Laura Henderson
GY 102

Precambrian Period – The beginning of life

Our planet formed from a cloud of dust and gas some 4.56 billion years ago. The Earth’s vast history is divided in four major eras: the Precambrian, the Paleozoic, the Mesozoic and the Cenozoic. The Precambrian era covers most of our planet’s history and lasted from Earth’s inception until 542 million years ago. It was during this era that the Earth became solid, the atmosphere became breathable and life emerged.
There is little evidence about the events that led to the creation of our planet billions of years ago. Most of the knowledge we have about Earth’s beginnings comes indirectly from meteorites that smashed into our planet or from rocks gathered from the Moon. The Earth is just a miniscule part of the immense Universe surrounding us but for us it constitutes the center of it and the only planet known to us that bears any form of life. Our planet is part of the Milky Way Galaxy and it constantly revolves around the Sun and around itself which causes the night and day alternation and the season changes.
The Precambrian period covers about 90% of Earth’s history and is divided into three periods: the Hadean, the Archean and the Proterozoic. The Earth formed some 4.56 billion years ago during the Hadean period which lasted from Earth’s inception to about 4 billion years ago. Initially, there was nothing but an enormous cloud of gas and dust. Progressively, the cloud began to rotate and contract and about 10 million years later the major planets that make up our Solar System - including the Earth - formed. During the Haden period our planet had an atmosphere deficient in oxygen, there was no life on it, its surface was mainly molted lava and the planet was constantly bombarded by meteorites. The Earth’s atmosphere was mainly water vapor, nitrogen, carbon dioxide and a combination of other gases in small percentages. There is no evidence of rocks or any form of life from the Hadean period.
The Archean period lasted from about 4.0 billion until 2.5 billion years ago and major changes happened during this period. Although the Earth’s surface was still filled with active volcanoes, during the Archean the planet began cooling down, the crust started shaping up, the ocean and the ocean floor began forming and life appeared. The earliest life form emerged on the surface of the ocean and it was the microscopic green algae known as cyanobacteria. The oldest proof of life discovered comes in the form of cyanobacteria colonies named stromatolites and is dated some 3.5 billion years ago. Although rare, stromatolites still exist today and some can be seen in shallow, salty waters such as Shark Bay, Australia. Before the appearance of cyanobacteria there was no oxygen in the atmosphere. As cyanobacteria released more and more oxygen into the atmosphere through photosynthesis, eventually the level of free oxygen reached 10% by Cambrian.
During the Archean period the core of today’s continents and the plate tectonics formed. The continents were much smaller than today, were covered by igneous rocks and there were no living organisms on them. Although most of the Archean rocks have been eroded or covered by younger strata, Precambrian rocks can still be found in the center of the continents. Those rocks are called cratons. At the center of the craton sits the continental shield. In North America such rocks can be seen north of Michigan where the Canadian Shield covers about 3 million square miles extending into eastern and central Canada. The Canadian Shield forms the core of the North American continent.
The Archean was followed by the Proterozoic period. The Proterozoic lasted from about 2.5 billion to 592 million years ago and it resembles our world as we know it today. The Proterozoic is divided into three periods: the Paleoproterozoic (lasted from about 2.5 to 1.6 billion years ago), Mesoproterozoic (lasted from about 1.6 to 1 billion years ago), and Neoproterozoic (lasted from about 1 billion to 542 million years ago).
During the Proterozoic period two major continents began shaping up. The two continents were stretching along the equator on opposite sides of the Earth. There were less active volcanoes and the Earth’s core cooled down some more. Although life on land was still inexistent at the beginning of Proterozoic, it was during this era that the first single-celled being with a nucleus emerged. Furthermore, about 30 million years before the end of the Proterozoic era, multi-celled life forms appeared. Those organisms are called eukaryotes and some of the earliest found eukaryotes fossils are acritarchs. Acritarchs were found in rocks from 1.6 billion years ago. The acritarchs became almost extinct by about 675 million years ago and they are considered index fossils for the Proterozoic period. The Proterozoic living creatures began releasing oxygen in the atmosphere. Initially, the oxygen was absorbed by the Earth’s rocks and evidence of this is found in Proterozoic banded iron formations (BIF). Banded iron formations are rocks composed of alternating layers of hematite, magnetite, chert and shale and some of the oldest once discovered are dated from about 3 billion years ago. There is no evidence of banded iron formations after the Archean period. Eventually, some 800 million years ago, our planet’s oxygen level built up to levels high enough to make the development of more complex creatures possible. The first multi-celled animals appeared about 600 million years ago and they are known as Ediacarans, named after the Ediacaran hills of South Australia where they were found.
Smaller land masses continued accreting and the first supercontinent, Rodinia, formed some 1 billion years ago. The Iapetus Ocean also formed during the Proterozoic period, around 600 million years ago. It was also during the Proterozoic that three major orogens that shaped North America occurred: the Hudsonian Orogeny to the east, the Wopmay Orogeny in Northwest Canada, and the Greenville Orogeny which extends from the Labrador Trough to Texas.
Even though there were living creatures and the continents began shaping up, the Earth during the Proterozoic era was still a harsh place. The weather was very cold and towards the end of the Proterozoic most of our planet’s surface was covered by ice sheets. The extended Ice Age led to mass extinction and many of the feeble life forms created during the Precambrian ceased to exist towards the end of Proterozoic. However, the Precambrian era was a very important period in our planet’s history, a period when the Earth developed into a solid sphere and it cooled down, the plate tectonics developed, the atmosphere became breathable and life emerged for the first time.

Interview – Ask a Geologist

1. How did you get interested in science?
Julie Dumoulin: I became interested in dinosaurs as a kindergartener, based on picture books about prehistoric life. My mom and dad often took me hiking and we picked up various rocks and fossils, which sparked a life-long interest in geology.
2. What do you like about your field?
Julie Dumoulin: I love the fact that geology combines aspects of many sciences--e.g., biology, chemistry, and physics--in a single field.
3. What interests you about the Precambrian Period?
Julie Dumoulin: I am particularly interested in the origins of life.
4. What do you think is the most fascinating discovery of the Precambrian Period?
Julie Dumoulin: The earliest evidence for life.
5. As a student, what recommendations do you have for me?
Julie Dumoulin: Find a field or fields that interest you and follow your passion(s)! Never stop asking questions and looking for answers.

Works Cited

Levin, H. (2012). The Earth Through Time. Danvers: John Wiley & Sons, Inc.
Precambrian Time. (2013). Retrieved July 1, 2013, from National Geographic Webpage: http://science.nationalgeographic.com/science/prehistoric-world/precambrian/
Schaetzl, R. (n.d.). The Precambrian Era. Retrieved July 1, 2013, from Michigan State University: http://web2.geo.msu.edu/geogmich/Precambrian.html
Speer, B. (1997, March 9). The Archean Eon and the Hadean. Retrieved July 1, 2013, from University of Claifornian Museum of Paleontology (Berkley) website: http://www.ucmp.berkeley.edu/precambrian/archean_hadean.php
The Proterozoic. (2013). Retrieved July 1, 2013, from Smithsonian National Museum of Natural History Web site: http://paleobiology.si.edu/geotime/main/htmlversion/proterozoic3.html
Windley, B. F. (2013). Archean Eon. Retrieved July 1, 2013, from Encyclopaedia Britannica: http://www.britannica.com/EBchecked/topic/32681/Archean-Eon