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A volcano erupts out of the water, like in the Archaean Eon

Archaean Eon: A volcano erupts out of the water

The beginning of the Archaean Eon

After the end of the Hadean Eon is the beginning of the Archaean Eon. That’s about 3.8 billion years ago. Earth was still about three times as hot as it is today. But it was no longer hot enough to boil water.

Go back to the Hadean Eon
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Most of the Earth was covered with oceans. Earth’s atmosphere was mainly carbon dioxide with very little oxygen in it. Just a little bit of land was forming as volcanoes began to poke out of the water.

Read more about volcanoes

granite - irregular gray and black stone

Granite rock

Most of the rocks were igneous or metamorphic like granite or quartz. But the earliest sedimentary rocks like sandstone also formed during this time. These rocks were mainly in the oceans.

When did life begin on Earth?

About this time – around the beginning of the Archaean Eon, about 3.8 billion years ago – the earliest living cells formed on Earth. These cells all lived in the oceans, which were probably much warmer and more acidic than they are now.

Read more about how life got started

chloroplasts - green dots

Chloroplasts (seen through a microscope) can photosynthesize food from sunlight, water, and carbon monoxide.

By about 3.5 billion years ago, these early cells had evolved into simple prokaryote cells. For the rest of the Archaean Eon, there were only prokaryote cells on Earth (and the vast majority of cells on Earth are still prokaryotes).

Read more about prokaryote cells

red and gray striped rock

Archaean Eon: Iron that has turned red because of oxygen in the water

Photosynthesis gets started

By three billion years ago, some of these prokaryote cells evolved to be able to make their own food out of sunlight, water, and carbon dioxide. We call this process photosynthesis.

Read more about photosynthesis

Cells that got their energy by photosynthesis excreted (pooped out) oxygen, and once a lot of cells were photosynthesizing there started to be more and more oxygen on Earth.
But during the Archaean Eon almost none of that oxygen was in the atmosphere – instead, iron and sulfur rocks mixed with early oxygen atoms to make rusty red rocks and limestone.

Millions of one-celled creatures with silicon and oxygen in their cell wallsprokaryotes – died and sank to the bottom of the ocean, where the silicon and oxygen was squashed into chert and flint.

Read more about flint

Asteroids hit the Earth

There were probably many large asteroids that hit the Earth during the Archaean Eon. These asteroids would have killed many small living creatures – prokaryote cells – and may have encouraged others to evolve.

The end of the Archaean Eon

About 2.5 (two and a half) billion years ago, the Archaean Eon ended and the Proterozoic Eon began.

Do you need a second source to cite? Check out this excellent article in the Encyclopedia Britannica!

Learn by doing: look at bacteria through a microscope
More about prokaryote cells
Go on to the Proterozoic Era

Bibliography and further reading about the Archaean era:

Go on to the Proterozoic
List of Geological Eras home