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How did life alter the carbon dioxide concentration of Earth’s atmosphere?

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abundance of oxygen — evolution of the atmosphere A “best guess” reconstruction of the abundance of O₂ in Earth's atmosphere as a function of time. The O₂ abundance axis is logarithmic.

Life has played a fundamental role in regulating Earth’s carbon dioxide ( $C O_{2}$ ) levels through the biological carbon cycle. Early in Earth's history, the atmosphere contained significantly higher concentrations of $C O_{2}$ than today, likely exceeding current levels by at least 100 times.1

The primary mechanism by which life altered this concentration is photosynthesis. Primitive organisms, such as cyanobacteria, evolved to use $C O_{2}$ and water to produce carbohydrates, a process that effectively removed $C O_{2}$ from the atmosphere and released oxygen as a byproduct. Over millions of years, a portion of this organic matter was not recycled through respiration but was instead buried in Earth’s crust via sedimentation. This long-term burial of organic carbon—which eventually formed fossil fuels—sequestered carbon that had been extracted from the atmosphere, contributing to a significant reduction in atmospheric $C O_{2}$ over geologic time.123

Conversely, when biological productivity is inhibited or organic matter is oxidized, $C O_{2}$ levels can rise. Today, human activity is rapidly returning this historically sequestered carbon to the atmosphere, reversing the long-term cooling influence of these biological sinks.2