Welcome to the second eon of Earth’s history, the Archean Eon. In this blog we will explore the eon when life evolved, and the events that brought oxygen into the atmosphere. We will further consider other key features and the subdivisions of the eon.
Future blogs will explore in detail the other eons of Earth’s history.
Overview of the Archean Eon
The Archean Eon was named after the Greek work ‘archē’ meaning ‘beginning’ or ‘origin’. This eon was named by geologists in the 19th century.
The Archean Eon lasted from 4 billion years ago to around 2.5 billion years ago. It lasted an amazing 1.5 billion years.
For this blog, we will be referring to the image below as our timeline.
Key Features of the Eon
This eon provided the earliest evidence of fossils and rocks for us to examine. The Earth transitioned during this time from the hellish place we saw in the previous eon, to a more stable environment with a solid crust, oceans and early life.
As the atmosphere stabilised and was held in place with the gravity of Earth, the molten state of the surface cooled, solidified and formed a stable crust. There were repeated processes of melting and cooling of certain parts of the crust and tectonic activity, which eventually created small landmasses called protocontinents (early continents).
The volcanic activity from the previous eon, the Hadean, brought to the surface water vapor. As the Earth cooled and the water vapor condensed, it changed state from gas to liquid and formed the first oceans of the world. The early oceans were anoxic, meaning there was no free oxygen. The early oceans were rich in iron and silica.
Life therefore evolved in this early anoxic atmosphere and in the primordial (early) seas. Early organisms are classed as anaerobic microbes, organisms that evolved without oxygen. The evidence of this can be found in Stromatolites which are layered structures of these early organisms – these can be found in rocks dating back to this period of time.
Towards the latter half of this eon we get cyanobacteria evolving. These were among the first organisms to photosynthesise, that is to get their energy from the sun. The by-product of photosynthesis is oxygen. In a world that is anoxic (devoid of oxygen), this was a crucial event that shaped the end of this eon and the world for the next few billion years. The free oxygen that was released by these organisms flooded the atmosphere, creating an oxygen-rich atmosphere. This event actually caused a mass extinction, the first one we know of that was caused by a species and not necessarily some geological event.
Subdivisions
This eon sees confirmed subdivisions of time where there is evidence of different geological events occurring. The large pink dots on the timeline above show the subdivisions of time.
Eoarchean Era – 4 to 3.6 billion years ago (400 million years long)
This was when the first stable crust formed and the oldest rocks we know of, formed.
Paleoarchean Era – 3.6 to 3.2 billion years ago (400 million years long)
This is the era we find the earliest evidence of life.
Mesoarchean Era – 3.2 – 2.8 billion years ago (400 million years long)
Microbial life continues to evolve.
Neoarchean Era – 2.8 to 2.5 billion years ago (300 million years long)
When the cyanobacteria increased in numbers and flooded the oceans and atmosphere with oxygen by the end of the eon.
Summary of the Eon
This eon builds on the events of the first eon, the Hadean. We see stability in the general state of Earth with a stable crust laying the foundation for plate tectonics, continents and oceans.
It can be argued that the evolution of the first life forms is the most crucial part of the story of the Earth and of life. Without life, the planet wouldn’t have seen all the subsequent major changes.
The early organisms evolved in an anoxic ocean and atmosphere. Therefore the introduction of free oxygen and the process of photosynthesis was the next crucial turning point, creating the basis for all of life to evolve in an oxygen rich atmosphere.
Lesson to Take Away
Just like the previous eon, we can take away some crucial lessons that we can apply to living and coping with modernity.
Building on the Small Things
In this eon we see the continuation of building on things that came before. That chaotic beginning stabilised once the initial heat and high energy activity ran out of steam. However there are still small things happening, it’s a continuation of the same theme: small things build up to big things.
This can be seen in the innovations we create and have created in the past. There is usually a period of trial and error in the creation of a new invention, when the creator has to go through uncertainty and chaos to find something that flows and is stable in nature. When the invention is rolled out to society, there is a period of uncertainty and caution of ‘what is this new thing?’. Then slowly, the invention is integrated into society creating stability and integration in ways that cannot be predicted, changing the very fabric of society.
Just like the stability of the continents and oceans integrated across the planet, the result was a calm environment where life flourished.
Harsh Environments Can Still Lead To Growth
This eon sees what we would consider to be harsh environments. At the bottom of the oceans was where live first evolved, from the heat and chemical reactions of the ocean floor and hot vents. Life was still able to evolve, there is always a way to draw out energy from the environment, no matter how harsh.
When we are faced with a challenge, some of us flee and some of us fight. It is somewhat easier however to find an excuse, to go into a hole, to live with pain and not try to fight out of it. To bandage the pains of the harsh environment, escape it and deal with it later even though later doesn’t come.
Further Exploration
We have explored the second eon of Earth’s history, the timeline and subdivisions of time, the features of this eon that made it special and some lessons we can take away from it.
To further your education on this topic outside of Evolution of Stuff, you can explore deeper into the development of continents and tectonic shifts, an anoxic environment and the effects of it, the effect of free oxygen and the mass extinction that followed.
At the end of this blog are the links to some of our other blogs, so you can continue your exploration and education.
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