The age of the earth is around 4 - 5 billion years. We came from a long journey and we don't even know how exactly our earth came into the existence, but we made several theories, several hypothesis but the overall conclusion that we all agree is that once the earth was a ball of fire, lots of events took place, billion years have passed and so many things happened. Slowly the earth became colder, oceans formed, geological events happened lots of activities took place like rain, earthquake, volcanic eruptions.
Life started from water, unicellular to multicellular and millions and millions types of species evolved, some of them became amphibians and then this process goes for millions of years and they came on the earth's surface. Lots of destruction happened because of different different events like ice age, some became fossils some started a new age, a new era and in this process of evolution some of our companions lost the race. Earth itself is a box of mystery, layer-by-layer lots of enigmas are buried. Human came so much later, but when we study fossils, we come to know that many eras have passed on this earth, many animals have arrived.
Geological history has so much importance if we want to know how these events happened and what was the order of these events and the history of life on the earth, and to understand this evolution we made Geological Time Scale.
Geologic Time Scale is a chronological parameter of events in the past which categories and classifieds them in a certain order that helps scientists, geologists to understand the history, extinction and the evolution of life on the earth.
In the year 1760 an Italian geologist Giovanni Arduino did the first classification of geological time his scale was based on his study of northern Italy, but for the world it was developed in Bologna city in International Geological Congress in 1881 and after that it has been gone through lots of improvements. Arthur Holmes who was a british geoscientist he made the first complete geological time scale in 1913.
When we studies about the history of the earth we need a geological time scale so we have different different classification of the geological time according to the events, species that we found studying the fossils, we divided this time in Era, Epoch and Periods.
1. Archean Era
This era was 4,000 million years ago - 2,500 million years ago, this was the first era of the earth and it had gone through the most geological changes, the environment was not capable to form life earthquake, volcanic eruption and tectonic plates movement was the major part of this era, but by the end of this era the earth started cooling down and the life had began.
2. Paleozoic Era
This era was 541 million years ago - 252 million years ago, and this era is known for development of flora and fauna. This era is also know as the era of invertebrates. This era can be divided into six periods.
(i) Cambrian Period
This period was 541 million years ago - 485.4 million years ago, for the first time the encroachment of the sea took place at the land and lots of land had submerged. Creatures of this period were invertebrates. e.g Metaspriggina, Neodrepanura, Ovatiovermis.
(ii) Ordovician Period
This period was 485.4 million years ago - 443.8 million years ago, this period had increased the speed of sea encroachment and the creature of Cambrian period were more developed.
(iii) Silurian Period
This period was 443.8 million years ago - 419.2 million years ago, this period is known for rise of vertebrates that's why it's also called age of vertebrates.
(iv) Devonian Period
This period was 419.2 million years ago - 358.9 million years ago this period is also know as age of fishes because the fossils of fishes are found more in the rocks of this period than any other period.
(v) Carboniferous Period
This period was 358.9 million years ago - 298.9 million years ago, most of the world's coal was produced in this era that's why this is know as Carboniferous period but this period is also known as the of amphibians because in this period animals living both in water and land were born, and the development of amphibians beings.
(vi) Permian Period
This period was 298.9 million years ago - 251.902 million years ago, in this period the development of reptiles begins and hercynian orography (mountain formation) was major.
3. Mesozoic Era
This era was 251.902 million years ago - 65 million years ago, and mesozoic era is also known as the Age of Reptiles and the Age of Conifers. This era can be classified into three periods.
(i) Triassic Period
This period was 251.902 million years ago - 201.3 million years ago, in this period the weather was arid which leads to the worse extinction event and majorly the life was destroyed which is known as the Great Dying. Mammals evolved in this period and the period is known as the age of reptiles, dinosaurs and crocodiles were particularly found.
(ii) Jurassic Period
This period was 201.3 million years ago - 145 million years ago, Brongniart named this period in 1829 after the jura mountains of alps. In the beginning of this Jurassic period the supercontinent Pangaea had started rifting into two landmasses Laurasia and Gondwana.
(iii) Cretaceous Period
This period was 145.5 million years ago - 66 million years ago, dinosaurs and lots of other creatures were extincted at the end of this period and after that the mammals got the opportunity to emerge.
4. Cenozoic Era
This era was 66 million years ago - 2.58 million years ago, in this era the temperature started decreasing and many parts were covered with snow which leads to the extinction of ancient creatures and new mammals and birds evolved in their place.
This era can be divided into five periods.
(i) Paleocene Period
It was 66 million years ago - 56 million years ago, In this period the process of building the Rocky Mountains of north America started due to the Laramide orogeny. This period is know as the age of mammals.
(ii) Eocene Period
It was 56 million years ago - 33.9 million years ago, during this period volcanic lava eruption eradicated the Reptiles.
(iii) Oligocene Period
It was 33.9 million years ago - 23.03 million years ago, The Great Himalayas were formed during this period.
(iv) Miocene Period
It was 23.03 million years ago - 5.333 million years ago, The Lower Himalaya or The Lesser Himalaya were formed during this period.
(v) Pliocene Period
It was 5.333 million years ago - 2.58 million years ago, The Shivalik Himalaya were formed during this period and modern mammals evolved during this period.
5. Neozoic Era
This era was 2.58 million years ago - present day, It is divided into two periods.
(i) Pleistocene Period
It was
2.58 million years ago - 0.012 million years ago, There have been at least five documented ice ages during 4.5 billions years since the earth was formed and the most recent one occurred in this period.
(ii) Holocene Period
This period is 0.012 million years - present day, this period is the most recent period and also know as " Recent Period". This is the period that we are living in.
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