Archean Earth – Signs of Life

The Archean Aeon

4 to 2.5 Billion Years Ago

Peter Sawyer

An artist’s impression of a typical scene from the mid- to late-Archean when microbial colonies had begun to establish themselves, even though volcanism was rife throughout the era. Note the stromatolites and bacterial colonies in the shallow waters.

In the second part of the series ‘‘A Journey through the History of Earth’, we’ll be exploring the formation of the continents and the first signs of life that characterised the second of four aeons in the history of our world. The Archean, from the Greek word for ‘origin’, was named so because it was once thought to be the earliest aeon. Let’s step back in time 4,000- to 2,500-million years ago to this lonely young Earth.

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

The Archean Earth was a fantastically beautiful place, albeit one that would be quite unfamiliar to us now. 3.8- to 4-billion years ago, the fiery morass of the primordial world started to settle, and oceans and landmasses started to form, hailing in the end of the chaotic Hadean aeon. Although inhospitable to all modern forms of life, the early Archean was a far calmer world, characterised by a global ocean with small, scattered islands, red-tinged skies and a Moon much closer than it is today.

Earth’s earliest rocks date from the Archean aeon, although little remains from these times, extremely distant as they are. Nonetheless, we now know that the most basic forms of life managed to find a foothold in this lonely alien world, and evidence points to photosynthesis occurring about half way through this time, or approximately 3-billion years ago. Albeit at a glacial place, the foundations of the world with which we’re familiar today came into being.

Highlights of the Archean

• End of the Late Heavy Bombardment
• The formation of the first rocks
• Volcanic activity creates the second atmosphere
• Continental land masses appear
• First bacteria appear; the tree of life begins
• Photosynthesis occurs for the first time

Chaos Begets Order in a Cooling Earth

As the global magmatic ocean of the Hadean started to cool, continents rose from the fiery seas, and the Archean aeon began. The first part of this 1.5-billion-year-long aeon in Earth’s ancient history began with the Eoarchean era, a period when the surface of our world had cooled enough for a solid crust to form. It’s also from this time that the oldest rocks on Earth were formed, the oldest of which haven been dated to just over 4-billion years.

Very gradually, the Earth’s surface started to resemble that of the terrestrial planet we inhabit today. The first rocks were igneous ones, meaning that they were formed as magma solidified to form rocky structures and, thus, the first consistently solid landmasses. According to the two most common hypotheses, the first oceans formed around the same time, during which water became stable on the Earth’s surface as such that it didn’t evaporate in the much higher temperatures.

We would not be able to survive the surface of the Earth at any point during the Archean aeon, and the Eoarchean was most inhospitable era of all. For the first 400-million years of the Archean, the Earth’s second atmosphere was forming as lighter gasses, such as hydrogen and helium, escaped into space and got swept away by the solar wind. The intense volcanic activity that marked the late Hadean continued into the Archean, with outgassing allowing heavier elements to form in the Earth’s atmosphere.

The second atmosphere would be completely unbreathable to us, since it consisted mostly of volcanic gasses such as CO2, rather like the current surface of Venus. The atmospheric pressure was also crushingly high at the beginning of the Archean, perhaps as much as 100 times that of modern times, which is also comparable to that of modern Venus. Although the surface continued to cool, current simulations show that the Archean climate probably sported temperatures of around 200°C due to the intense greenhouse effect and the fact that the Earth’s mantle was still far hotter than it is today.

Our Ancestors Are Born

Didier Descouens

Dating from the Paleoarchean era, this stromatolite shows the fossilised remains of layered structures that were likely created by cyanobacteria that lived some 3.2-billion years ago.

The Eoarchean era ended with the last years of the Late Heavy Bombardment, a period characterised by the constant bombardment of asteroids since the formation of the Earth 4.57-billion years ago. The era that followed was the Paleoarchean (3.6- to 3.2-billion years ago), Greek for ‘ancient origin,’ which seems apt, since it’s back to this far-off time that humanity and, indeed, all life on Earth, can definitively trace their ancestors. It was also during this era that the first supercontinent formed, a barren expanse known as Vaalbara.

