MPE - Precambrian History Lesson
Precambrian History
The Earth was formed 4.6 billion years ago and 90% of that history is included in what is known as the Precambrian time. The name means "before the Cambrian period" spans from the formation of Earth, 4.6 billion years ago to about 541 million years ago and includes three eons: The Hadean, Archean, and Proterozoic eons.
Though the Precambrian time frame is about 90% of Earth's history, it contains the least amount of evidence of past life from fossils or rocks. When the earth formed, the environment was hostile and the atmosphere, oceans, and continents were changing and evolving. Many rocks and fossils formed during this time period were probably destroyed because of erosion, heat, and pressure so we have less evidence than from the Cambrian time period.
Haden Eon - 4.6 to 3.9 billion years ago
The Haden eon began with the formation of Earth which developed from a cloud of gas and dust. During this time period the force of gravity pulled ice, dust, and rock materials from outer space together towards the center of a large massive object that eventually formed the planet Earth. The surface of the Earth consisted of erupting volcanoes, craters from the bombardment of meteorites, and extremely hot pools of lava.
Though the atmosphere lacked oxygen, it was rich in the gases released from the volcanoes: carbon dioxide, water vapor, nitrogen, and sulfur. Any rocks that did form from cooling lavas were quickly buried under new lava flows or blasted to bits by meteorites from space. No one has found any rocks on earth from this era as only meteorites from space and moon rocks are this old. If any life formed on earth during this era, it was probably destroyed in the hostile environment. About 4.3 billion years ago, the atmosphere began to cool down and rain fell to the earth helping to start the creation of the oceans.
Archean Eon - 3.9 to 2.5 billion years ago
Archean refers to the "Ancient" or "Primitive" time. This eon marks the beginning of geologic time as erosion and plate tectonics probably destroyed all evidence for the previous eon. The Earth continues to cool down; water evaporated into the atmosphere to form clouds and continued to find it's way to earth in the form of rain. The water filled oceans and started creating water reservoirs on the land masses that were forming. Most of the carbon dioxide has been chemically changed into limestone and deposited into the bottom of the ocean and the atmosphere is now mainly nitrogen.
While the surface of the Earth is much cooler, the interior of the Earth is still hot and active causing many volcanoes to still erupt. These volcanoes create long chains of small islands. These islands are carried over the surface of the earth by movements of rocks deep in the Earth's interior. This movement is called plate tectonics. These islands begin colliding with each other, forming micro-continents.
Since the interior of the Earth was very hot, scientists believe that Precambrian rocks would consist of metamorphosed sedimentary and igneous rocks. These rocks are also known as Precambrian shields, large areas of exposed rocks. Erosion, volcanic activity, and mountain building from millions of years ago has exposed the Precambrian shield on Earth's surface. Precambrian rocks are the oldest rocks in Earth's history and which can be found on every continent.
Life during the Archean time most likely evolved in the oceans. There is evidence of prokaryotic organisms, which were blue-green algae known as cyanobacteria. Other than that there is no other life on Earth. The oldest known fossils - the remains of different types of bacteria - are in Archean rocks about 3.5 billion years old.
Proterozoic Eon - 2.5 billion years ago to 540 million years ago
The Proterozoic or "Early Life" eon is the longest time period in geologic history lasting around 2 billion years and there are lots that has happened.
The Proterozoic eon is divided into three distinct eons:
- Paleoproterozoic
- Mesoproterozoic
- Neoproterozoic
One of the most important features of the Proterozoic eon was the buildup of oxygen in the atmosphere. Oxygen released by the blue-green algae floating in the ocean started to collect in the air. Though oxygen has been released for the past two billion years, it has been combining with other elements such as iron to form mineral deposits around the world. This build up of oxygen is going to help foster the great change in life that is about to occur in the next eon.
Though there are fewer volcanoes, this time period is marked with lots of plate tectonic activity and more land masses. Continental collisions are still frequent, every few hundred million years or so, and it is commonly accepted that during this period of time, the Earth went through several building and breakups of these land masses. In the late Proterozoic time period, the dominant supercontinent was Rodina, which consisted of several continents attached to a central craton. There is an example of orogeny, or mountain building, associated with the creation of Rodina that can be seen across the United stated today called the Grenville orogeny located in Eastern North America.
Life is still only found in the oceans, but there is now the formation of eukaryotic cells or cells that have a nucleus (1.7 million years ago). After the formation of a nucleus, multi-celled life was able to appear about 30 million years ago before the end of this time period. Because these multicellular organisms have no hard shells, bones or teeth, finding fossils is almost impossible. There is evidence of the first symbiotic relationships between mitochondria and chloroplasts and their hosts. The evolution of eukaryotes did not stop the expansion of cynobacteria. In fact, stromatolites reached their greatest abundance and diversity roughly 1200 years ago.
Other important events include the first of four major glaciations or ice age events occurred 800 million years ago. 600 million years ago, the atmosphere, with the build up of oxygen, started to form a protective ozone layer over the planet. The first mass extinction event happened during this time period almost 540 million years ago affecting single celled algae and soft bodied organisms.
IMAGES CREATED BY GAVS OR OPENSOURCE