Archean and Hadeon Eons

Archean and Hadean Eons

In the previous article, we discussed the geological timeline. Now it is time to go through all the periods and explore the history of life on our planet. The Precambrian eons (Hadean, Archean, and Proterozoic) cover 85% of Earth’s history, from the formation of Earth to the first skeletons.

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History of life
Archean 
Hadean
Visualization of Precambrian
Image courtesy: YouTube

The Hadean- before life emerged

The Hadean started before even water appeared on our planet, with the formation of the Earth-Moon system. Firstly, the collision with hypothetical protoplanet Theia caused the formation of the Moon and vaporization of Earth’s crust. As a result, came changes in Earth’s atmosphere and plate tectonics. Later on, solar wind swiped a lot of debris from the planet, and the first Earth’s crust formed. Moreover, about 4.4 billion years ago it cooled enough, to enable the ocean’s formation. With liquid water, and atmosphere reduced methane and ammonia the beginning of life was only a matter of time. The oldest known mineral, found in Jack Hills, gave also samples of organic matter, possibly sign of life. The eon ends with late heavy bombarding probably caused by the migration of Neptune into the Kuiper belt. The Hadean’s Earth was way hotter, more unstable, and rawer than the Blue Planet we know today.

Earth during Hadean and Archean eons
Earth during the Hadean Eon
Image courtesy: Pinterest

The Archean and origins of life

The Archean eon started about 4 billion years ago. The condition started being rather hospitable, which led to the beginning of life. During that time Earth was still cooling, which allowed the formation of continents. Earth was still an unfamiliar place- there was no oxygen and only a little bit of life. Processes that led to the beginning of life are still nature’s secret, though we know a little about the first life forms. The Procaryota- simple, single-celled organisms with genetic material flowing in the cytoplasm. Later on, the domain Bacteria formed- organisms extremely adaptive to various changes. There is no evidence of viruses, and the first Eukaryota are thought to evolve in the next eon. Evidence from that eon includes fossilized terrestrial microbes, which prove that life quite quickly adapted to exist on the land.

First Archean organisms were probably bacteria
First life forms on Earth were bacteria

Archean bacteria

The next eon is marked mostly by the rise of the level of oxygen in the atmosphere. Moreover, other elements started cycling around, which enabled new life forms to show. It happened due to autotrophic bacteria. First, cyanobacteria photosynthesize to convert light energy to chemical, one of the side products is oxygen. The other types of bacteria collect energy from chemosynthesis- using energy from inorganic reaction to produce organic matter, which leads to freeing many elements. Some bacteria release sulfur and nitrogen- the basic compounds we are all built of, the necessary biological building material. The entire life evolution started with those single-celled organisms, and we definitely should remember about it. The evidence includes stromatolites- sheets of microorganisms, produced mostly by cyanobacteria. There is no evidence of viruses or first Eukaryotes at that moment, though it is not impossible they existed.

Stomatolites from the Archean eon
Fossil evidence- stromatolites
Image Courtesy: Pinterest

Author’s message

I hope the second article from the History of Life series entertained you and helped you understand the origin on our planet and life. If you enjoyed it please share it with family and friends. In the next article we will talk about many fascinating life forms, so stay tuned. Also, if you want to contact me, feel free to email, you can find the address in the Author’s section of the page.

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