The Origin and Evolution of Earth

When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun. Earth has a central core, a rocky mantle, and a solid crust. (definition from NASA)

The Earth is our home planet the third planet from the Sun, and the only planet we of so far that’s inhabited by living. The origin of the earth was a mystery for several years. Many scholars tried to short out the mystery and put forward several concepts and theories.

Many hypothetical theories were given by different scholars and scientists regarding the origin of the earth. Some early theories tried to reveal the mystery and after that modern theories came to light that tried to solve the mystery of the origin of the Universe.

Early Theories:

In the early period, scientists focused on the origin of the Earth or planets rather than the origin of the Universe.

The early theories describing the origin of the earth are as follows:

Nebular Hypothesis

The nebular hypothesis was given by German philosopher Immanuel Kant. The theory was revised by mathematician Laplace in 1796. The Nebular Hypothesis considered that the planets were formed out of a cloud of material associated with a youthful Sun, which was slowly rotating.

In 1950, the Nebular Hypothesis was again revised by Otto Schmidt in Russia and Carl Weizascar in Germany who considered that the sun was surrounded by a solar nebula. It contained mostly hydrogen and helium along with dust. The friction and collision of particles led to the formation of a disk–shaped cloud and the planets were formed through the process of accretion.

Binary Theories:

The binary theories are some opposite arguments to the Nebular Hypothesis.

  • Chamberlain and Moulton in 1900, considered that a wandering star approached the Sun. s a result a cigar-shaped extension of material was separated from the solar surface.
  • As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and slowly condensed into planets.
  • This theory was later supported by Sir James Jeans and Sir Harold Jeffrey. These theories are considered that the companion of the sun co-existed.

Modern Theories:

In the later period, scientists proposed theories regarding the origin of the Universe, Stars, and planets.

Origin of Universe

Big Bang Theory:

In 1920, Edwin Hubble provided evidence that the universe is expanding and galaxies move further and further apart as time passes.

The Big Bang theory is the most popular theory regarding the origin of the universe in modern theories. It is also called the expanding universe hypothesis.

The Big Bang theory considers the following stages in the development of the universe:

  1. All matters forming the universe existed in one place in the form of a tiny ball with an unimaginably small volume, infinite temperature and infinite density in the beginning.
  2. The “tiny ball” exploded violently which led to a huge expansion during the Big Bang. It took place 13.7 billion years ago.
    • The expansion continues even to the present day.
    • The expansion was rapid in the first few seconds after the bang but it slowed down during the later stage.
    • Between the first three minutes of the Big Bang event, the first atom began to form.
  3. In the course of the process, within 300,000 years from the Big Bang temperature dropped to 4,500K which gave rise to atomic matter and finally after that, the universe started to come in shape and became transparent.

Hoyle’s Concept of Steady State:

Sir Fred Hoyle (born June 24, 1915) was a British mathematician and also known as the defender of the steady state theory of the universe.

The steady-state universe theory: This theory holds that the universe is expanding and that matter is being continuously created to keep the mean density of matter in space constant.

  • The steady-state universe theory is put forward as an alternative to the Big Bang theory.
  • The steady-state model asserts that although the universe is expanding, it nevertheless does not change its appearance over time; the universe has no beginning and no end.
  • The research paper on steady-state cosmologies was published by Herman Bondi, Thomas Gold, and Fred Hoyle in 1948. Similar models had been proposed earlier by William Duncan Macmillan among others.
  • Albert Einstein considered a steady-state model of the expanding universe, indicated in a 1931 manuscript, many years before Hoyle, Bondi and Gold. However, he quickly abandoned the idea.

The Star Formation

The distribution of matter and energy was not even in the early universe. Initially, there was a difference in the density of matter which gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed are based on the development of galaxies. Galaxy contains a large number of stars.

  • Galaxies spread over vast distances that are measured in thousands of light-years. The diameter of individual galaxies ranges from 80,000 to 150,000 light-years.
  • A galaxy starts to form through the accumulation of hydrogen gas in the form of a very large cloud called a Nebula.
  • The Nebula develops localised clumps of gas, which continue to grow into even denser gaseous bodies.
  • Finally, it gave rise to the formation of stars which is believed to have taken place 5-6 billion years ago.

Formation of Planets:

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The dust around a star is critical to forming celestial objects around it. Dust around stars contains elements such as carbon and iron which can help the planetary system.

