Photosynthesis is the fundamental process that transforms light energy, typically from the sun, into stored chemical energy within organic compounds. Tracing the origin of this light-harvesting capability leads back billions of years to the simplest single-celled organisms. The question of which organism first evolved photosynthesis requires understanding a major evolutionary transition from a primitive form of energy capture to the oxygen-producing mechanism that dominates the planet today.
The Earliest Forms of Energy Capture
The Earth’s early atmosphere was largely anaerobic, containing no free oxygen. The first photosynthetic life forms used a simpler method known as anoxygenic photosynthesis because it does not generate oxygen as a byproduct. These ancient organisms, likely appearing around 3.5 to 3.8 billion years ago, utilized compounds other than water to fuel their metabolism.
Instead of splitting water molecules for electrons, these phototrophs used readily available reduced compounds. Common electron donors included hydrogen sulfide (\(\text{H}_2\text{S}\)), molecular hydrogen (\(\text{H}_2\)), or ferrous iron (\(\text{Fe}^{2+}\)). This process converted carbon dioxide into organic matter using sunlight, releasing elemental sulfur or oxidized iron rather than oxygen.
Different groups of bacteria, such as the green sulfur bacteria (Chlorobiaceae) and the purple sulfur bacteria (Chromatiaceae), still perform this type of energy capture today. These organisms thrive in oxygen-poor habitats like deep sediments or hot springs. Anoxygenic photosynthesis established the initial blueprint for capturing light, but it was limited by the scarcity of suitable electron donor molecules.
The Evolutionary Turning Point
The organism that introduced the evolutionary turning point was Cyanobacteria, a group of prokaryotes that developed the ability to perform oxygenic photosynthesis. This innovation allowed them to use water (\(\text{H}_2\text{O}\)), the most abundant resource on Earth, as the source of electrons for their light-driven reactions. Splitting water releases oxygen (\(\text{O}_2\)) as a waste product, fundamentally changing the planet.
The biochemical breakthrough centered on the evolution of Photosystem II (PSII). While anoxygenic phototrophs use only one photosystem, Cyanobacteria link two reaction centers, Photosystem I and Photosystem II, in series. PSII contains a specialized manganese-calcium cluster (\(\text{Mn}_4\text{CaO}_5\)) required to break the strong chemical bonds of the water molecule.
This development freed life from dependence on scarce electron donors like sulfide or iron. The successful deployment of this mechanism by Cyanobacteria, which proliferated in the ancient oceans, ensured a stable energy supply. The excess oxygen produced by these microbes eventually transformed the chemistry of the entire planet.
The Great Oxygenation Event
The success of oxygenic photosynthesis by Cyanobacteria led to the environmental upheaval known as the Great Oxygenation Event (GOE). This event, sometimes referred to as the Oxygen Catastrophe, spanned hundreds of millions of years, altering the composition of Earth’s atmosphere and oceans. For nearly a billion years, the newly produced oxygen was consumed by reacting with reduced minerals like iron and sulfur in the oceans.
Once these oxygen sinks were saturated, free oxygen began to accumulate in the atmosphere, shifting the planet from a reducing environment to an oxidizing one. This buildup of oxygen was toxic to the anaerobic life forms that had dominated the planet, triggering a mass extinction event. The GOE also affected climate, as the oxygen reacted with and destroyed atmospheric methane, a powerful greenhouse gas.
The resulting loss of atmospheric heat retention led to a global cooling and the planet’s most severe ice age, the Huronian glaciation, which occurred between approximately 2.45 and 2.22 billion years ago. The new oxygen-rich environment paved the way for complex life by enabling the evolution of aerobic respiration. This metabolic process extracts more energy from organic compounds than anaerobic pathways.
Geological Evidence and Timeline
The evolutionary timeline of oxygenic photosynthesis is supported by two types of geological evidence found in ancient rock formations. The direct fossil evidence comes from structures called stromatolites. These are layered, dome-shaped sedimentary structures created by the growth of successive microbial mats, primarily composed of Cyanobacteria.
Fossilized stromatolites provide a record of microbial communities that flourished as far back as 3.4 billion years ago. The geological record also contains Banded Iron Formations (BIFs), which are distinctive units of rock with alternating layers of red iron oxides and silica. The formation of these layers is directly linked to the rise of oxygen.
As Cyanobacteria released oxygen into the ancient oceans, the gas reacted with dissolved ferrous iron (\(\text{Fe}^{2+}\)) present in the seawater. This reaction caused the iron to rust and precipitate out as insoluble ferric iron oxide (\(\text{Fe}^{3+}\)), which settled on the ocean floor. The most extensive BIFs date to the late Archean and early Paleoproterozoic, with the peak of their deposition coinciding with the first major rise in atmospheric oxygen between 2.4 and 2.3 billion years ago.