Lecture 6 Hadean was the first time on earth, nicknamed hell on earth, beacuase the rocks were molten, no ocean, no atmosphere But then the world differentiated, heavy elements sank down becoming the core, mantle, crust, etc. This also allowed a magnetic field to be developed, which created the conditions to build an atmosphere by constricting the gases and oceans Four major steps to life on earth 1. The earliest bacteria 3.8 Ga 2. The Cambrian explosion 542 Ma (multicellular organisms with shells, backbones, etc.) 3. Diversification during the Paleozoic c.400-240Ma 4. Role of meteorites A fossil is formed by any cavities filled in with silica, calcium carbonate, iron a process called mineralization Permineralization is the process that forms fossils: 1. Organism dies in or near water 2.