Lect32_Life_Biogeography_2013_NKW

Indicang persistence also carbonate deposits predicted

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Unformatted text preview: ar@facts in calcula@ng divergences –  In fact, later studies based on many more genes and many more lineages have reduced the discrepancy –  Es@mated animal age at ~700 mya •  Animals existed but were @ny or soe and lee no fossils –  Snowball Earth Snowball Earth •  Models 1960’s: Lowering of greenhouse gases  ­> cooling  ­> glacial ice and snow reflect heat ­ ­> posi@ve feedback for temperature drop •  Albedo = frac@on of incoming radia@on reflected back, increases with more ice •  Self ­reversing: Escape from the snowball through CO2 effect (J Kirschvink 1992) •  Evidence: synchronous glacial deposits globally, at or near equator, through reversals of magne@c fields (indica@ng persistence) –  Also carbonate deposits predicted in the greenhouse sequel to the Snowball Earth have been found at many sites, capping the glacial deposits –  Many other predic@ons now being tested and extended –  Snowball Earth view is increasingly accepted •  Implica@ons for Life on Earth? •  One of the hypothesized Snowball stages was about 550 Mya, beginning of Cambrian 6 4/9/13 Science 2004 Vernanimalcula - “Small spring animal” (after the ‘winter’ of Snowball Earth) ~ 180 microns long = 580-600 Mya Implies Bilateria arose well before Cam...
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