The History of Life on Earth

Geologic Timescale

Era

Period

Epoch

From - To
(millions of years ago)

Duration (millions of years)

% Time
Wikipedia Reference
Cenozoic
Quaternary
Holocene
0.0117
0
0.01

0.0%

Holocene
Pleistocene
2.588
0.0117
2.58
0.1%
Pleistocene
Tertiary
Pliocene
5.3
2.588
2.71
0.1%
Pliocene
Miocene
23
5.3

17.7

0.4%
Miocene
Oligocene
33.9
23
10.9
0.2%
Oligocene
Eocene
56
33.9
22.1
0.5%
Eocene
Paleocene
65.5
56
9.5
0.2%
Paleocene
Mesozoic
Cretaceous
145.5
65.5
80.0

1.7%

Cretaceous
Jurassic
199.6
145.5
54.1
1.2%
Jurassic
Triassic
251
199.6
51.4
1.1%
Triassic
Paleozoic
Permian
299
251
48.0
1.0%
Permian
Carboniferous
359.2

299

60.2
1.3%
Carboniferous
Devonian
416
359.2
56.8
1.2%
Devonian
Silurian
443.7
416
27.7
0.6%
Silurian
Ordovician
488.3
443.7
44.6
1.0%
Ordovician
Cambrian
542
488.3
53.7
1.2%
Cambrian
Precambrian
Proterozoic
2500
542
1958
42.6%
Archean   3800 2500 1300 28.3% Archean
Hadean   4600 3800 800 17.4% Hadean
4600
100%

The Earth is estimated to have formed about 4.6 billion (4600 million) years ago, and yet by 3.9 billion years ago, only shortly after the molten planet solidified, the oceans formed, and the asteroid bombardment ceased, there is evidence of the first primitive life. Only in the last 500 million years or so did complex life begin to evolve, and that represents just 12% of Earth's history. The human lineage only diverged from our most recent common ancestor about 5 million years ago; less than half of 1% of that time, and modern Homo sapiens is only between 200,000 and 50,000 years old, depending on your definition. Such vast spans of time are hard for us to comprehend. Even the shapes and locations of the continents, which have been virtually the same throughout all of human history, are now known to have undergone dramatic shifts that influenced ocean currents, animal migration routes, and even the climate. The Earth has undergone major changes since its beginning, and is very, very old. We've only been here for the wink of an eye. We are living now in the midst of the largest plant and animal extinction since the end of the Cretaceous, when the dinosaurs died out. The cause, this time, is not asteroid impact or ice age, but us. Given the ways in which humans have been changing the planet, you have to wonder whether we'll last as long as the dinosaurs did!

For more detailed information about the geologic timescale, and for a timeline of major biological events, see Wikipedia. Here are some questions to research using the links to each time period in the table above.
  1. How old is the Earth? Describe conditions on the primitive Earth.
  2. When did the first life appear? Describe the earliest forms of life.
  3. How did the Earth's atmosphere come to be so rich in oxygen?
  4. What is a "eukaryote"? When did they appear? How do they differ from prokaryotes?
  5. When did the first multi-cellular forms of life appear? How did they arise?
  6. What are the characteristics of a Protist?
  7. Describe the life forms of the Cambrian.
  8. What is a chordate? What is a vertebrate?
  9. What happened during the Ordovician?
  10. What is a mass extinction? What could cause one to occur?
  11. Describe the life forms of the Silurian.
  12. What adaptations are necessary in order to make the transition from aquatic (water) to terrestrial (land) habitats?
  13. Why is the Devonian called the Age of Fishes? How have fishes changed since the Ordovician?
  14. Describe the Carboniferous period. How did forests of that time form today's coal and oil deposits?
  15. What happened at the end of the Permian? What was the result?
  16. Reptiles dominated the Triassic, Jurassic, and Cretaceous. How did they change during this time?
  17. Why is it thought that the dinosaurs went extinct? What was the result of this extinction?
  18. How is the Tertiary different from modern times?
  19. What is an ice age? Why do they occur? When was the last big one?
  20. When did the first humans appear?

The 24 Hour Clock Analogy

piechart timescale

To help you put the age of the Earth in perspective, this analogy may be helpful. If you think of all of Earth's history as having taken place in just one day, then the pie-chart above is a 24 hour clock. The Earth forms at 12:00 am, and cools down from a molten state over the next several hours. The oceans form and the asteroid bombardment diminishes. Very primitive single celled life appears very quickly, before 4:00 am, and photosynthetic organisms appear before 6:00 am. Right around noon, the atmosphere becomes oxygen rich. By 1:00 pm (13:00 h) the single-celled eukaryotes have arrived. By 5:00 pm (17:00 h) the first multicellular forms appear. The first aquatic animals don't arrive until after 8:00 pm. Plants colonize land at 9:30 pm and land animals follow at 10:00 pm. The dinosaurs are the life of the party for about an hour; from 10:40 pm to 11:40 pm. Human ancestors split off from the rest of the hominids at only two minutes to midnight, and modern humans arrive as the clock strikes midnight. You can download this Excel document to see how the chart was made.

24 hour clock

If we overlay a 24 hour clock on the history of the Earth, the great span of time becomes easier to comprehend.

Period
Begins
Ends
Major Events
Hadean 12:00:00 AM 4:10:26 AM Uni-cellular life appears before 4:00 am
Archean 4:10:26 AM 10:57:23 AM Photosynthesis begins around 5:30 am
Proterozoic
10:57:23 AM
9:10:20 PM
First eukaryotes, around 1:00 pm, first multicellular life around 5:30 pm
Cambrian
9:10:20 PM
9:27:08 PM
The Cambrian explosion happens at 9:00 pm
Ordovician
9:27:08 PM
9:41:06 PM
Molluscs and arthropods dominate the seas
Silurian
9:41:06 PM
9:49:46 PM
First land plants
Devonian
9:49:46 PM
10:07:33 PM
The age of bony fishes
Carboniferous
10:07:33 PM
10:26:24 PM
Coal age swamps, amphibians and insects invade land
Permian
10:26:24 PM
10:41:26 PM
Reptiles rising
Triassic
10:41:26 PM
10:57:31 PM
Dinosaurs arrive
Jurassic
10:57:31 PM
11:14:27 PM
Dinosaurs dominate
Cretaceous
11:14:27 PM
11:39:30 PM
Dinosaur extinction
Paleocene
11:39:30 PM
11:42:28 PM
Early mammals
Eocene
11:42:28 PM
11:49:23 PM
Warm, wet climate, modern mammal families
Oligocene
11:49:23 PM
11:52:48 PM
Drying period, continents nearing current positions
Miocene
11:52:48 PM
11:58:20 PM
Hominids appear
Pliocene
11:58:20 PM
11:59:11 PM
Cooling period
Pleistocene
11:59:11 PM
12:00:00 AM
Major ice ages
Holocene
12:00:00 AM
12:00:00 AM
Modern humans arrive and quickly trash the place ;)

Activity: Take a look at this project on building a scale model of the solar system. You'll be drawing a similar timeline highlighting the major periods of time since the formation of the Earth, scaled accurately for length, highlighting the organisms and environmental features that characterized each period. Here's a geologic timeline calculator that will help you scale each period to the correct length after you decide how long your timeline will be overall.