A History of Life on Earth - Major Divisions of Time

What a struggle to find a simple demonstration of the major divisions of time as we understand it today. One of the issues I found is that it is broken up into 4 EONs, which are geographical periods based on billions of years, and ERAs, which are major subdivisions of an EON, and PERIODs, which are subdivisions of an ERA. For example, the Jurassic PERIOD is a period of time within the Mesozoic ERA, which sits within the Phanerozoic EON.

After hours of research, I found that using the proper search criteria (terminology), like PERIOD, EON, ERA or even EPOCH or AGE is very important to finding exactly what I was seeking. I found several visuals, but none made it really clear - they weren't logically organized. They mixed EONs and ERAs and didn't have a good visual of the PERIODs I was interested in, so I have organized it here in an easy to understand format for future reference.

EON: Eonothems have the same names as their corresponding eons, which is based on 4 major geologic time scales. Oldest to newest these are the Hadean, Archean, Proterozoic, and Phanerozoic. The fact that eons are broken up in geologic time scales was new to me. I just had a vague sense that an EON was a really long time.

SUPEREON: There is also a "super" eon, which represents the first three eons, and is called the Precambrian SUPEREON.

Here are a couple conventions that will help your understanding if you research this further.

Ga = Billions of years ago
Ma = Millions of years ago

Here are the 4 EONs:

1. Hadean Eon - 3800 Ma and earlier.
2. Archean Eon - 3800 Ma – 2500 Ma
3. Proterozoic Eon - 2500 Ma – 542 Ma
4. Phanerozoic Eon - 542 Ma – present

I have 3 graphics below to represent these timelines. Note that the second and third timelines are each subsections of their preceding timeline as indicated by asterisks.

The Phanerozoic (latest) Eon, which is the EON I was interested in, literally means the "period of well-displayed life", and marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, which are divided by major mass extinctions.

I was originally looking for a sequence of PERIODs to determine when the bugs ruled the earth. I had seen some History Channel show about a year ago and had forgotten which period this was and where it fit into other periods. I don't know if you get these types of curiosities that bug you for some long period of time until you finally take a few hours to resolve it, but that's something I do on a regular basis. I also document it for future reference.

The graphic above shows it a little, but I have broken it down the latest PERIODs nicely here:

• Cambrian period (542-488 Ma)
• Ordovician (488-444 Ma)
• Silurian (444-416 Ma)
• Devonian period (416-359 Ma)
• Carboniferous (359-299 Ma)
• Permian (299-251 Ma)
• Triassic (251-199 Ma)
• Jurassic (199-145 Ma)
• Cretaceous (145-65 Ma)
• Paleogene (65-23 Ma)
• Neogene (23-2.5 Ma)
• Quaternary (2.5 Ma to now)

To make it more complicated, the above periods are broken down into epochs and ages, such as the Permian period's Cisuralian epoch, and the Cisuralian epoch's Asselian age. The current Quaternary period shows up as just a Q on the right of the graphic below, and the Holocene epoch (the latest epoch) is too short to be show up on this timeline.

During the late Carboniferous and Early Permian periods, some giant dragonfly-like creatures reached wingspans of 55 to 70 cm (22 to 28 in), making them far larger than any living insect. I have read many studies and seen documentaries about this gigantism in other insect-like creatures too. It is suggested that the higher atmospheric oxygen levels (up to 80% above modern levels during the Carboniferous period) allowed increased respiratory efficiency relative to today and allowed this gigantism. I have also read many studies over the years that show modern insects grow much larger in higher oxygenated environments.

So, now that I know the Carboniferous PERIOD is what I am interested in, I can continue my research with more efficiency. I can't tell you how difficult it is to find information now without using the specifics in your search criteria. Inappropriate use of simple words like "period", "era" or "epoch" can get you lost in an ocean of useless information. I hope this helps you with your research!

I have posted the most overall useful link I found on this subject below.