Question: when did the first major extinction event take place
Answer: a Viluy Traps Silurian Lau event 420 Ma Changes in sea level and chemistry ? Mulde event 424 Ma Global drop in sea level ? Ireviken event 428 Ma Deep - ocean anoxia ; Milankovitch cycles ? Ordovician Ordovician -- Silurian extinction events 450 -- 440 Ma Global cooling and sea level drop ; Gamma - ray burst ? Cambrian Cambrian -- Ordovician extinction event 488 Ma Dresbachian extinction event 502 Ma End - Botomian extinction event 517 Ma Precambrian End - Ediacaran extinction 542 Ma Great Oxygenation Event 2400 Ma Rising oxygen levels in the atmosphere due to the development of photosynthesis