On the grossest scale, the planet’s history is measured in eons. As begun in Chapter 10, the Archean Eon spans 3.8 – 2.5 BYA, when Earth’s crust was solid and life was entirely prokaryotic. The Proterozoic Eon was the age of protists, before (most) animal life, 2.5 BYA – 500 MYA. The eon of “animal life”, the Phanerozoic Eon, picks up 500 MYA and continues today.
The Phanerozoic Eon is divided into three eras. The Chapter 9 time scale takes us no further than the Paleozoic Era of “old life”, beginning about 500 MYA. Finally, we will see the first five periods of the Paleozoic Era: Cambrian = 540 – 490 MYA | Ordovician = 490 – 440 MYA | Silurian = 440 – 420 MYA | Devonian = 420 – 360 MYA | Carboniferous = 360 – 300 MYA. These periods are distinguished by broad differences in their overall fossil records. They are not necessarily indicative of milestones in proto-human history.
The timeline on top, from red to yellow, shows the full span of Chapter 9 from 3 BYA to 300 MYA (linear time scale). You can see that this span of time fairly well squares with the Proterozoic eon. Phanerozoic evolution became more fast-paced, so it earns disproportionate coverage. Below the main line is a close-up of the Phanerozoic Eon between 540 and 300 MYA.
Time-helix: Graham, Joseph; Newman, William; and Stacy, John, 2008, The geologic time spiral—A path to the past (ver. 1.1): U.S. Geological Survey General Information Product 58, poster, 1 sheet. Available online at http://pubs.usgs.gov/gip/2008/58/ . This image is released into the public domain as a work of a United States agency.
Timelines: By Scot Fagerland, 6/14/15.
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