The Precambrian

I thought it would be good to go through the geological eras so that you can get an idea of the fossil fauna, flora and major events.

So firstly, I will be going through the Precambrian as a whole. The Precambrian begins 4.6 Ga (Ga = giga years ago or billion years ago) in the Hadean, its here where the formation of the Earth begins.  Approximately 4.3 Ga a small planet, named Theia, collided with our planet, this is thought to be due to the orbits of the

Above: Artists impression of Theia colliding with the early
Earth. Credit: The Huffington Post

planets being too close. The resulting debris from the impact coalesced to form the early moon. We know this because rock samples from the Apollo Moon landings showed remarkable similarities to the composition of rocks on Earth. For 600 My (My = million years) the Earth remain in a molten state.

4 Ga the Archean begins, this signals the arrival of a more stable but still inhospitable Earth, especially at the surface. A subdivision of the Archean, the Paleoarchean (3.6 to 3.2 Ga), heralds the arrival of something wondrous; life.

The earliest fossils on record come from the middle of the Paleoarchean approximately 3.4 Ga. These are fossils of unicellular bacteria, this bacteria uses fuels to survive. However, as the numbers of bacteria soar, the fuel sources plummet. This prompts the evolution of a new bacteria, Cyanobacteria, a leap forward for life on Earth as these bacteria use the sunlight as fuel for respiration, a process called photosynthesis. These cyanobacteria are the blueprints for the plant life we see today.

During this time, there was virtually 0% of oxygen at the Earth's surface. Approximately 2.4 Ga, the vast amount of photosynthesising Cyanobacteria causes a release of oxygen from the oceans into the atmosphere. The oxygen levels climb until 800 Ma (Ma= million years ago) where they reach 21%. At this time the Ozone layer is forming because of the new gas in the atmosphere, this filters the harmful UV rays that the sun gives off and continues to do so today. This also helps the Earth's surface temperature to lower.

A new gas composition is not without its downfalls. It is believed that due to this increase in oxygen, a vast glaciation of the planet was triggered. This is what we refer to as 'The Snowball Earth Theory'. It took place in the Cryogenian period, during the Proterozoic eon, approximately 850 Ma. This mass glaciation nearly wiped out all life on Earth, if it wasn't for secluded pockets of water that survived the freeze the Earth may have been devoid of life for many more millenia.

The final period of the Proterozoic and the Precambrian as a whole is called the Ediacaran, 635 to 541 Ma. This period is very exciting, this is where we find the very first multicellular organisms. Some of the most alien looking life in the fossil record, these animals were mere centimetres across. The largest of which is the Dickinsonia costata, this odd creature is found in the Ediacara Hills of Southern Australia and Northern Russia. Dickinsonia.

Above: Fossil of Dickinsonia costata from the Winter Coast of the White
Sea in Northern Russia. Credit: ucmp.berkeley.edu

These animals lacked distinguishable body parts, i.e. head, gut etc and are therefore referred to as being 'simple life forms'. It is unknown what happened to these animals, we are unsure as to whether they evolved into the more complex organisms of the Cambrian Period or whether they simply became extinct at the end of the Ediacaran. It is theorised that the Ediacaran ended with a global ice age and it was this that was the reason for the disappearance of animals such as Dickinsonia.