3 The implications of an oxygenated atmosphere were immense for both Earth's climate and its inhabitants. Methane, a greenhouse gas, traps heat from sunlight and keeps the Earth warm enough for organisms to survive. Therefore, when methane was displaced by oxygen, global temperatures dropped, causing Earth to enter a series of ice ages known as the Huronian glaciation [8]. Meanwhile, ultraviolet radiation (UV) from the Sun split oxygen molecules (O2) into individual atoms, which then reacted with other oxygen molecules to create ozone (O3), forming the ozone layer that now protects life on Earth from harmful UV radiation. The omnipresence of oxygen on Earth also fundamentally changed the planet’s biological landscape. To the anaerobic bacteria and archaea of the time, oxygen was toxic. This led to a mass extinction in which most anaerobes were wiped out. However, some survivors found ways to adapt and even thrive in the newly oxygen-rich environment. They developed ingenious solutions in terms of oxygen binding, aerobic respiration, and oxygen detoxification. To protect themselves from oxygen, these anaerobic organisms made use of certain proteins to bind oxygen and incorporate it into other molecules they need such as melanin [9]. Scientists believe that some of these ancient proteins eventually evolved into oxygen-transporting respiratory pigments found in animal blood today [9, 10]. For example, hemocyanin was likely derived from the oxygen-binding protein tyrosinase. These organisms also harnessed the power of oxygen as the terminal electron acceptor in respiration, which releases much more energy than anaerobic respiration. On the other hand, they evolved more effective versions of detoxifying enzymes, including superoxide dismutase and catalase (footnote 1), to deal with the harmful reactive oxygen species resulting from aerobic respiration [1]. For those unable to adapt, alternative strategies were employed. Some chose to remain in anaerobic environments, while others “acquired” the ability to perform aerobic respiration by engulfing smaller aerobically respiring cells, as suggested by the famous endosymbiotic theory [11, 12]. The latter gave rise to the ancestors of eukaryotic cells, with the engulfed aerobically respiring cells eventually becoming today's mitochondria. And the story of cyanobacteria did not end with the GOE – the endosymbiotic theory also suggests that they were engulfed by early non-photosynthetic eukaryotes [11] and became chloroplasts in modern plants and algae. 地球生命史有著許多關鍵時刻，但也許除了生命 起源本身，沒有一件大事較大氧化事件（The Great Oxidation Event / GOE）的影響更為深遠。大氧化事件 標誌著早期地球大氣層開始充滿遊離氧的時期，為需氧生 物的出現以及最終現代人類興起奠定了基礎 [1, 2]。 試想像回到 45億年前地球剛形成的時候，當時的 大氣層與我們今天擁有的截然不同 — 它由水蒸氣、二 氧化碳和甲烷組成，但不含氧氣。因此，最早約於38億 年前出現的生物均是厭氧生物。 但在大約34 億年前，一群細菌從這些厭氧祖先分化出 一個分支，改變了整個局面 [3, 4]。這 些獨特的微生物發展出地球生命史 上其中一種最創新的能力 — 以 光合作用製造氧氣，而這些微 生物最終演化成我們現 在熟知的藍綠菌（通常 被稱作藍綠藻，但它們 在分類上並不屬於藻 類）（圖一）。 1. Editor’s note: Superoxide dismutase converts harmful superoxide radicals (O2 _. ) to molecular oxygen (O2) and hydrogen peroxide (H2O2). Catalase further converts H2O2 to O2 and water.
RkJQdWJsaXNoZXIy NDk5Njg=