The Scando Review
The Scando Review
How the Earth evolved into a habitable planet

How the Earth evolved into a habitable planet

The Earth has provided us with everything we need to survive and thrive. We have air to breathe, enough water to quench our thirst, enough light from the Sun, and good soil to grow food.  

However, the Earth did not become habitable for humans and other living beings overnight. Billions of years ago, planet Earth was nothing more than a globular mass in the universe that was filled with dust and gas. Let us find out how the planet evolved from that to the Earth as we know it today.

From nothingness

The universe came into being from nothingness, and that was the beginning of everything. There are billions of galaxies in the universe and even more stars in those galaxies. There were clouds of dust and gas all around.

According to National Geographic, scientists believes that the clouds of dust and gas began to spin owing to the impact of a large explosion such as that of a supernova. This rotating cloud of gas and dust, called the solar nebula,  contracted because of its own gravitational pull, and pulled matter inward towards the centre of gravity. The hydrogen atoms in the centre fused into helium and expelled large amounts of energy.

This gravitational collapse caused most of the collapsing mass to collect in the centre to form the Sun while the rest of the interstellar matter became planets, moons, asteroids, and other small solar-system bodies. The Sun thus became the centre of our solar system. Other planets and celestial bodies in the solar system formed only after the Sun, came into existence about 4.603 billion years ago.

The swirling clouds and other gases collided and some of them fused.  This fusion resulted in the formation of some planets. Some of the matter that fused were able to hold the resulting mass in place by virtue of its own gravitational pull, thus forming planets. At the centre of our solar system – formed 4.571 billion years ago – is a red giant star we call the Sun, which is orbited by planets and satellites. Among those planets, the third planet from the Sun, which looks like a blue marble, is the one where we live.

It has been 4.543 billion years since planet Earth came into existence. This is the only known astronomical object in the universe that is known to harbour life. The four planets closest to the Sun, namely, Mercury, Venus, Earth and Mars are known as the Inner Planets. All of these planets are smaller than other planets and have a rocky surface. They are the only ones capable of withstanding the heat of the Sun.

Evolution of the Earth

In the beginning, the Earth was not a planet that was rich in water, rocky landscapes, and life forms. The surface of the Earth was originally covered with molten rock. It took hundreds of thousands of years for the Earth to cool down and for the oceans to form. The planet then began to develop many layers. The outermost layer, which is also the hardest and the thinnest layer, is called the crust. The inner layers became thicker and partially solid.

According to the giant-impact theory, the Moon was formed as the Earth’s satellite during a collision between the Earth and a smaller planet.

Scientists speculate that the Earth, like the other three Inner Planets that are close to the Sun, reached its present state in three stages. This view is supported by the theory of the core accretion model.This theory was proposed in 1944 by Otto Schmidt. According to the Core Accretion model, earth and other terrestrial planets were formed from meteoric materials.

These studies show that the Earth was formed in several stages. In the first stage, known as accretion, clusters of gases, dust, rocks, and ice all came together to form planetesimals. The larger planetesimals accreted some smaller ones while others shattered in collisions.

According to the giant-impact theory, the Moon was formed as the Earth’s satellite during a collision between the Earth and a smaller planet. Studies have shown that the Earth inherited water from the asteroids and comets that crashed into it.

Asteroids did more than just help shape the Earth. They also helped our planet grow and form at various stages. Scientists believe that during the formative phase, asteroids frequently crashed into the planet, and that a large percentage of water from these asteroids found its way to the Earth through such collisions. The minerals on the asteroids had water inside, and when these asteroids hit the Earth’s surface, the rock fragments were scattered on the planet.


Later, a layer of gas around the Earth was created by gravity and it came to be known as the atmosphere. At first, there were only light gases such as hydrogen and helium in the Earth’s atmosphere. Oxygen was not present in the atmosphere then.

As the Earth was constantly changing, its landscape also began to change. Volcanic eruptions began to happen. Thus some new gases like ammonia and carbon dioxide were formed. Water, which was formed by steam, began to combine and turn into oceans. Thus the first version of life took shape in the oceans.

Bacteria came into existence about 3,000 million years ago. They also began to emit oxygen through photosynthesis. Thus, slowly, oxygen began to reach the atmosphere. That is how the Earth’s atmosphere began to change over a period of hundreds of millions of years.

Mercury, Venus, Earth, and Mars are terrestrial planets made of rock and metals, and the Earth is the largest planet among them.

Today, our atmosphere is 21% oxygen and 78% nitrogen. The remaining small percentage is other gases. It was this atmosphere that made the planet conducive to life. Studies show that about 580 million years ago, there were only simple life forms on the planet. The evolution of these simple organisms into multicellular organisms happened later.

The one and only Earth

The Earth, the third planet from the Sun, is the only habitable planet we know of today. It is the fifth largest planet in the solar system and the only planet which sustains liquid surface water. Water forms 70 per cent of the Earth’s surface.

Mercury, Venus, Earth, and Mars are terrestrial planets made of rock and metals, and the Earth is the largest planet among them. The name Earth originated about 1,000 years ago. All planets except the Earth are named after Greek or Roman gods or goddesses. Our planet came to be known by the German word ‘erde’ which means ‘ground’.

The radius of the Earth is 6,371 kilometres. The distance between the Sun and the Earth is one astronomical unit (AU), which is about 150 million kilometres. (AU is used to measure the distance from the Sun to the planets). It takes 8 minutes and 20 seconds for light from the Sun to reach the Earth.

The Earth, while orbiting the Sun, rotates on its axis. It takes the Earth 23.9 hours to complete one rotation on its axis. It takes 365.25 days for  the Earth to orbit the Sun once. We see 365 days as a year because it is practically impossible in our calendar to calculate a quarter of a day every year. That is why we add an extra day to the calendar every four years and call it a Leap Year.

The Earth has only one satellite – the Moon. The Moon is also the only satellite we can see in the night sky clearly. The Moon is 3,84,400 kilometres from the Earth. 

Even though there are many theories about the origin and evolution of earth, it still remains a strange, scientific mystery. As scientists continue to study planets inside and outside the solar system, they would be able to explain it better.

Now put on your thinking hats and think about the following questions for a couple of minutes.

How would you explain the “core-accretion model” to your students?

Can you think of the reasons why earth is considered as the only planet that supports life?

Write down your thoughts and discuss them with your students, children and your colleagues. Listen to their views and compare them with your own. As you listen to others, note how similar or different your views are to others’.

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Happy Teaching!