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Significance Earth progressed from an oxygen-free planet at birth to the well-oxygenated planet we have today. Life enabled Earth’s oxygenation through the evolution of cyanobacteria, and the subsequent history of oxygen enabled further developments in the evolution of eukaryotic life; mammals, for example, need relatively high oxygen levels to breathe. The relationship between oxygen and life, however, is hampered by uncertainties in the history of oxygen concentrations, particularly during the middle of the Proterozoic Eon when eukaryote ecosystems were first emerging. We explore ancient organic matter cycling through Raman spectrographic analysis of nine geologic formations spanning 1 billion years of Proterozoic time. Our results, when combined with modeling, show that emerging eukaryote ecosystems were not limited by oxygen concentrations.

Petrographic carbon in ancient sediments constrains Proterozoic Era atmospheric oxygen levels