Life on Earth hinges on three essentials: liquid water, organic molecules, and a reliable energy source. It’s not surprising, then, that a critical breakthrough by early life forms was mastering the conversion of sunlight into energy—a process known as photosynthesis. This ability has had a profound impact on our planet.
Determining the exact origin of photosynthesis is challenging for geobiologists, as evidence of various precursor chemical reactions exists both in the fossil record and in living organisms. However, fossils of 3.4-billion-year-old microorganisms known as Filamentous Anoxygenic Phototrophs (FAPs) provide early evidence of photosynthesis. This initial form of photosynthesis, termed “anoxygenic,” did not produce free oxygen as a byproduct.
In the Archean era, organisms like FAPs and green sulfur bacteria thrived in the hydrogen-rich atmosphere of early Earth, utilizing anoxygenic photosynthesis. This process involved sunlight triggering electron release in reaction centers composed of proteins, pigments, and molecules. The breakdown of carbon dioxide and hydrogen sulfide formed complex organic molecules, eventually converted into glucose. The byproducts of this process were primarily water and elemental sulfur.
Later in the Archean, organisms such as cyanobacteria evolved oxygenic photosynthesis. This similar process also relied on sunlight-induced electron transfers but differed in using carbon dioxide and water as inputs, producing glucose and oxygen as outputs. Over time, this form of photosynthesis became integral to eukaryotes—organisms with complex internal cell structures, including plants—through a process known as endosymbiosis.
The emergence and proliferation of oxygenic photosynthesizing cyanobacteria towards the Archean’s end led to a significant atmospheric change known as “The Great Oxidation.” This event marked the first of several instances where life on Earth drastically altered the planet’s atmosphere.
