Timeline 4 to 2.5 Billion Years Ago (Archean)

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.
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.

As the global magmatic ocean of the Hadean started to cool, continents rose from the fiery seas, and the Archean aeon began. Green slime filled the air with oxygen

The Archean Earth was a fantastically beautiful place, 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. 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.

  • End of the Late Heavy Bombardment
  • Volcanic activity creates the second atmosphere
  • Continental land masses appear
  • Yggdrasil the tree of life begins to grow

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.

Very gradually, the Earth’s surface started to resemble that of the terrestrial planet we inhabit today. The first rocks were formed as magma solidified to form rocky structures and, thus, the first consistently solid landmasses. The first oceans formed around the same time, during which water became stable on the Earth’s surface.

For the first 400-million years of the Archean, the Earth’s second atmosphere was forming. The intense volcanic activity that marked the late Hadean continued into the Archean.

The second atmosphere consisted of poisonous volcanic gasses, the atmospheric pressure was crushingly high at the beginning of the Archean, perhaps as much as 100 times that of modern times. 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.

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) during this era that the first supercontinent formed, a barren expanse known as Vaalbara.

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.

Climate Change Changes Everything

The Mesoarchean,  lasted from 3.2- to 2.8-billion years ago characterised by the biggest ever changes in the Earth’s climate and topology 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.

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 Vaalbarasupercontinent 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.