Origin of Earth – NDA Geography Notes

How did the Earth form? How old is it? When did life first appear? These are questions that scientists have been trying to answer for centuries. This chapter covers the theories about the origin of the Universe and the Earth, the geological time scale, and the major eras of Earth’s history.

NDA does not ask many questions from this chapter, but the ones it does ask are very specific, particularly about which era dinosaurs lived in, what happened in the Pleistocene epoch, and the geological time scale. These questions are straightforward if you know the basic framework clearly.

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How Did the Universe Begin?

Most scientists today accept the Big Bang Theory as the best explanation for the origin of the Universe.

About 13.8 billion years ago, the entire Universe existed as an incredibly hot and dense single point. In a massive explosion, the Big Bang, this point expanded rapidly. Space, time, matter, and energy all came into existence at that moment. The Universe has been expanding ever since. [NDA 2019-II]

As the Universe expanded and cooled, hydrogen and helium, the simplest elements, formed first. Over billions of years, gravity pulled these gases together to form stars. Inside stars, nuclear reactions created heavier elements like carbon, oxygen, and iron. When massive stars exploded at the end of their lives, they scattered these elements into space. New stars and planets, including our Solar System, formed from this enriched material.

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How Did the Earth Form?

The Earth formed about 4.6 billion years ago from the same cloud of gas and dust that formed the Sun and the rest of the Solar System.

As the Sun formed at the centre of this cloud, the remaining material began to clump together. Small particles collided and stuck, gradually building up larger bodies that eventually became the planets.

The early Earth was extremely hot. The interior was entirely molten. Heavier materials like iron and nickel sank toward the centre, forming the core. Lighter materials rose and cooled to form the crust. Over billions of years, the Earth cooled further, oceans formed, and eventually life appeared.

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Theories About Earth’s Origin

Nebular Hypothesis was proposed by Kant and Laplace. A rotating cloud of gas and dust (called a nebula) cooled and contracted, throwing off rings of material that condensed into planets. This was an early theory that has since been refined.

Planetesimal Hypothesis was proposed by Chamberlin. A passing star pulled material from the Sun. This material formed small solid bodies called planetesimals. These collided and gradually built up into planets.

★ IMPORTANT The Big Bang Theory explains the origin of the Universe (13.8 billion years ago). The Nebular and Planetesimal hypotheses explain the origin of the Solar System (4.6 billion years ago). These are different events. Do not confuse them. [NDA 2019-II]

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The Geological Time Scale

The geological time scale organises Earth’s 4.6-billion-year history into named time periods. Scientists determined this by studying rock layers and the fossils found within them. Older rocks are generally found below newer rocks. Different fossils appear in different layers, helping scientists identify when different forms of life existed.

Divisions from largest to smallest: Eons → Eras → Periods → Epochs

The Four Major Eons

Eon	Time	Key Feature
Hadean	4.6 to 4.0 billion years ago	No life; molten surface
Archean	4.0 to 2.5 billion years ago	First simple life (bacteria)
Proterozoic	2.5 billion to 541 million years ago	Simple multicellular life begins
Phanerozoic	541 million years ago to present	Complex life; divided into three eras

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The Three Major Eras of the Phanerozoic Eon

Era	Time	Known As	Key Life
Palaeozoic	541–252 Ma	Age of Ancient Life	Marine invertebrates → fish → amphibians → reptiles
Mesozoic	252–66 Ma	Age of Reptiles	Dinosaurs
Cenozoic	66 Ma–present	Age of Mammals	Mammals → Humans

Palaeozoic Era (541 to 252 million years ago)

Palaeozoic means “ancient life.” This era saw the explosion of complex marine life, followed by the appearance of fish, amphibians, reptiles, and land plants.

Cambrian Period: the first explosion of complex invertebrates (trilobites, molluscs). Life was almost entirely in the sea.

Permian Period: the era ended with the largest mass extinction in Earth’s history, wiping out about 96% of all marine species.

Mesozoic Era (252 to 66 million years ago)

Mesozoic means “middle life.” This is the famous Age of Reptiles, the era of dinosaurs.

Dinosaurs first appeared in the Triassic Period (252 to 201 million years ago). They dominated the Earth through the Jurassic and Cretaceous periods. The Mesozoic Era ended about 66 million years ago with a mass extinction, most likely caused by a large asteroid striking the Earth, which wiped out the dinosaurs and about 75% of all species.

Appeared in Triassic. Went extinct at end of Cretaceous. Dinosaurs did not go extinct in the Triassic. They only first appeared there.

Cenozoic Era (66 million years ago to present)

Cenozoic means “recent life.” After the extinction of dinosaurs, mammals rapidly diversified and became the dominant animals, the Age of Mammals.

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Important Epochs of the Cenozoic

Palaeocene Epoch (66 to 56 million years ago), the first epoch of the Cenozoic. Early mammals spread rapidly after the dinosaurs disappeared.

Pleistocene Epoch (2.6 million to 11,700 years ago). This is when the Ice Ages occurred, periods when large parts of the Earth were covered in glaciers. Early humans (the genus Homo) appeared during this epoch. The Karewas of Kashmir, the flat-topped lacustrine deposits, formed during the Pleistocene when the Kashmir Valley was a large lake.

Holocene Epoch (11,700 years ago to present), the current epoch. The Ice Ages ended. Human civilisation began. All of recorded history falls within the Holocene.

Epoch	Time	Key Event
Palaeocene	66–56 Ma	Early mammals spread after dinosaur extinction
Pleistocene	2.6 Ma–11,700 years ago	Ice Ages; early humans (Homo); Karewas formed
Holocene	11,700 years ago–present	Current epoch; all recorded history

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Palaeoclimatology

Palaeoclimatology is the study of ancient climates: what the Earth’s climate was like in the distant past. Scientists use ice cores, fossil pollen, tree rings, and ocean sediments to reconstruct past climates. [NDA 2022-II]

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How Do We Know the Age of the Earth?

The answer: 4.6 billion years: comes from a technique called radiometric dating.

Certain elements are radioactive. They slowly change from one element to another at a known, fixed rate. For example, uranium slowly changes into lead. By measuring how much uranium and how much lead are in a rock, scientists can calculate how long the rock has been forming. The oldest rocks found on Earth are about 4 billion years old. Meteorites give ages of about 4.6 billion years. This tells us the Solar System formed about 4.6 billion years ago.

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