The increase in oxygen levels in Earth's atmosphere, particularly during the Great Oxidation Event (approximately 2.4 billion years ago), is primarily attributed to the photosynthetic activities of cyanobacteria (formerly known as blue-green algae). The evidence supporting this includes several lines of geological and biological findings:
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Stromatolites: These layered structures formed by the activity of cyanobacteria provide fossil evidence of ancient microbial life that conducted photosynthesis. The rise of stromatolites in the geological record coincides with increasing oxygen levels.
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Banding in Iron Deposits: The presence of banded iron formations (BIFs) is a significant indicator of oxygen levels in ancient oceans. Before the accumulation of free oxygen in the atmosphere, iron in the ocean was soluble. As oxygen began to accumulate due to photosynthesis, it reacted with dissolved iron, leading to the precipitation of insoluble iron oxides. The transition from BIFs to more oxidized sediment layers suggests an increase in atmospheric oxygen produced by photosynthetic bacteria.
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Fossil Evidence of Eukaryotes: The emergence of oxygen-dependent life forms, particularly eukaryotes, is associated with rising oxygen levels. Fossils of early eukaryotic organisms that require oxygen for cellular respiration indicate the presence of sufficient atmospheric oxygen produced by photosynthetic cyanobacteria.
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Isotopic Signatures: The carbon isotopic compositions of ancient rocks can indicate shifts in biological activity. A decrease in the ratio of carbon-12 to carbon-13 in sedimentary rocks suggests an increase in biological productivity (like photosynthesis), leading to elevated oxygen levels.
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Geochemical Models: Modern geochemical modeling of ancient environments supports the idea that increased photosynthetic activity from cyanobacteria led to the massive production of oxygen, drastically altering Earth's atmosphere and oceans over geological timescales.
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Atmospheric and Oceanic Composition Studies: Studies of ancient atmospheric composition through ice cores and mineral deposits provide indirect evidence of the oxygen levels at various geological periods, aligning with the timing of significant cyanobacterial activity.
In summary, the evidence that oxygen levels in a carbon-rich environment increased due to photosynthetic bacteria is substantial and comes from geological formations, fossil records, isotopic studies, and geochemical models, all of which highlight the pivotal role that cyanobacteria played in shaping Earth's atmosphere and enabling the evolution of aerobic life forms.