9.I: Geological Time Periods

If you haven’t seen this great time-helix before, click it to study in detail. Chronologically, most of Chapter 9 was pre-Cambrian when life was single-celled or very primitive. We will also break through the Cambrian Explosion into the Paleozoic Era.

When geologists and paleontologists talk about Earth’s past, they label eras with a variety of adjectives that sound like they come from a Tolkien or Rowling novel.  Many of these names are taken from places where fossils were discovered, offering no clues about their timeline.  In this book, I prefer to measure time in plain English terms, ie millions or billions of years ago.  Sometimes it does make sense to refer to the particulars of an age, so I can’t avoid using some common geological terms.  For reference, here is a breakdown of the geological ages of the last few billion years.

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.



Back to Chapter 9

Continue to Section 9.II:  Oxygen and Eukaryotic Cells


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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