207_2009_Lec_17Seventeen - Biology 207 2009 Lecture...

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10/21/2009 1 Biology 207, 2009 Lecture seventeen Friday, 23 October Microbial evolution Evolution – a theme that unifies all of biology We consider: •the possible conditions under which life arose •the processes that might have given rise to the first cellular life •its divergence into two evolutionary lineages, Bacteria and Archaea Much remains speculative, but geological and molecular evidence is providing an increasingly clear view of how life may have arisen and diversified. Origin of Earth (and other planets of our solar system) (origin of universe, circa 12.5 billion years ago) Earth, about 4.54 billion years ago, based on data from slowly decaying radioactive isotopes from accretion of materials making up a disc-shaped nebular cloud of dust and gases released by the supernova of a massive old star; a new star, our Sun, formed within this cloud, compacting in on itself and beginning to undergo nuclear fusion and to release heat and light; materials left over in the nebular cloud began to clump and fuse due to collisions and gravitational pull forming tiny accretions that gradually collisions and gravitational pull, forming tiny accretions that gradually grew larger, forming clumps that eventually coalesced into planets. Earth – initially was a fiery hot, molten mass; conditions were completely inhospitable for life; a molten surface under intense bombardment from space by masses of accreted materials. Earth’s early atmosphere – “reducing” (e.g., no free O 2 ) containing mostly water vapor, hydrogen, nitrogen, methane, ammonia, and some carbon dioxide.
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10/21/2009 2 Life requires water. Where did the water come from that formed the oceans of early Earth? Collisions with icy comets and asteroids, volcanic out-gassing of water vapor from the planet's interior. At first, the surface of Earth was hot (several hundred degrees C), so water from comets, asteroids, and volcanoes was always in the vapor phase (steam). Life requires relatively cool temperatures (less than 150 ° C). When did Earth cool enough for water vapor to condense to the liquid state? As the Earth cooled over millions of years, the water vapor condensed as rain and began accumulating as liquid. This rain, because it was somewhat acidic, containing carbon dioxide, sulfur dioxide etc (which form acid when dissolved in water) sulfur dioxide, etc., (which form acid when dissolved in water) dissolved some of the rock material of the surface of Earth, leading to formation of salty water that now exists the Earth’s oceans (ca. 34 parts per thousand salt). The oldest sedimentary rocks, which form under liquid water, are dated to 3.86 billion years old; in the Itsaq gneiss complex, in southwestern Greenland. The sedimentary nature of these rocks indicates that at the latest by that time (3.86 billion years ago) Earth had cooled sufficiently for the water vapor to have condensed and formed the early oceans.
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