- The presence and gradual increase of oxygen in Earth’s atmosphere stands as one of the most profound events in the history of life.
- Initially, Earth’s atmosphere—about 4.5 billion years ago—was largely devoid of free molecular oxygen (O₂). It consisted mainly of carbon dioxide, nitrogen, methane, ammonia, and water vapor.
- Life began in this anaerobic environment, with the earliest microorganisms thriving in the absence of oxygen. However, the emergence of oxygenic photosynthesis, performed by cyanobacteria around 2.5 to 3 billion years ago, triggered a planetary transformation that would eventually enable complex, multicellular life to evolve.
- The first major shift is known as the Great Oxidation Event (GOE), occurring around 2.4 billion years ago. During this period, photosynthetic cyanobacteria began producing oxygen as a byproduct of splitting water molecules to capture energy from sunlight. At first, this oxygen reacted with abundant iron and other reduced minerals in the oceans and crust, forming banded iron formations and rust-colored deposits. Once these sinks were saturated, free oxygen began to accumulate in the atmosphere. This was initially catastrophic for many anaerobic life forms, as oxygen is toxic to cells unadapted to it. Thus, the GOE marked both a mass extinction and a turning point in evolution, as organisms that could tolerate or exploit oxygen began to dominate.
- Over the next billion years, oxygen levels slowly rose, allowing for the evolution of more efficient cellular respiration. Compared to anaerobic respiration, aerobic respiration produces significantly more ATP—the energy currency of the cell—making it a key factor in the development of eukaryotic cells, which require high energy to support complex internal structures like nuclei and organelles. This energetic advantage was critical to the eventual emergence of multicellular life during the Ediacaran Period (around 600 million years ago) and the explosive biodiversity of the Cambrian Explosion (~541 million years ago), when most major animal groups first appeared in the fossil record.
- Oxygen not only enabled complex life but also influenced Earth’s climate and geological processes. With oxygen, the formation of ozone (O₃) in the upper atmosphere became possible, creating a protective layer that shields the surface from harmful ultraviolet (UV) radiation. This protection allowed life to migrate from oceans to land, expanding biodiversity even further. Oxygen also affected the cycling of elements such as carbon, nitrogen, and sulfur, regulating the Earth’s climate through long-term feedback mechanisms involving photosynthesis, respiration, and weathering.
- Fluctuations in atmospheric oxygen levels have been linked to major evolutionary and extinction events. For instance, periods of oxygen increase often correspond with the rise of large, mobile organisms, while drops in oxygen levels may have contributed to mass extinctions. The peak of atmospheric oxygen, estimated to have occurred during the Carboniferous Period (about 300 million years ago), may have supported giant insects and amphibians due to the higher oxygen diffusion capacity in their respiratory systems.
- Today, oxygen continues to play a central role in life. It is essential for the metabolism of nearly all animals, drives combustion, supports ecosystems through the oxygen-carbon cycle, and remains a key parameter in environmental monitoring. Modern research continues to explore the influence of oxygen on early Earth and its implications for the search for life on other planets. If oxygen is detected in an exoplanet’s atmosphere, it may suggest the presence of biological activity similar to that on Earth.