Stages in the formation of planets

The following stages take place in the formation of planets:

  • The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud. Finally, a huge rotating disk of gas and dust develops around the gas core.
  • The dust clumps become pebbles, the pebbles become larger rocks that grind together to expand. The presence of gas particles of solid material stick together. Some break apart, but some hold on. These are the building blocks of planets, sometimes called “planetessimals.”
  • These planetesimals are a large number of small bodies. Large bodies start forming by collision, and gravitational attraction causes the material to stick together. At final, these large numbers of small planetesimals accrete to form fewer large bodies in the form of planets.
Some pieces of information taken from NASA:
  • The rocky planets begin to form in the warmer part of the disk closer to the sun. Thus the rocky planets like Mercury, Venus, Earth, and Mars may be taken tens of millions of years after the birth of the star. The exact details of the formation of planets from disks are still a mystery and an ongoing area of research.
  • The planets like Jupiter, Saturn, Uranus, and Neptune, the gas giants of our solar system are thought to form by the colder regions that also allow the gas molecule to slow down enough to be drawn onto a planet.
Solar system:

Our solar system consists of eight planets. Our solar system is supposed to form by nebula which started its collapse and core formation sometime 5-5.6 billion years ago. The solar system possesses the Sun (our star), eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), moons, asteroids, comets, and a huge quantity of dust grains and gases.

(Read in detail about the Solar system:

Evolution of the Earth

The earth was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. The development of the earth started 4600 million years ago.

The earth consisted of a layered structure. From the outermost end of the atmosphere to the centre of the earth, the material is not uniform.

Evolution of Lithosphere:

The earth was in the most unstable state during its primordial stage. Due to a gradual increase in the density, the temperature inside has increased. As a result, the materials inside started getting separated depending on their density.

  • The heavier materials (like iron) sink towards the centre of the earth and the lighter ones move towards the surface.
  • With the passage of time, it cooled further and solidified and condensed into a smaller size which later led t the development of the outer surface in the form of a crust.
  • During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that the earth-forming material gets separated into different layers.
  • Starting from the surface to the central parts, it has layers like the crust, mantle, outer core and inner core.
  • The density of the material increases from the crust to the core.

Evolution of Atmosphere:

(click here to read the composition and structure of the atmosphere:

At present, the atmosphere is chiefly composed of the gases nitrogen and oxygen. The stages involved in the evolution of the atmosphere are:

  • The first stage is marked by the loss of the primordial (primary) atmosphere.
  • The hot interior of the earth contributed to the evolution of the atmosphere in the second stage.
  • the final stage was the composition of the atmosphere was modified by the living world through the process of photosynthesis.

The early atmosphere with hydrogen and helium is considered to have been stripped off as a result of solar winds.

At the time of the cooling of the earth, gases and water vapour were released from the interior solid earth starting the evolution of the present atmosphere.

The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little free oxygen.

The process through which the gases were outpoured from the interior is called degassing.

Evolution of Hydrosphere:

Continuous volcanic eruptions contribute water vapour and gases to the atmosphere. As the earth cooled, the water vapour released started getting condensed.

The carbon dioxide got dissolved in rainwater and the temperature further decreased causing more condensation and more rain. The rainwater falling onto the surface got collected in the depression to give rise to the oceans.

  • The earth’s oceans were formed within 500 million years of the formation of the earth. So, oceans are as old as 4000 million years.
  • Life was confined to the ocean for a long time. Ocean began to contribute by the presence of oxygen through the process of photosynthesis around 2500-3000 million years ago. Eventually, oceans were saturated with oxygen and 2,000 million years ago, oxygen began to flood the atmosphere.

Origin of life:

The last phase in the evolution of the earth relates to the origin and evolution of life. Modern scientists refer to the origin of life as a kind of chemical reaction, that first generated complex organic molecules and assembled them.

  • By this assemblage, the molecules were able to duplicate themselves by converting inanimate matter into living substances.
  • The record of life that existed on this planet in different periods is found in rocks in the form of fossils.
  • The microscopic structures closely related to the present form of the blue algae have been found in geological formations much older than some 3,000 million years.
  • It can be assumed that life began to evolve 3,800 million years ago.
  • Click here to read: geological time scale (depicting the evolution of life from unicellular bacteria to modern man).

The origin of life on earth is yet not solved and still is an unsolved question. There are many ideas, but few clear facts. Most experts agree that all life today evolved by common descent from a single primitive lifeform. Although it is not known how this early life evolved, scientists think it was a natural process which happened about 3,900 million years ago.

This is in accord with the philosophy of naturalism; where only natural causes are admitted. Many scientists worked on it and gave their points on the origin of life on Earth.

Important theories on the Origin of Life:

The main names of the scientists who worked on it and gave important theories on the origin of life are:

  • Aristotle gave the theory of the “Spontaneous generation” of life.
  • Louis Pasteur: He disproved the theory of “Spontaneous generation” in which it was believed that the spontaneous generation of life from non-living matter. Pasteur showed that without spores no bacteria or viruses grew on sterile material.
  • Charles Darwin proposed “a natural process for the origin of life”
  • Haldane and Oparin: Alexander Oparin reasoned that atmospheric oxygen prevented the synthesis of organic molecules. Organic molecules are the necessary building blocks for the evolution of life. Oparin argued in his work ” The Origin of Life” that a “primordial soup” of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine in an ever more complex fashion to form droplets and these droplets would grow by fusion with other droplets to reproduce through fission into daughter droplets and so have a primitive metabolism in which those factors that promote “cell integrity” survive, those that do not become extinct.
    • Many scientists and modern theories of the origin of life still take Oparin’s idea as a starting point.


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