Timeline 2.5 Billion to 541 Million Years The Proterozoic Aeon (The age of slime)

Spanning almost 2-billion years, the Proterozoic era this fascinating aeon started in a time when the air wasn’t even breathable to us and when the only life consisted of microbial mats covering the ocean beds. The planet was a lonely place at the dawn of the aeon, an inhospitable world.

The Proterozoic that gave birth to the first true animals and set the precedent for the Cambrian Explosion, perhaps the most amazing event in the history of our world, that which heralded the beginning of the Cambrian period and the beginning of the aeon we still live in to this day.

Highlights of the Proterozoic

  • The Oxygen Catastrophe transforms Earth
  • Several major glaciations occur
  • Supercontinents rise and fall
  • Multicellular life takes over
  • First sexual reproduction
  • First primitive animals evolve

Oxygen Becomes a Weapon of Mass Destruction

CyanobacteriaCharles | Earthly Universe

Cyanobacteria, often incorrectly referred to as ‘green algae’, is the photosynthetic organism believed to have been responsible for the transformation of the Earth’s atmosphere in the so-called Oxygen Catastrophe.

At the turn of the Archean, methane in the atmosphere meant the sky was tinted with shades of red or orange. The oceans had a distinct greenish tint in them, due to cyanobacteria and large quantities of iron.

The first occurrence of photosynthesis, a process that produces oxygen, occurred almost a billion years before the beginning of the Proterozoic. For near a billion years, the Earth’s atmosphere managed to retain a balance, but that all changed in the beginning of the Paleoproterozoic era 2.5- to 2.3-billion years ago. As microbial mats of photosynthetic cyanobacteria, which still exist to this day, colonised the entire globe, the amount of oxygen produced reached critical mass.

As the Earth’s atmosphere shifted completely out of balance, what is commonly known as the oxygen holocaust was responsible for one of the most apocalyptic events that the world has ever seen. Oxygen, created by photosynthetic cyanobacteria, was thusly responsible for completely transforming the entire biological makeup of all life on Earth most early lifeforms perished.

While free oxygen is toxic to anaerobic bacteria, thus largely wiping them out during the oxygen holocaust, bacteria that relied on photosynthesis, such as the genocidal cyanobacteria, managed to thrive, creating an entirely new ecosystem in which new species could evolve.

The Rise of Rodinia

After the breakup of the Paleoproterozoic supercontinent Columbia in the beginning of the Mesoproterozoic some 1.6-billion years ago, the fragmented lands came together to form the first recognizable Precambrian supercontinent, and the land of Rodinia was born. Surrounded by an ocean named Mirovia, Rodinia was particularly special, since it was the first supercontinent to be reconstructed by geologists with a reasonable degree of accuracy. The rocks that made up Rodinia are now spread all over the world, making it extremely challenging to put the pieces together and envisage what the surface of the Earth looked like all those aeons ago. Accompanying the formation of Rodina was the Grenville orogeny, the formation of mountains that partially survive to this day. As lands collided to build Rodina, mountains, valleys and lakes formed, including what is now Lake Superior, the largest freshwater lake in the world.

Sexual Reproduction Paves the Way for Complex Life

One of the most important events of the entire Precambrian period occurred in the Mesoproterozoic era, and that was the first sexual reproduction, a vital precursor to the development of complex life as we know it. A eukaryotic red alga called bangiomorpha pubescens was the first organism to sexually reproduce some 1.2-billion years ago, and thus the separation of male and female came to be. Life on Earth was no longer restricted to bacteria, archaea and simple multicellular organisms living together in colonies like microbial mats. All of today’s eukaryotes, which include all animals, plants and fungi, originate from a single-celled common ancestor, the first organism to sexually reproduce.

Although it was still hundreds of millions of years before they would colonise the land, the very first plants also evolved during the Mesoproterozoic in the form of green algal mats along the shorelines of Rodinia. At the same time, freshwater lakes also became home to a multitude of these communities of primordial plants, though inland regions remained stark and lonely places that were largely void of all but the most basic forms of life, such as bacteria and archaea. The rapid spreading of photosynthetic green algae also meant that the oxygen content of the Earth’s atmosphere continued to rocket, although it was probably still no more than 1% of today’s levels during the middle of the Proterozoic.

The World Freezes Over

Snowball EarthChris Butler

An artist’s impression of the so-called snowball Earth, when the planet was almost entirely covered by ice.

The Mesoproterozoic era ended a billion years ago with the beginning of the Neoproterozoic, the last geological era before the Phanærozoic aeon, which we live in today. During the Tonian period, the first of three periods of the Neoproterozoic, the first carnivorous habits appeared, as evidenced by the decline of stromatolite microbial colonies and the apparent development of defensive characteristics in certain mysterious eukaryotic organisms known as acritarchs.

Some 850-million years ago, Rodinia began to break up, heralding the beginning of the Cryogenian period, a time of some of the most extreme climate change in the history of the Earth. Quite literally, the Earth froze over no less than three times during this period, which lasted until 650-million years ago. The Cryogenian is one of the most important geologic periods of Earth’s history, not least because it was also during this time that the first animals appeared.

The average global temperature during the Cryogenian was only 5°C, some 9°C lower than it is today. For millions of years at a time the world was entirely, or almost entirely, covered with ice and snow and that even the oceans were mostly frozen solid. The first of these so-called Snowball Earths occurred at the beginning of the period, gradually growing more severe with each major glaciation event. At the same time, the amount of oxygen in the atmosphere continued to mount, affording life the opportunity to evolve to an entirely new level of sophistication.

Some 720-million years ago, the Sturtian glaciation event saw ice sheets covering most of the planet, even the equatorial regions of Earth were covered with ice during this time. However, due to large amounts of volcanic activity, the planet was probably more of a muddy ball of slush rather than a snowball. The final major glaciation ended around 635-million years ago, as the oceans defrosted and the glaciers receded, finally lifting the restrictions on the development of plant and animal life.

The First Animals

Around, 650-million years ago, towards the end of the Cryogenian period, the first animal evolved from simple eukaryotic organisms. The very first animal on Earth was a humble sea sponge and, by the time the Cryogenian ended 635-million years ago, life was already well on its way to developing complex biological systems throughout the world’s oceans.

As the Earth thawed, life radiated and complex biodiversity finally became a reality after almost three-billion years of relative simplicity. Complex multicellular organisms appeared in the form of the earliest plants and animals. By the end of the Ediacaran period, the Earth’s oceans were teeming with life, including the very first jellyfish and a multitude of organisms that are now long extinct. Various symmetrical animals also evolved during this period, including the mysterious Dickinsonia, a jellyfish-like creature.

Conclusion

By the end of the Ediacaran, and the end of the Proterozoic and the Precambrian supereon, the Earth would have been relatively familiar to us. Early animals started to change the environment, dominating global ecosystems and bringing oxygen levels right up to 63% of modern levels. The world became a warmer place and, for the first time, one that we could actually survive, were we to travel back in time to the end of the Proterozoic aeon, 541-million years ago.