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University of Toronto Scarborough
Biological Sciences
Rudy Boonstra

Chap 3 The Hadean and Archeaen eons 4625 bya Pre biotic earth 4638 bya Earth and solar system formed made from dust and gas cloud some dust was size of small planets Thea collided with earth and made the moon Luna LunarEarth ratio of mass 1183 greater than any ratio in solar system Corpernican principle nothing cosmologically special about Earth having lifeMoon believed to cause axial stability to Earth tilt angle hence climate stability planets with no large moon have half year of dark half year of sunlight on earth only polar regions are like this After collision water came from asteroids and protoearth rocks world was filled with CO water vapour and methane instead of oxygen 21 today 2and nitrogen like today Early sun only 30 as bright as todays so very powerful greenhouse gas effects was needed to keep earth warm enough for life back then Problem1 CO enough to keep the early world from freezing needs to be 3001000X 2more then today and if that was true it would mix with Iron to make iron carbonate and we dont find much of that Problem2One view NH3 ammonia was a major greenhouse gas but this is sensitive to sunlight and needs methane haze reflective layer in atmosphere and methane layer reflects light so things cancel out Also if earth too warm then no methane haze eitherPossibly life itself kept earth warm through methanogenic bacteria which produce 177ppmv methane today but likely 1000X more back then Methane low in atmosphere warms earth opposite of methane haze high in atmosphere cools earth methane only lasts decade today cuz of oxygen but back then anaerobic earth would last 2000 years and combined with CO warms earth up to perfect conditions and so no need for any 2methane haze to protect ammonia Early biotic earth 3823 bya Life arose quickly btwn 3542 bya when heavy bombardment of earth stopped 4bya original life probably resembles hypothermophiles Life likely started on mars and hitched ride to earth on asteroid mars had shallower water less gravity less asteroid collisions etcanaerobic prokaryotes came first bacteria algae then aerobes than eukaryotes 17byaProterozoic eon 250542 bya Anaerobicaerobic transition 2117 bya Photosynthetic organisms evolved 2bya caused oxygenation of the world 6CO 6HO 22 CHO6O From 223bya for about half a billion years oxygen content increased 61262and methane was slowly used up to todays levelMajor glaciationhappened 22bya Snowball Earth 1 Everything between 6 and 16degree latitudes of the equator was ice and stayed like that till sun got hotter volcanoes 2started erupting Current Solar constant 1373Wm Percentage change in solar
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