyanobacteria to outcompete single-cell organisms. The resulting growth of their populations increased the overall amount of oxygen released during photosynthesis. In a quick 150 million years, the planet’s atmosphere became oxygen-rich. Cyanobacteria were responsible for the most significant climate change in our planet’s history, creating the air we breathe and a protective shield from the Sun’s harmful rays.
Of course, 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. The other side of the coin is that cyanobacteria caused nearly every other living thing to become extinct. Small quantities of anaerobic life exist 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 more creative than 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 humans.
Cyanobacteria are also considered by many scientists to be the ancestors of all plants. The theory goes that, at some point, a single-celled organism ingested a cyanobacteria but did not digest it, and the cyanobacteria continued to photosynthesize inside its host — like chloroplasts do inside plants.
Today, cyanobacteria 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, anywhere there’s moisture, even in Antarctica and the fur of sloths.