<P> This is a list of extinction events: </P> <Table> <Tr> <Th> Period or supereon </Th> <Th> Extinction </Th> <Th> Date </Th> <Th> Possible causes </Th> </Tr> <Tr> <Td> Quaternary </Td> <Td> Holocene extinction </Td> <Td> c . 10,000 BCE--Ongoing </Td> <Td> Humans </Td> </Tr> <Tr> <Td> Quaternary extinction event </Td> <Td> 640,000, 74,000, and 13,000 years ago </Td> <Td> Unknown; may include climate changes, massive volcanic eruptions and human overhunting </Td> </Tr> <Tr> <Td> Neogene </Td> <Td> Pliocene--Pleistocene boundary extinction </Td> <Td> 2 Ma </Td> <Td> Supernova? Eltanin impact? </Td> </Tr> <Tr> <Td> Middle Miocene disruption </Td> <Td> 14.5 Ma </Td> <Td>--climate change due to change of ocean circulation patterns and perhaps related to the Milankovitch cycles? . </Td> </Tr> <Tr> <Td> Paleogene </Td> <Td> Eocene--Oligocene extinction event </Td> <Td> 33.9 Ma </Td> <Td> Popigai impactor? </Td> </Tr> <Tr> <Td> Cretaceous </Td> <Td> Cretaceous--Paleogene extinction event </Td> <Td> 66 Ma </Td> <Td> Chicxulub impactor; Deccan Traps? </Td> </Tr> <Tr> <Td> Cenomanian - Turonian boundary event </Td> <Td> 94 Ma </Td> <Td> Caribbean large igneous province </Td> </Tr> <Tr> <Td> Aptian extinction </Td> <Td> 117 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Jurassic </Td> <Td> End - Jurassic (Tithonian) extinction </Td> <Td> 145 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Toarcian turnover </Td> <Td> 183 Ma </Td> <Td> Karoo - Ferrar Provinces </Td> </Tr> <Tr> <Td> Triassic </Td> <Td> Triassic--Jurassic extinction event </Td> <Td> 201 Ma </Td> <Td> Central Atlantic magmatic province; impactor </Td> </Tr> <Tr> <Td> Carnian Pluvial Event </Td> <Td> 230 Ma </Td> <Td> Wrangellia flood basalts </Td> </Tr> <Tr> <Td> Permian </Td> <Td> Permian--Triassic extinction event </Td> <Td> 252 Ma </Td> <Td> Siberian Traps; Wilkes Land Crater; Anoxic event </Td> </Tr> <Tr> <Td> End - Capitanian extinction event </Td> <Td> 260 Ma </Td> <Td> Emeishan Traps? </Td> </Tr> <Tr> <Td> Olson's Extinction </Td> <Td> 270 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Carboniferous </Td> <Td> Carboniferous rainforest collapse </Td> <Td> 305 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Devonian </Td> <Td> Late Devonian extinction </Td> <Td> 375--360 Ma </Td> <Td> Viluy Traps </Td> </Tr> <Tr> <Td> Silurian </Td> <Td> Lau event </Td> <Td> 420 Ma </Td> <Td> Changes in sea level and chemistry? </Td> </Tr> <Tr> <Td> Mulde event </Td> <Td> 424 Ma </Td> <Td> Global drop in sea level? </Td> </Tr> <Tr> <Td> Ireviken event </Td> <Td> 428 Ma </Td> <Td> Deep - ocean anoxia; Milankovitch cycles? </Td> </Tr> <Tr> <Td> Ordovician </Td> <Td> Ordovician--Silurian extinction events </Td> <Td> 450--440 Ma </Td> <Td> Global cooling and sea level drop; Gamma - ray burst? </Td> </Tr> <Tr> <Td> Cambrian </Td> <Td> Cambrian--Ordovician extinction event </Td> <Td> 488 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Dresbachian extinction event </Td> <Td> 502 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> End - Botomian extinction event </Td> <Td> 517 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Precambrian </Td> <Td> End - Ediacaran extinction </Td> <Td> 542 Ma </Td> <Td> </Td> </Tr> <Tr> <Td> Great Oxygenation Event </Td> <Td> 2400 Ma </Td> <Td> Rising oxygen levels in the atmosphere due to the development of photosynthesis </Td> </Tr> </Table> <Tr> <Th> Period or supereon </Th> <Th> Extinction </Th> <Th> Date </Th> <Th> Possible causes </Th> </Tr> <Tr> <Td> Quaternary </Td> <Td> Holocene extinction </Td> <Td> c . 10,000 BCE--Ongoing </Td> <Td> Humans </Td> </Tr>

What 4 occurrences in order needed to occur for life to appear on earth