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The Earth is over 4.

6 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.

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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 longer 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.

Precambrian

About 4.6 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 4.6 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 exists on early Earth.Following the increased production of oxygen, there was an increase in the number of organisms on Earth. This transition led into 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.The Cambrian period is within the Paleozoic Era. This era saw in increase in the number and diversity of life.

It started with an increase in animal life during the Cambrian explosion, but this increase was also seen in plants. As plants became more diverse and complex, other organisms also evolved. The first reptiles were seen towards the end of the period, along with the origin of most insects. However, during the end of the Paleozoic Era, there was a mass extinction of both land and water organisms. Now, the mass extinction doesn’t mean that everything died off, but many life forms did not survive.

An increase in mammals and primate diversity occurred during the Phanerozoic eon
Phanerozoic Era

The Mesozoic Era is known as the age of the dinosaurs.

These reptiles ranged greatly in size and function. You may automatically think of huge dinosaurs like tyrannosaurus or stegosaurus, but there was much diversity in the dinosaur population. This era saw mostly gymnosperm plants, which do not produce flowers.

However, much like the era before it, the Mesozoic Era ended with a mass extinction. This mass extinction is what killed the dinosaurs about 66 million years ago.The last, and current, era is the Cenozoic Era. This era ranges from the mass extinction of the dinosaurs to current day and is divided into smaller units. Early on, there was much diversity of mammals, birds and insects.

This was followed by an increase in mammals and the diversity of primates, including apes. This era includes the emergence and evolution of human’s early ancestors, the ice ages and even historical time – like that which you normally learn about in a history class.

Lesson Summary

Scientists determine how old things are both relatively and specifically.

In order to determine the specific age of rocks and fossils, radioactive dating is used. For most fossils of living things, carbon-14 is used.The Earth’s history is divided into three major eons: Archean, Proterozoic and Phanerozoic. The Archean and Proterozoic eons together are called the Precambrian. These eons saw the gradual appearance of life on Earth, followed by the gradual evolution of simple organisms.

The Phanerozoic eon started with the Cambrian explosion, which was a massive increase in the number and diversity of life on Earth. The Phanerozoic eon covers the most recent times in Earth’s history, and therefore, we know the most about it. We looked at three specific eras of time within this eon. The Paleozoic Era saw a continued increase in the diversity of life and the movement of organisms from water to land.

However, it ended with a mass extinction. The Mesozoic Era is best known as the age of the dinosaurs. These reptiles ruled the Earth until there was a mass extinction ending the era.

The last, and current, era is the Cenozoic. This era has seen a great increase in the diversity of mammals, including the evolution of humans, and covers Earth’s modern history. All of these eons, eras and periods in Earth’s history have helped define what we know about our planet and our past.

Learning Outcomes

After watching this lesson, you should be able to:

  • Explain how scientists learn the relative and specific age of rocks
  • Identify and describe the three eons of Earth’s history, including what kinds of life existed in each and how the eons are separated
  • Define the Cambrian explosion

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