Photosynthesis is how organisms convert light energy into chemical energy, stored in organic molecules like sugars, to power their growth and survival. On early Earth, this ability to harness the sun’s energy was transformative for nascent life.
Photosynthesis’s Earliest Beginnings
The earliest forms of photosynthesis were “anoxygenic,” meaning they did not produce oxygen. These ancient processes utilized light to convert carbon dioxide into organic compounds, relying on electron donors other than water. Organisms like purple non-sulfur bacteria and green sulfur bacteria are thought to be among these pioneers.
These bacteria thrived in environments rich in substances like hydrogen sulfide, hydrogen gas, or ferrous iron. For example, green sulfur bacteria used sulfide, while purple non-sulfur bacteria utilized a broader range of electron donors, including organic compounds. These early photosynthetic pathways did not significantly alter the planet’s atmospheric composition.
The Oxygen Revolutionaries
A profound shift occurred with the evolution of “oxygenic” photosynthesis, which produces oxygen. This change is attributed to ancient cyanobacteria, sometimes called blue-green algae. Unlike their anoxygenic predecessors, cyanobacteria used water as their electron donor.
Water is an abundant resource, making its use as an electron donor a significant advantage. This allowed photosynthetic organisms to flourish in new environments, no longer limited to specific chemical donors. The widespread proliferation of cyanobacteria, fueled by water, gradually released vast quantities of oxygen into the oceans and atmosphere.
Transforming Earth’s Atmosphere
The continuous oxygen release by cyanobacteria transformed Earth’s environment, leading to the Great Oxidation Event (GOE). This period, beginning approximately 2.4 to 2.1 billion years ago, marked the first significant accumulation of free oxygen in the atmosphere and shallow seas. Initially, much of the oxygen produced reacted with dissolved iron in the oceans.
This reaction formed insoluble iron oxides that precipitated, creating distinctive layered rock formations known as banded iron formations. These formations are geological evidence of the immense amount of oxygen being produced and consumed.
Once readily available iron in the oceans was oxidized, oxygen accumulated in the atmosphere, leading to the ozone layer, which shielded Earth from harmful ultraviolet radiation. This new oxygen-rich atmosphere was toxic to many existing anaerobic life forms, triggering a mass extinction, but it also paved the way for the evolution of aerobic respiration and, eventually, more complex life.
Uncovering Ancient Life’s Secrets
Scientists piece together the story of ancient photosynthetic organisms through various lines of evidence. Fossilized stromatolites provide compelling clues, appearing in the geological record as far back as 3.4 to 3.5 billion years ago. These layered structures are thought to have been formed by ancient microbial mats, predominantly composed of cyanobacteria, trapping sediments over time.
Further geological evidence comes from banded iron formations, a record of Earth’s early oxygenation. The alternating layers of iron-rich and silica-rich sediments reveal the cyclic nature of oxygen production and iron precipitation in ancient oceans. Additionally, molecular clock dating, which analyzes biomolecule mutation rates, helps scientists estimate when different life forms diverged. By calibrating these clocks with fossil and geological data, researchers infer approximate timelines for the evolution of photosynthesis and its forms.