To understand the history of the earth and the evolution of life it is essential to understand the geological time scale. in order to give answers to many questions of the past geologists use a special time line which is known as the geologic time scale. The geologic time scale helps to record the earth’s geologic history by studying the different layer of rocks. The geologic time line is separated into larger and smaller subdivisions, which help the scientists to get better idea of how various historical events fit together. The geological time is divided into four large categories these are Cenozoic era (age of recent life), Mesozoic era (age of medieval life), Paleozoic era (age of ancient life), and the Precambrian.

Eons

The major principal subdivision is called the eon. The time spans is hundred to thousands of millions of years. The scientists generally agree that there are two major eons the Precambrian eon and the Phanerozoic eon. The Precambrian eon gives an idea about the period of the formation of the earth to the time when the multicellular organism first occurred in the earth. The time span is long that is from 4500 million ago to just 543 million ago. After that, the Phanerozoic eon gives a brief description about the different patterns of the modern days. The eons are made up of eras that span periods of ten to hundred of millions of year. The four main eons are hadean, archean, Proterozoic, and the Phanerozoic.

Hadean

Among all the eons hadean is the oldest one, which began about 4.6 billion years ago with the formation of earth and ended about 4 billion years ago with the appearance of the first single celled organisms. The hadean eon is named after the Greek god hades that is addressed as the god of the ocean.

Achaean

The next geological eon the Achaean began about 4 billion years ago. During this time, the cooling of the earth’s crust allowed for the formation of the first oceans and the continent. The researchers are not sure about the shape of the continents looked like since there is very few evidence about the period. However, some of them believed that first landmass on earth were a supercontinent known as Vaalbara.

Scientists believe that the first single celled organism was developed during the archean. These tiny microbes left their mark in the layered rocks known as the  age of these stones were more than 3.5 billion years ago.

Unlike the hadean, the archean eon was classified into eras these are Eoarchean, Paleoarchean, Mesoarchean and Neoarchean. The Neoarchean was formed about 2.98 billion years ago it was the era in which oxygenic photosynthesis began. This process performed by the algae and other microorganism which helped in the formation of oxygen molecules in water and later it was released in the atmosphere. Before the oxygenic photosynthesis earths atmosphere had no free oxygen , a huge requirement for the evolution of life.

Proterozoic

The Proterozoic eon was started about 2.5 billion years ago and it ended about 500 million years ago when the first complex life forms appeared on the earth. During this time, the great oxygenation event transformed the earth’s atmosphere, which allows for the evolution of the aerobic organism. The Proterozoic was also the period the earth’s first glacier was formed.

Phanerozoic 

The most recent geologic eon is the Phanerozoic eon, which began about 540 million years ago. The eon was very different from the previous three types of eons, which are also known as the Precambrian era.

Era

The three main eras are the Paleozoic, the Mesozoic and the Cenozoic.

Paleozoic era

During the Paleozoic era, the first life was created on the earth surface. The evolution of oxygen on the atmosphere of the earth was occurred during this era.

Mesozoic era

The Mesozoic era was started about 250 million years ago, during this period the dinosaurs ruled the planet. During this period, the dinosaurs dominated all the other creatures on the earth. The dinosaurs were eventually wiped out during the K-2b extinction an event that killed about 75 percent of the life on the earth.

Cenozoic era

Following the Mesozoic era was the Cenozoic era, which began about 66 million years ago. This period was also known as the age of the mammals. After the extinction of the dinosaurs, the big mammals dominated the earth.  

Periods

A geographical period is on of the different subdivisions of geologic time which enables the cross referencing of rocks and geological events from different place to place. These periods form elements of a hierarchy of divisions into which geologists have split the history of earth.

Epochs 

In the geochronology, an epoch is a subdivision of the geologic time scale that is longer than an age but less than a period. The current epoch is the Holocene epoch of the quaternary period. Series are defined as the rock layers deposited during an