The Rains poured to form water bodies on Earth. The molecules of life were formed in the water.

From the molecules of life evolved bacteria, the earliest and simplest organisms.

The oldest fossils of bacteria which were the first living organisms on earth have been found in rocks that are 3-5 billion years old.

Chlorophyll

For almost two billion years, different kinds of bacteria lived on Earth.

One of these evolved a green pigment called chlorophyll.

These chlorophyll-containing bacteria used carbon dioxide and water and released oxygen through photosynthesis and started accumulating in the atmosphere.

Continued photosynthesis by such bacteria progressively accumulated oxygen in the atmosphere.

In this method, the atmosphere gradually transformed from reducing to oxidizing.

When oxygen content in the atmosphere becomes 21% it triggered biological evolution to begin and progress and this led to the invasion of land by living organisms.

With time protists evolved from bacteria which are unicellular organisms.

Then came multicellular organisms, the fungi followed by plants and animals.

Geological time scale

Geologists and paleontologists study the geologic time scale, the “calendar” for events in the earth’s history.

They record the Earth shaping events and life of the past.

But this record is incomplete as we do not have records of the time before Archean Eon, especially in the early parts.

According to geologists, the Earth is billions of years old.

Formal geologic time began at the start of the Archean Eon (4.0 billion to 2.5 billion years ago) and continues to the present day.

Modern geologic time scales additionally often include the Hadean Eon, which is an informal interval that extends from about 4.6 billion years ago (corresponding to Earth’s initial formation) to 4.0 billion years ago.

The Timescale Division

Super Eons

Super Eons contain several Eons in them and cover extremely large periods of time.

There has only been one Super Eon in Earth’s history. This one was the Precambrian Super Eon.

It could be having been technically said that we are at the beginning of the next Super Eon.

Precambrian Super Eon

The Precambrian Super Eon started about 4.56 billion years ago and ended about 541 million years ago.

It can be divided into 3 specific Eons which are the Hadean, the Archean, and the Proterozoic.

Hadeon Eon 

The Hadean eon (4,540 – 4,000 million years ago) represents the time before a reliable (fossil) record of life.

Temperatures were extremely high, and much of the Earth was molten because of frequent collisions with other bodies, extreme volcanism, and the abundance of short-lived radioactive elements.

A giant impact collision with a planet-sized body named Theia (approximately 4.5 billion years ago) is thought to have formed the Moon.

The moon was subjected to Late Heavy Bombardment(LHB – lunar cataclysm – 4 billion years ago).

Archean Eon

The beginning of life on Earth and evidence of cyanobacteria date to 3500 million years ago.

Life was limited to simple single-celled organisms lacking nuclei, called Prokaryote’s atmosphere was without oxygen, and the atmospheric pressure was around 10 to 100 atmospheres.

The Earth’s crust had cooled enough to allow the formation of continents. The oldest rock formations exposed on the surface of the Earth are Archean.

Volcanic activity was considerably higher than today, with numerous lava eruptions. The oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.

By the end of the Archaean, plate tectonics may have been similar to that of the modern Earth. Liquid water was prevalent, and deep oceanic basins are known to have existed.

The earliest stromatolites are found in 3.48-billion-year-old sandstone discovered in Western Australia.

The earliest identifiable fossil consists of stromatolites, which are microbial mats formed in shallow water by cyanobacteria.

Proterozoic Eon

It is the last eon of the Precambrian “supereon”.

It spans from the time of the appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as corals) on Earth.

Multicellular organisms

Bacteria begin producing oxygen, leading to the sudden rise of life forms.

Eukaryotes (have a nucleus), emerge, including some forms of soft-bodied multicellular. 

Earlier forms of fungi formed around this time. The early and late phases of this eon may have undergone Snowball Earth periods (the planet suffered below-zero temperatures, extensive glaciation, and as a result drop in sea levels).

It was a very tectonically active period in the Earth’s history. It featured the first definitive supercontinent cycles and modern orogeny (mountain building).

It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic.

In the late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 million years ago.).