If I had to pick the most important group of organisms in the history of life, I’d pick cyanobacteria. We used to call them blue-green algae, finding some of them now as a kind of pond scum, but they are real bacteria, simple cells with no nucleus.
Cyanobacteria evolved over 3 billion years ago. They originated photosynthesis, which liberated oxygen from water. That means that they created free oxygen in the air. That oxygen also created the ozone layer that made life on land possible, sheltering us landlubbers from ultraviolet radiation.
Cyanobacteria also almost “did in" themselves and all other life several times starting about 2.2 billion years ago. The oxygen they liberated oxidized away the methane greenhouse that kept Earth warm enough when the Sun was dimmer. Earth froze over, perhaps completely, until it was rescued after 50 million years by volcanoes pouring out CO2 to warm Earth once again.
Some cyanobacteria got captured by other cells, to evolve into chloroplasts now found in all green plants that provide our food, forage, wood, and more. Land plants now dominate in global total photosynthesis, leaving cyanobacteria and their colleagues in the oceans to play second fiddle. What a ride they have had, and the whole Earth, too.
This has been an outreach activity of the Las Cruces Academy, viewable at GreatSchools.org.
Source: My research and reading over about 40 years
