An oxygen-rich atmosphere is essential for complex animals. The early Earth had an anoxic atmosphere, and understanding the rise and maintenance of high O2 levels is critical for investigating what drove our own evolution and for assessing the likely habitability of exoplanets. A growing number of techniques aim to reproduce changes in O2 levels over the Phanerozoic Eon (the past 539 million years). We assess these methods and attempt to draw the reliable techniques together to form a consensus Phanerozoic O2 curve. We conclude that O2 probably made up around 5–10% of the atmosphere during the Cambrian and rose in pulses to ∼15–20% in the Devonian, reaching a further peak of greater than 25% in the Permo-Carboniferous before declining toward the present day. Evolutionary radiations in the Cambrian and Ordovician appear consistent with an oxygen driver, and the Devonian “Age of the Fishes” coincides with oxygen rising above 15% atm. ▪ An oxygen-rich atmosphere is essential for complex animals such as humans. ▪ We review the methods for reconstructing past variation in oxygen levels over the past 539 million years (the Phanerozoic Eon). ▪ We produce a consensus plot of the most likely evolution of atmospheric oxygen levels. ▪ Evolutionary radiations in the Cambrian, Ordovician, and Devonian periods may be linked to rises in oxygen concentration. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Phosphate is an essential macronutrient for all organisms with a key role in setting levels of marine primary productivity. Despite its importance for marine biogeochemical cycles and its role in shaping the evolution of marine organisms, the factors controlling phosphate bioavailability on geologic timescales remain poorly understood. Here we develop a statistical model of the coupled cycles of phosphate, carbon, oxygen and calcium to constrain the weathering-derived fluxes and seawater concentrations of phosphate through Phanerozoic time (541 million years ago to the present). Our model includes input parameters and time-dependent forcings derived from geologic and geochemical data. We find that the climate sensitivity of chemical weathering of the oceanic crust by low-temperature fluids exerts a first-order control on phosphate availability. Specifically, continental weathering is a source of the limiting nutrient phosphate, but seafloor weathering is considered to be a minor phosphate sink. Consequently, times in Earth history during which seafloor weathering constituted a large fraction of the total (seafloor + continental) weathering were also times during which phosphate influxes to and concentrations in the ocean were relatively low. Lower seawater phosphate levels during those times probably resulted in lower primary productivity and oceanic and atmospheric oxygen concentrations. Marine phosphate levels and biological productivity were lowest during the early Phanerozoic when seafloor weathering rates were high and continental weathering rates were muted, according to a statistical model of coupled elemental cycles.

The burial of organic carbon, which prevents its remineralization via oxygen-consuming processes, is considered one of the causes of Earth's oxygenation. Yet, higher levels of oxygen are thought to inhibit burial. Here we propose a resolution of this conundrum, wherein Earth's initial oxygenation is favored by oxidative metabolisms generating partially oxidized organic matter (POOM), increasing burial via interaction with minerals in sediments. First, we introduce the POOM hypothesis via a mathematical argument. Second, we reconstruct the evolutionary history of one key enzyme family, flavin-dependent Baeyer-Villiger monooxygenases, that generates POOM, and show the temporal consistency of its diversification with the Proterozoic and Phanerozoic atmospheric oxygenation. Finally, we propose that the expansion of oxidative metabolisms instigated a positive feedback, which was amplified by the chemical changes to minerals on Earth's surface. Collectively, these results suggest that Earth's oxygenation is an autocatalytic transition induced by a combination of biological innovations and geological changes.

sometime before the Early Devonian period 405 million years ago. Low atmospheric oxygen during the Middle and Late Devonian was accompanied by a decrease A wildfire, forest fire, brushfire or a bushfire is an unplanned and uncontrolled fire in an area of combustible vegetation. Some natural forest ecosystems depend on wildfire. Modern forest management often engages in prescribed burns to mitigate fire risk and promote natural forest cycles. Controlled burns can turn into wildfires if they escape containment lines. Wildfires can be classified by ca The first evidence of wildfires is fossils of the giant fungi Prototaxites preserved as charcoal, discovered in South Wales and Poland, dating to the Silurian period (about 430 million years ago). Smoldering surface fires started to occur sometime before the Early Devonian period 405 million years ago. Low atmospheric oxygen during the Middle and Late Devonian was accompanied by a decrease in charcoal abundance. Additional charcoal evidence suggests…