Although the Precambrian time makes up almost 90% of Earth’s history on the geologic time scale, it contains the least amount of evidence of past life from fossils or rocks. Many rocks and fossils were destroyed because of erosion, heat, and pressure. Scientists have been able to use radiometric dating to estimate that the oldest sedimentary rock layer on Earth contained minerals that are about 4.3 billion years old. The environment was hostile when the Earth formed about 4.56 billion years ago. The atmosphere, oceans, and continents were changing and evolving.
The Precambrian consist of three eras: Hadean, Archaean, and Proterozoic. The Earth was developing from a cloud of gas and dust during the Hadean Era. The force of gravity pulled ice, dust, and rock materials from outer space together towards the center of a large massive object which 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. The atmosphere lacked oxygen. It was rich in the gases released from the volcanoes: carbon dioxide, water vapor, nitrogen, and sulfur.
The Earth began to cool down during the Archaean era. The oceans formed as water evaporated in the atmosphere to form clouds and eventually fell down to the Earth. The continuous rainfall filled low lying areas that slowly became the oceans. A lot of the carbon dioxide in the air was removed during this process and dissolved in the oceans to form limestone. The atmosphere was mostly made up of nitrogen. During this time large islands were forming due to erupting volcanoes. They began colliding with each to form micro-continents because of the convection currents within the mantle. 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, mountain building from millions of years ago has exposed the Precambrian shield on Earth’s surface. They can be found on every continent. Precambrian rocks are the oldest rocks in Earth’s history. They contain many valuable minerals. Life most likely evolved in the oceans. The prokaryotic organisms were probably blue-green algae known as cyanobacteria. The fossil remains of cyanobacteria have been found in Archaean rocks that are about 3.5 billion years old.
The oldest eukaryotic fossil was found about 1.8 billion years ago. Based on the fossil remains, geologists believe that these organisms had soft bodies similar to jellyfish, worms, and corals. The atmosphere was gaining oxygen. Cyanobacteria used a process called photosynthesis to release oxygen into the air. Colonies of coral reef-like cyanobacteria are called stromatolites. They live in warm and shallow waters which covered most of Earth’s landmass during the Precambrian time. Scientists believe that the photosynthesizing stromatolites help produced enough oxygen to make a major impact on the atmosphere. The oldest and most common fossils found in Precambrian rocks are of stromatolites.