Earth's History: From Precambrian to Today

1. The Earth and a Long, Changing History

Sedimentary rocks cover most of the Earth's surface, and are found in strata. These layers often contain fossils. The history of the Earth is divided into two parts:

• Precambrian: The planet's formation, the emergence and development of life, the formation of the primitive atmosphere. It is divided into three eons: Hadean, Archean, and Proterozoic.
• Phanerozoic: At this stage, the Earth remained as we know it, with the current continents and existing life forms (including humans). It is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

2. Precambrian: The Most Ancient Times

2.1. Hadean: Ancient God of Hades = Hell

The oldest rocks (3800 Ma) are magmatic and metamorphic. There was intense volcanic activity. Slowly, the Earth began to cool down and formed a solid surface.

2.2. Archean

The primitive atmosphere formed, as did the first oceans. The first life forms were very simple bacteria, and their fossils are the oldest, found in the oceans. Between 3200 and 3300 Ma, the Earth was cooling, and the first continental lithosphere appeared. The breaking of the lithosphere marked the beginning of plate tectonics. The first continents emerged. Oxygen began to be released into the atmosphere.

2.3. Proterozoic

Life diversified in both oceans and continents. A large continent called Pangea I formed. The climate alternated between warm and cold periods. The first glaciation occurred. The first eukaryotic cells appeared. The most abundant fossils are stromatolites (algae). A very thin layer of oxygen developed. The first metaphytes (multicellular algae, red and green) spread everywhere. The first animals emerged. The first great extinction of living beings took place. Pangea I began to fragment at the end of the Proterozoic.

3. Phanerozoic

A relatively short eon, divided into three eras:

• Paleozoic: Divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
• Mesozoic: Divided into three periods: Triassic, Jurassic, and Cretaceous.
• Cenozoic: Divided into two periods: Paleogene (Paleocene, Eocene, Oligocene) and Neogene (Miocene, Pliocene, Pleistocene, and Holocene).

3.1. Paleozoic

In the Cambrian, Pangea I fragmented into Laurasia (modern North America) and Gondwana (South America, Africa, Asia, the Middle East, and Australia). Warm periods alternated with very cold periods. There was a drop in sea level and the extinction of some species. Life was diversifying. Trilobites appeared in the oceans (marine arthropods).

In the Ordovician, the first fish appeared.

In the middle of the Silurian, terrestrial life was diversifying. Ferns developed, and the first terrestrial insects, amphibians, and plants with seeds appeared.

During the Devonian, freshwater fish and the first trees appeared.

In the Carboniferous, large, giant fern forests appeared. The first reptiles and gymnosperms (pine, fir) appeared.

In the Permian, the largest species extinction in Earth's history occurred. Trilobites disappeared, along with many amphibians, reptiles, and plants without flowers. The Hercynian orogeny (Appalachian) took place. The regrouping of the continents formed Pangea II.

3.2. Mesozoic