During the Archean eon life started and was dominated by one-celled prokaryotic life forms. Eukaryotic one-celled organisms became dominant in the Proterozoic eon . Multicellular organisms ruled the planet during the eon known as the Phanerozoic . Table 10b-1 describes some of the important geologic events that have occurred since the Earth's formation some 4.6 billion years ago. Geologic time scale.
Modern humans develop.
Pleistocene Ice Age Interglacial. Pleistocene Ice Age. Extinction of many species of large mammals and birds. Cascade Mountains began forming. Chimpanzee and hominid lines evolve.