Proterozoic Earth – The First Animals

The Proterozoic Aeon

2.5 Billion to 541 Million Years Ago

Ryan Somma

An artists representation of life in the Ediacaran seas during the last period of the Proterozoic aeon, including the mysterious Dickinsonia and primitive jellyfish.

In the third part of the series ‘‘A Journey through the History of Earth”, we’ll be exploring one of the most intriguing yet mysterious periods of our planet’s past. From the formation of the atmosphere we breathe today to the glaciation of the entire world to the birth of the very first animals, let’s start our journey by stepping 2.5-billion years back in time to the dawn of the Proterozoic aeon.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

Spanning almost 2-billion years, the Proterozoic era saw the evolution of a world we could almost recognise today. 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 that had little in common with our own but, over hundreds of millions of years, it transformed beyond measure.

The Proterozoic is Greek for ‘earlier life’, which is a bit of a misnomer given that the first definitive evidence for the existence of life on Earth dates from some 1.5-billion years earlier. However, it’s 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

Charles | 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, Earth would have still looked very alien compared to what we know today. Methane in the atmosphere meant the sky was tinted with shades of red or orange due to the blue wavelengths of the Sun’s light being reflected back into space. The oceans had a distinct greenish tint in them, due to the presence of cyanobacteria and large quantities of iron. There’s also evidence that the day was 12.3 hours long, and there were 714 days per year at the end of the Archaean. In other words, the Earth was a very alien world at the dawn of the Proterozoic aeon.

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.

We complain today, very justifiably I might add, about the devastating impact that overpopulation and industrialisation has on the Earth but, instead of CO2 being the problem back in the early Proterozoic, it was O2. 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, with most phylums being forever lost to us all these billions of years later. As such, there’s a lot about life on Earth before the Great Oxygenation Event that we will simply never know.

As I’ve touched on several times already over this series, one of the commonly repeated themes in natural history is that chaos and destruction form the foundations for new life. 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. According to Harvard palaeontologist Andrew Knoll, the first eukaryotes (of which all animals, plants and fungi are members of) developed between 2.1- and 1.6-billion years ago.

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

Chris 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, which is Greek for ‘cold birth’ 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, despite how inhospitable the world was.

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, glaciations occurred at low latitudes, and there’s a popular theory that 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, temporarily putting the brakes on the development of more complex life. According to findings published in the National Geographic in 2010, 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

Matteo De Stefano/MUSE Science Museum

Growing to over three feet (1 metre) in length, Dickinsonia was the most iconic Ediacaran organism and also one of the most mysterious lifeforms that ever existed.

For a long time, it was widely believed that complex life developed in the Cambrian period, which started the Phanerozoic aeon 541-million years ago. However, new evidence has shown that life in the Ediacaran (also known as the Vendian) period was far more advanced than previously thought, to the extent that it set the precedent for the Cambrian explosion.

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 that may have even belonged to a taxonomic kingdom of its own, one that is now long extinct.

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.

Be sure to stay tuned for the upcoming additions to this series where we’ll be exploring the migration of life from the sea to the land, the rise and fall of the dinosaurs and the evolution of mankind itself. In the following articles, we’ll be journeying through the eleven periods that make up the three geological eras of the Phanerozoic, the current geological aeon. If you’d like to receive automatic alerts as I publish new posts, please subscribe to my blog newsletter.

Part 4: Cambrian Earth – An Explosion of Evolution