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13.1 Precambrian Time -Earth formed 4.56 b.y.a. (billion years ago) -atmosphere formed -ocean formed -tectonic plates began to build up continental land Earth Forms -Gravity pulled dust, rock, & ice in -Gravity increased, pulling in more of these -High-velocity impact of rocks from space caused planet to melt -Dense materials (iron & nickel) sank toward center (core) -Less dense materials (silicates) “floated” forming layers (mantle & crust) Atmosphere Evolves -Earth’s original atmosphere was similar to gases released in volcanic eruption: water vapor, carbon dioxide, nitrogen, & trace gases (NO oxygen) -Earliest life forms began to change atmosphere by using carbon dioxide and releasing oxygen -Slowly OXYGEN increased Oceans Form -As planet cooled, water vapor condensed to form clouds and great rains began -This sped up the cooling of Earth’s surface -Torrential rains slowly filled low areas = OCEANS Continents Develop -mantle convection drove the process of plate tectonics -Rock Cycle began -early continents were granitic igneous rocks -weathering & erosion led to sedimentary rock formation -tectonic forces & volcanic activity helped form metamorphic rock Shields -large, relatively flat expanse of ancient metamorphic rock within the stable continent interior -shaped like a warrior’s shield Precambrian Life -earliest life probably evolved in oceans -near hydrothermal vents -one-celled organisms evolved that use photosynthesis (Prokaryotes) -similar to cyanobacteria -photosynthesis- use CO₂ and sunlight to make food (carbohydrates) Stromatolites -trace fossils left by cyanobacteria (Precambrian time) -are layered mounds of calcium carbonate deposited by cyanobacteria Prokaryotes & Eukaryotes -Prokaryotes- cell that lacks a nucleus -one celled Eukaryotes - Cells that have a nucleus -more complex multi-cellular organisms evolved Eukaryotes with soft bodies evolved. -can be multi-cellular Multicelled Organisms Ediacaran fossils (Australia) -molds and casts of soft bodied animals -resemble worms, jellyfish, coral *To be preserved as fossils, they must have been rapidly buried in fine sediment