The History of Life on Earth: Timeline and Characteristics of Major Eras

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  • 0:32 Radioactive Dating
  • 2:26 Precambrian
  • 3:38 Phanerozoic Eon
  • 6:10 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

The Earth is over 4 billion years old. Ever wonder what happened during certain time periods in that large amount of time? We will look at the major eons and eras of Earth's history along with important characteristics of each.

Radioactive Dating and the Fossil Record

Generally, when you think about history, you may think of learning about things like wars, invasions and empires. However, in comparison with the history of Earth, these things are all very recent events. While current history is definitely important, we are going to look at the history of Earth, which starts more than 4.6 billion years ago. Before we get to the specific eons and eras and their characteristics, let's first talk about how we know when things happened.

Scientists sometimes look at the relative age of rocks - that is, they compare a few things to decide which is the oldest and which is the newest. They know that layers buried deeper are older than layers near the surface. This is kind of like if you look at dirty clothes in a laundry hamper. The clothes that you wore long ago will be towards the bottom, while clothes that you just wore will be near the top. This same idea is used to determine the relative age of rock layers and of the fossils within those rocks.

For more specific information about the age of a rock layer or fossil, scientists use radioactive dating. This process is based on the half-life of radioactive substances. You may be unfamiliar with these terms, so let's go over half-life and radioactivity. Half-life is the length of time it takes for half of a sample of a substance to decay or break down. For example, if we have ten grams of something, however long it takes for five grams to break down is the half-life. Radioactive decay is basically when, over time, one element breaks down and becomes another element. Different elements have different rates of radioactive decay. For most of the work that scientists do concerning the history of life on Earth, carbon-14 is used.

Timeline of Major Eons and Eras

Timeline of the eons and eras of Earth
Eons and Eras Timeline

Now that we know how scientists determine how old things are - both relatively and more specifically - let's look at a general timeline of the history of life on Earth. We will come back to this image as we discuss specific characteristics and events in each era. Let's look at a few general things on this timeline first.

We can see that the Earth's history is broken into three eons: Archean, Proterozoic and Phanerozoic. Some of these eons are separated by mass extinction - that is, a large loss of life - while others are separated by a great increase in the diversity of life. These decreases and increases in diversity may have been caused by different things but, nonetheless, they help define periods within Earth's history.


About 4 billion years ago, life forms on Earth were merely simple cells
Early Archean Eon

The Precambrian groups together both the Archean and Proterozoic eons and lasted about four billion years. Let's look at the Archean eon first.

The Archean eon started with the formation of Earth about 4 billion years ago. During this time period, Earth looked very different than it does today. We know this because of rocks, fossils and other samples, such as core samples taken from glaciers. Within these rocks, scientists have found fossils of early life forms that are over 3.5 billion years old! These old life forms were simple but helped to change the atmosphere of early Earth. They helped to produce more oxygen, making the atmosphere more similar to what we have today. It also allowed for changes in the forms of life that could exist on early Earth.

Following the increased production of oxygen, there was an increase in the number of organisms on Earth. This transition led in to the Proterozoic eon, which began around 2.7 billion years ago. During this eon, the simple cells that were found in the Archean eon continued to evolve, and some became more complex. Instead of just simple single-celled organisms, multicellular organisms - and even soft-bodied invertebrates like worms, sponges and jellyfish - were found towards the end of the Proterozoic eon.

Eras of the Phanerozoic Eon

The Phanerozoic eon covers a small portion of Earth's history, but we have the most information about this eon. Covering only about the last half-billion years, this eon is divided into three smaller units known as eras: the Paleozoic, Mesozoic and Cenozoic. Now, even more confusing than that, each of those eras is divided into smaller units known as periods. We will not look at most of these periods but will rather focus on the eras and their defining characteristics. However, we do need to look at the period that started the Phanerozoic eon.

The Cambrian period is the first division of time in the Phanerozoic eon. It is important because of the mass increase in diversity of life that helps separate the Phanerozoic eon from the previous Proterozoic eon. The Cambrian explosion was a huge increase in the number and diversity of life. Instead of just a few simple organisms, this time period saw an abundance of new life - including the movement of life from water to life on land.

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