Geologic time scale showing the geologic eons, eras, periods, epochs, and associated dates in millions of years ago (MYA). The time scale also shows the onset of major evolutionary and tectonic events affecting the North American continent and the Northern Cordillera (SCAK, south-central Alaska; SEAK, southeast Alaska; NAK, northern Alaska; CAK central Alaska). The following subdivisions and events are included on the time scale, from oldest to youngest. The oldest subdivision of the time scale is the Precambrian (symbolized by PC, X, Y, or Z in the GRI GIS data). The Precambrian is split into three eons: Hadean (4600-4000 MYA), Archean (4000-2500 MYA), and Proterozoic (2500-541 MYA). Global evolutionary and tectonic events that occurred during the Precambrian include (organized from oldest to youngest and including the eon in which the event occurred): formation of the Earth 4,600 MYA (Hadean); formation of the Earth’s crust (Hadean); origin of life (Hadean); oldest known Earth rocks (Archean); early bacteria and algae (stromatolies; Archean); simple multicelled organisms (Proterozoic); Kanektok Metamorphic Complex (oldest known rocks in Alaska; Proterozoic); and complex multicellular organisms (Proterozoic). The next subdivision of the timescale is the Phanerozoic Eon (541.0 MYA-present). The Phanerozoic Eon is split into three eras: Paleozoic (541.0-252.2 MYA; symbolized by PZ in the GRI GIS data), Mesozoic (252.2-66.0 MYA; symbolized by MZ in the GRI GIS data), and Cenozoic (66.0 MYA-present; symbolized by CZ in the GRI GIS data). The Paleozoic Era is split into seven periods (organized from oldest to youngest and including the geologic map symbol used in the GRI GIS data): Cambrian (541.0-485.4 MYA; C); Ordovician (485.4-443.4 MYA; O); Silurian (443.3-419.2 MYA; S); Devonian (419.2-358.9 MYA; D); Mississippian (358.9-323.2 MYA; M); Pennsylvanian (323.2-298.9 MYA; PN; the Mississippian and Pennsylvanian are also collectively known as the Carboniferous); and Permian (298.9-252.2; P). Major evolutionary and tectonic events that occurred during the Paleozoic include (organized from oldest to youngest and including the period in which the event occurred): Wales Orogeny (SEAK; Cambrian); early shelled organisms (Cambrian); rise of corals (Cambrian); trilobite maximum (Ordovician); primitive fish (Ordovician); mass extinction (Ordovician-Silurian); first land plants (Silurian), Kakas Orogeny (SEAK; Silurian); first forests (evergreens; Devonian); extensive plutonism and volcanism in the Yukon-Tanana and Brooks Range (Devonian); first amphibians (Devonian); mass extinction (Devonian); Ellesemerian Orogeny/Antler Orogeny (Devonian-Mississippian); ancestral Rocky Mountains (Mississippian); first reptiles (Mississippian); sharks abundant (Pennsylvanian); coal-forming swamps (Pennsylvanian); supercontinent Pangaea and Tethys Ocean (Pennsylvanian-Permian); and mass extinction (end Permian). The Mesozoic Era is split into three periods (organized from oldest to youngest and including the geologic map symbol used in the GRI GIS data): Triassic (252.2-201.3 MYA; Tr), Jurassic (201.3-145.0 MYA; J), and Cretaceous (145.0-66.0 MYA; K). Global evolutionary and tectonic events that occurred during the Mesozoic include (organized from oldest to youngest and including the period in which the event occurred): flying reptiles (Triassic); first dinosaurs and first mammals (Triassic); breakup of Pangaea begins (Triassic); mass extinction (end Triassic); Talkeetna arc (Jurassic); dinosaurs diverse and abundant (Jurassic); Brookian Orogeny (Jurassic-Cretaceous); early flowering plants (Cretaceous); opening of the Canada Basin and rotation of Arctic Alaska (Cretaceous); placental mammals (Cretaceous); exhumation of the Nome Complex (Cretaceous); extensive plutonism (Cretaceous), mass extinction (end Cretaceous). The Cenozoic Era is split into three periods (organized from oldest to youngest and including the geologic map symbol used in the GRI GIS data): Paleogene (66.0-23.0 MYA; PG), Neogene (23.0-2.6 MYA; N; together the Paleogene and Neogene are also known as the Tertiary [T]), and Quaternary (2.6 MYA-present; Q). The Paleogene is split into three shorter subdivisions called epochs: Paleocene (66.0-56.0 MYA; EP), Eocene (56.0-33.9 MYA; E), and Oligocene (33.9-23.0 OL). The Neogene is split into two epochs: Miocene (23.0-5.3 MYA; MI), and Pliocene (5.3-2.6 MYA; PL). The Quaternary is split into two epochs: Pleistocene (2.6-0.01 MYA; PE), and Holocene (0.01 MYA-present, H). Global evolutionary and tectonic events that occurred during the Mesozoic include (organized from oldest to youngest and including the epoch in which the event occurred): early primates (Paleocene); slab-window subduction (SCAK; Paleocene-Eocene), start of Bering Sea Volcanic eruptions (Oligocene); Alaska Range uplift (CAK; Oligocene); spread of grassy ecosystems (Miocene-Pliocene); modern humans (Pleistocene); ice age glaciations (Pleistocene); extinction of large mammals and birds (end Pleistocene); end of the ice age (end Pleistocene).