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.
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.
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.
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).
Photosynthesis gets started
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.
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.