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. 1

The predominant theme of the last few billion years was the maturation of life.  In this chapter, our ancestors evolved from bacteria to back-boned animals!   As we trace this history, we will refer to the standard geological / paleontological divisions of time.  As begun in Chapter 10, the Archean Eon spans 2.5 – 3.8 BYA, when Earth’s crust was solid and life was entirely prokaryotic.  The Proterozoic Eon, 0.5 – 2.5 BYA, was the age of protists, the next wave of single-celled life.  Protists are introduced in Section II, and their most popular invention, sexual reproduction, is the subject of Section III. The eon of plants and animals, the Phanerozoic, picks up 500 MYA and continues today. The Phanerozoic Eon is divided into three eras, the first of which was the Paleozoic or “ancient life” era. 

Most major body features of animals, from the skin to the brain, appeared in the Paleozoic Era. Section IV will discuss the fundamentals of sexual evolution before Section V surveys the specific biohistory of human ancestors.  With a developing brain came self-awareness and primordial behaviors such as mating, hunting, and communication.  Animal behavior is covered in Section VI.  Finally, Section VII reviews contemporaneous developments in the rest of the world:  the geophysical planet as well as the plants and animals that are not part of our direct lineage. 

To measure out a billion years, we’d need an hourglass 20,000 feet tall — the size of a high mountain.

Click here for attributions. 2    


Continue to Section 9.II:  Oxygen And Eukaryotic Cells


  1. 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.
  2. Mountain icon by Memed_Nurrohmad, Pixabay license, https://pixabay.com/illustrations/mountain-icon-sign-design-symbol-1849091/.  Egg / sperm icon “reproduction” by Edwin PM, https://thenounproject.com/term/reproduction/1332876/ .  “Yellow sponge with green algae” by Luayana, https://www.dreamstime.com/yellow-sponge-green-algae-white-background-yellow-sponge-green-algae-image120195101 .  Fish icon by kreatikar, Pixabay license, https://pixabay.com/illustrations/clipart-fish-sea-water-swim-3418189/.  All accessed, saved, and archived 9/12/20.
Please Like or Share!

Facebook comments preferred; negative anonymous comments will not display. Please read this page / post fully before commenting, thanks!

Powered by Facebook Comments

One thought on “9.I: Geological Time Periods

Leave a Reply

Your email address will not be published. Required fields are marked *