Although the oldest undisputed evidence of life on Earth appeared 3-billion years ago in the following Mesoarchean era, strong evidence points to the first cyanobacteria appearing around 3.5-billion years ago. Incredibly ancient fossils from this time have been found in rocks dating from the Paleoarchean period in what is now Australia. It’s from this amazing discovery that we can deduce that the last universal common ancestor of absolutely all life on Earth from microbes to humans lived during the early Archean Earth.

Our most distant ancestors were, unsurprisingly, extremely simple forms of life, but much more interesting ones than many people give them credit for. In fact, the cyanobacteria phylum exists to this day, and it was likely the very first form of life to obtain energy through photosynthesis. This is especially important, since it’s the process of photosynthesis that has been crucial for the evolution of our species and all those who came before us. After all, photosynthesis creates oxygen as a by-product, thus forming the atmosphere that we breathe today and creating entirely new niches for life to evolve into.

Towards the end of the Paleoarchean era, one of the largest impact events in the history of the Earth occurred when an asteroid the size of a large city crashed into the planet some 3.26-billion years ago. However, despite the impact being orders of magnitude greater than that which wiped out the dinosaurs billions of years later, life somehow managed to survive.

Climate Change Changes Everything

We worry a great deal about man-made climate change today, but at few times in history has natural climate change been so profound as it was in the era following the Paleoarchean. The Mesoarchean, meaning ‘middle origin’ in Greek, lasted from 3.2- to 2.8-billion years ago, and was characterised by some of the biggest ever changes in the Earth’s climate and topology ever known throughout its history.

The two main factors influencing the rapidly changing world of the Mesoarchean were the impact event of the Paleoarchean and the evolution of the atmosphere due to oxygenic photosynthesis of Archean cyanobacteria. In other words, the oxygen levels produced by these primitive lifeforms, although likely still extremely low compared to today’s levels, reduced the amount of CO2 in Earth’s second atmosphere, thus reducing the greenhouse effect. The consequences were rapid cooling and lowering air pressures, and although we certainly still wouldn’t have been able to breath the Mesoarchean air, Earth rapidly became a lot more hospitable to life as we know it than that of the early Archean.

Palaeontologists estimate that the first glaciations occurred in the Mesoarchean, some 2.9-billion years ago, ultimately leading to the hypothesised snowball Earth in the early stages of the Proterozoic, the aeon that followed the Archean. The Mesoarchean era ended with the gradual breaking up of the Vaalbara supercontinent 2.8-billion years ago.

The Ingredients for Complex Life

In the Neoarchean era, which marked the last 300-million years of the Archean aeon, oxygenic photosynthesis continued to evolve, paving the way not only for the oxygen catastrophe that occurred in some 200-million years later, but also for the development of complex life. Although oxygen was still nothing more than a trace gas in the Earth’s atmosphere even by the end of the Archean, prokaryotic life was now well-established. The ocean floors were covered in living microbial mats, and there is even evidence pointing to land-based bacterial colonies existing.

Although Earth was mostly covered by ocean during the Archean, the end of the Archean saw the formation of the second supercontinent, known as Kenorland, from the scattered islands left over by the splitting of the Vaalbara supercontinent.

There is evidence to suggest that the Earth suffered another apocalyptic event during the last era of the Archean, a natural phenomenon known as mantle overturn, which occurs when the two layers of the Earth’s mantle, reverse their positions. The result, rampant volcanism, may have wiped out many species of bacteria and, perhaps, any other forms of life (that we so far don’t know about) that may have found a niche at the time.

Conclusion

Despite accounting for about a third of Earth’s entire existence, the Archean, like the Hadean before it, remains shrouded in mystery. However, thanks to the fact that rocks and, consequently, bacterial fossils in ancient stromatolites, exist from this period, we know for sure that simple life managed to thrive through most of these 1.5-billion years. It was during this time that we can all definitively trace our origins, even though the Archean world was a very different, and much less hospitable, one to our own.

Part 3: Proterozoic Earth – The First Animals