By Helen Wong 王思齊 The history of life on Earth saw many pivotal moments, but perhaps none, other than the origin of life itself, is more significant than the Great Oxidation Event (GOE). Marking the period when the early Earth’s atmosphere started to fill with free oxygen, the GOE set the foundation for the rise of aerobic life and ultimately, present-day humans [1, 2]. Imagine traveling back to 4.5 billion years ago, when Earth had just formed. The atmosphere was vastly different from what we have today – it consisted of water vapor, carbon dioxide, and methane, but not oxygen. Consequently, the earliest life forms that emerged approximately 3.8 billion years ago were anaerobic. But the entire game changed when a group of bacteria diverged from their anaerobic ancestors around 3.4 billion years ago [3, 4]. These unique microbes developed one of the most crucial innovations in the history of life on Earth – oxygenic photosynthesis – and evolved into what we now know as cyanobacteria (commonly called blue-green algae, although they are not technically algae) (Figure 1). The Great Oxidation Event: How Earth’s Atmosphere Became Oxygen-Rich 大氧化事件： 地球大氣層的氧從何而來 Through oxygenic photosynthesis, oxygen was generated as a by-product of water splitting. Initially, the oxygen levels in the atmosphere remained low, as the first oxygen released into seawater by cyanobacteria was quickly sequestered by chemical reactions with other elements, such as iron [2] (Figure 2). Over a period of 200–300 million years [1], seawater oxygen levels gradually increased, possibly due to a rapid expansion of cyanobacterial populations [3, 4], until the accumulated oxygen began to escape into the atmosphere. The escaped oxygen displaced the abundant methane, kicking off the GOE that took place between 2.4 and 2.1 billion years ago [1]. The implications of an oxygenated atmosphere Figure 1 A stromatolite fossil of cyanobacteria. The layered structure was formed from mats of cyanobacteria. 圖一 屬於疊層石的藍綠菌化石，當中的 層狀結構由多層藍綠菌堆疊而成。 Photo credit 圖片來源: James St. John [5] Figure 2 Banded iron formation as evidence of the GOE. Iron (II) ions in the ocean are thought to be oxidized and precipitated as red iron (III) oxides in the GOE [6]. 圖二 條狀鐵層是大氧化事件的證據之一。科學家認為在大氧 化事件中，海洋中的鐵（II）離子被氧化並沉澱為紅色的氧化鐵 （III）[6]。 Photo credit 圖片來源: Graeme Churchard [7]
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