animal annum archean banded iron formation biostratigraphy cambria cambrian cambrian explosion cenozoic cryogenian earth ediacaran eoarchean eon geologic time scale geologist geology of the moon giant impact hypothesis global boundary stratotype section and point hadean international union of geological sciences isua greenstone belt late heavy bombardment mesoarchean mesoproterozoic mesozoic metamorphic rock meteorite moon neoarchean neoproterozoic oxygen oxygen catastrophe paleoarchean paleoproterozoic paleozoic phanerozoic photosynthesis planetesimal plate tectonics proterozoic radiometric dating red beds redox rodinia small shelly fauna snowball earth supercontinent the ocean collective timetable of the precambrian tonian wales wikimedia commons zircon