The earth as we know it today is shaped by what happened during the Precambrian period, which covers four billion years! This lesson covers the events that shaped this important period.
Eons of the Precambrian
The primary way we get an understanding of Earth’s history is by studying the rocks and fossils leftover from various time periods. The difficulty in studying the Precambrian period is that many of the rocks from that time have eroded away or metamorphosed so they are not recognizable in their original form.The beginning of the Precambrian period starts with the formation of Earth about 4.5 billion years ago and ends at the first sign of complex life about 540 million years ago. Though the Precambrian Period is often referred to as a period, it’s actually the only supereon, which means that it spans multiple eons. The Precambrian has been divided into three eons: the Hadean, the Archean, and the Proterozoic.
The Hadean eon took place 4,500 – 4,000 million years ago. As our planet formed, gravity pulled gas and dust from the solar system into what we know now as Earth. Friction then heated the material that would eventually become Earth into a sphere of hot molten lava. At this time, the earth was a very harsh environment; there was no solid crust, just molten lava covering the surface. All the water that we see in the oceans, lakes, rivers, and glaciers were evaporated in the atmosphere.
Just imagine the humidity!If this wasn’t enough, asteroid impacts were a common occurrence on the young planet. Some scientists believe the moon was formed by one such collision. It is theorized that a Mars-sized object crashed into Earth early in the Hadean eon. The theory states that the impact caused a large amount of debris to be ejected from Earth, which later combined to form the moon.
The Archean eon took place 4,000 – 2,500 million years ago. Over time, as the earth cooled, the outer edge of the planet solidified from molten lava to a solid crust. Although it wasn’t perfectly solid, large volcanoes were erupting all over Earth and forming new land. With the crust shifting and cracking below, collisions formed the beginnings of early continents.As the earth cooled even more, the water in the atmosphere began to rain out.
All the water that was evaporated in the atmosphere began to pour down onto the newly formed solid ground. By this eon in the Precambrian, most of the water had rained down from the atmosphere and covered Earth with a global ocean.It is believed that the first signs of life formed during the Archean eon in these oceans. Carbon dating from fossils found in Western Australia show evidence that a small single-cell organism named cyanobacteria, or more commonly known as blue-green algae, was present in the ocean around 3,000 million years ago.
The Proterozoic eon took place 2,500 – 541 million years ago. The start of the Proterozoic eon is based off the earliest carbon dating of the continental rock that has not been deformed. By the Proterozoic eon, the earth had cooled even more, and the number of volcanoes had decreased from what was seen in the Archean eon.Plate tectonics were still very active, and by this eon, there were two supercontinents on either side of the planet. However, these continents look nothing like the ones we recognize today. Not only were they shaped differently, but they were completely void of life.All life was still confined to the oceans.
Around 1,700 million (or 1.7 billion) years ago, the only known life on Earth was still the single-celled blue-green algae, cyanobacteria. It would take another billion years for the algae to evolve into multi-cell organisms.Cyanobacteria were extremely important in adding oxygen to the atmosphere. The early atmosphere was composed of nitrogen and carbon dioxide, but no oxygen. The algae got its energy from the sun and survived by the process of photosynthesis, just like plants today. For two billion years, small single-celled bacteria were pumping oxygen into the atmosphere bit by small bit.
As a cool side note, you might be interested to know that cyanobacteria are still around today!
The Precambrian supereon was a harsh and violent time in Earth’s history. This formative period is broken down into the Hadean, Archean, and Proterozoic eons. Starting with the formation of the earth from gas and dust, to the first signs of life with single-cell algae, the Precambrian has shaped the earth as we know it today. Although much of the evidence of the Precambrian has been lost to time, new technologies and techniques are unlocking the secrets of the past.
The Precambrian Period Overview
An eon is a unit of geological time, and it often refers to a span of one billion years.The Precambrian period refers to the earliest part of Earth’s history. This period is broken into three eons: Hadean, Archean and Proterozoic.
The Hadean eon took place 4,500 – 4,000 million years ago. The earth was a sphere of molten lava.The Archean eon took place 4,000 – 2,500 million years ago. The earth started cooling and the outer edge of the planet solidified from molten lava to a solid crust.
Water rained from the atmosphere and created oceans. The first form of life on our planet was created during the Archean eon in these oceans.The Proterozoic eon took place 2,500 – 541 million years ago. Plate tectonics were very active. Aside from the oceans, only two continents were located on either side of the planet. Single-celled organisms confined to the oceans were the only form of life on Earth.
These organisms slowly pumped oxygen into the atmosphere.
Express your knowledge of this lesson by doing the following things:
- Name the three eons of the Precambrian period
- Provide specific details about each eon