You can see craters from the Late Heavy Bombardment on the moon.

You can see craters from the Late Heavy Bombardment on the moon.

     During the end of the Hadean Eon, the Earth was pummeled by rocks from space. The planets were still moving into their orbits. This caused the asteroid belt to move too. As a result, Earth and the moon were hit with thousands of asteroids. The impact craters from this event can still be seen on the moon. The event was called the Late Heavy Bombardment.

    The Hadean Eon ended after the Late Heavy Bombardment. The Archean Eon began around 3.8 billion years ago. The Archean Eon began when Earth’s crust formed.  The oldest rocks on Earth formed during the beginning of the Archean Eon.

    The beginning of the Archean Eon was also the beginning of life on Earth. Life first appeared in the oceans. It was prokaryotic. A prokaryote is a living thing with no organelles. Bacteria are prokaryotic.

During the Archean Eon, life on Earth was single-celled.

During the Archean Eon, life on Earth was single-celled.

During the Archean Eon , Earth had a red sky.

During the Archean Eon , Earth had a red sky.

    The first organisms on Earth didn’t breathe oxygen. They got energy from the chemicals in the environment. Earth’s atmosphere was filled with methane, nitrogen, carbon dioxide, and ammonia. These chemicals made the sky red instead of blue. Organisms that get energy without oxygen are called anaerobic. Oxygen was toxic to these early prokaryotes.

     The first aerobic organisms appeared about 3.5 billion years ago. Oxygen is not toxic to aerobic organisms. These aerobic organisms were called cyanobacteria. They lived deep under the ocean.

     As they made energy, they would release oxygen into the water. However, the oxygen couldn’t get into the atmosphere because it would bond with the iron in the oceans.

Hermatite

Hermatite

    Volcanoes erupted frequently during the Hadean and Archean Eons. When they erupt, volcanoes release iron. As a result, there was a lot of iron dissolved in the oceans. The oxygen and iron in the oceans bonded to form hematite. Hematite is red. You can still see a band of red hematite in rocks from the Archean Eon.

    Around 2.7 billion years ago, Kenorland formed. Kenorland was a massive supercontinent. There were no large continents before Kenorland. As Kenorland formed, it pushed the seafloor up around it. The cyanobacteria were now exposed to the Sun.

A stromatolite

A stromatolite

     The cyanobacteria were able to make energy using sunlight. This is called photosynthesis. As a result, they pumped more oxygen into the water than before. The extra oxygen was pushed into the atmosphere.

A mat of cyanobacteria

A mat of cyanobacteria

    Cyanobacteria have only one cell, but groups of cyanobacteria clump together to form mats. These mats trap sediments. Eventually, it becomes sedimentary rock. We call the rocks stromatolites. There are stromatolite fossils from the Archean Eon. There are also stromatolites formed by living cyanobacteria in Shark Bay in Australia.

     The Archean Eon ended just before one of the greatest extinction events in Earth’s history. The cyanobacteria, a tiny, one-celled organism, would change the fate of everything on Earth.