Origin and Evolution of the Earth

The term “evolution” usually refers to the biological evolution of living things. But the processes by which planets, stars, galaxies, and the universe form and change over time are also types of “evolution.” In all of these cases there is change over time, although the processes involved are quite different.

The Big bang theory

• The most popular argument regarding the origin of the universe is the Big Bang Theory.
• It is also called expanding universe hypothesis.
• Edwin Hubble, in 1920, provided evidence that the universe is expanding.
• As time passes, galaxies move further and further apart. Similarly, the distance between the galaxies is also found to be increasing and thereby, the universe is considered to be expanding. Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies.
• The Big Bang Theory considers the following stages in the development of the universe.

1. In the beginning, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.
2. At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. It is now generally accepted that the event of big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.

• Within 300,000 years from the Big Bang, temperature dropped to 4,500K (Kelvin) and gave rise to atomic matter. The universe became transparent.

• The expansion of universe means increase in space between the galaxies.
• An alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time.
• However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe.

• The distribution of matter and energy was not even in the early universe.
• These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together.
• These formed the bases for development of galaxies.
• A galaxy contains a large number of stars.
• Galaxies spread over vast distances that are measured in thousands of light-years.
• The diameters of individual galaxies range from 80,000-150,000 light years.
• A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
• Eventually, growing nebula develops localised clumps of gas.
• These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars.
• The formation of stars is believed to have taken place some 5-6 billion years ago.

The following are considered to be the stages in the development of planets:

(i) 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 and a huge rotating disc of gas and dust develops around the gas core.

(ii) In the next stage, the gas cloud starts getting condensed and the matter around the core develops into smallrounded objects. These small-rounded objects by the process of cohesion develop into what is called planetesimals. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.

(iii) In the final stage, these large number of small planetesimals accrete to form a fewer large bodies in the form of planets.

• Our Solar system consists of eight planets.
• The nebula from which our Solar system is supposed to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago.
• Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.
• Out of the eight planets, mercury, venus, earth and mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outer planets.
• Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.
• The rest four are called Jovian or Gas Giant planets. Jovian means jupiter-like.
• Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.
• The difference between terrestrial and jovian planets can be attributed to the following conditions:
  • The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.
  • The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.
  • The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.
• All the planets were formed in the same period sometime about 4.6 billion years ago.
• Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (2003 UB313)discovered in recent past may be called ‘dwarf planet’
• Let us briefly review the conditions on the other seven planets of the solar system.
  • Mercury is closest to the sun. It has a temperature range of 427ºC on its side facing the Sun and – 270ºC, on its dark side. It has no atmosphere.
  • Venus is the closest neighbour of the earth. It is about 40 mk away. It is an extremely hot planet with a temperature of 480ºC. Its atmosphere has 96% carbon dioxide and poisonous gases like sulphur dioxide and carbon monoxide.
  • Earth is the only planet known to sustain life.
  • Mars is also close to earth. It is called the red planet. It has 95% carbon monoxide and reddish dust. It is relatively a very cold planet and as of now presence of life on it has not been conclusively established.
  • Jupiter is the largest planet of the solar system. It is mainly a rapidly spinning ball of gas specially clouds of ammonia, and has no solid surface.
  • Saturn consists mainly of hydrogen and helium. Its atmosphere has 90% nitrogen and a temperature of (-184ºC). It is also made up of hydrogen cyanide which is a highly poisonous gas. It is characterized by a ring that surrounds it.
  • Uranus is also a very cold planet. Uranus is a distant planet of solar system and 7th in order from the sun. Uranus and Neptune are the outermost planets of the solar system. Uranus has a highly tilted rotational axis.
  • Neptune is cold and dark with its surface coated with frozen methane.

• The moon is the only natural satellite of the earth.
• Like the origin of the earth, there have been attempts to explain how the moon was formed.
• In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body.
• The whole mass became a dumb-bell-shaped body and eventually it broke.
• It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean.
• However, the present scientists do not accept either of the explanations.
• It is now generally believed that the formation of moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”.
• A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed.
• It blasted a large part of the earth into space.
• This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

• The earth broke off about 4.5 billion years ago with an explosion.
• It was a burning hot white mass of gas and dust.
• Over a long period of time, dust and gas gradually condensed to form solid rock.
• Such condensation and shrinking made the earth heat up so much that the rock melted into a gluey liquid.
• After millions of years, the outer surface of the earth or the earth’s crust cooled and formed hard rock again, just as melted chocolate or wax solidifies upon cooling.
• The interior of the earth is still very hot.
• The crust of the earth was formed from cooling and hardening of the molten matter and hot gases.
• With cooling of the earth the crust hardened and formed the land.
• Cooling of the earth also condensed water vapour into liquid water filling the depressions to form seas.

• The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the earth.
• Hence, there must have been some events– processes, which may have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water and conducive atmosphere favouring the existence of life.
• The earth has a layered structure.
• From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform.
• The atmospheric matter has the least density.
• From the surface to deeper depths, the earth’s interior has different zones and each of these contains materials with different characteristics.

Evolution of Lithosphere

• The earth was mostly in a volatile state during its primordial stage.
• Due to gradual increase in density the temperature inside has increased.
• As a result the material inside started getting separated depending on their densities.
• This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface.
• With passage of time it cooled further and solidified and condensed into a smaller size.
• This later led to 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 got separated into different layers.
• Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core.
• From the crust to the core, the density of the material increases.

Evolution of Atmosphere and Hydrosphere

• The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.
• There are three stages in the evolution of the present atmosphere.
• The first stage is marked by the loss of primordial atmosphere.
• In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere.
• Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis.
• The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.
• This happened not only in case of the earth, but also in all the terrestrial planets, which were supposed to have lost their primordial atmosphere through the impact of solar winds.
• During the cooling of the earth, gases and water vapour were released from the interior solid earth.
• This started the evolution of the present atmosphere.
• The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen.
• The process through which the gases were outpoured from the interior is called degassing.
• Continuous volcanic eruptions contributed water vapour and gases to the atmosphere.
• As the earth cooled, the water vapour released started getting condensed.
• The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains.
• The rainwater falling onto the surface got collected in the depressions to give rise to oceans.
• The earth’s oceans were formed within 500 million years from the formation of the earth. This tells us that the oceans are as old as 4,000 million years.
• Sometime around 3,800 million years ago, life began to evolve.
• However, around 2,500-3,000 million years before the present, the process of photosynthesis got evolved.
• Life was confined to the oceans for a long time.
• Oceans began to have the contribution of oxygen through the process of photosynthesis.
• Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.

Origin and evolution of Life on Earth

• In The Origin of Species, Charles Darwin (1859) hypothesized that new species arise by the modification of existing ones—that the raw material of life is life. But somehow and somewhere, the tree of life had to take root from nonliving precursors. When, where, and in what form did life first appear? The origin of life is one of the most intriguing, difficult, and enduring questions in science.
• To begin with, conditions on earth were inhospitable for life.
• Gases of the primitive atmosphere were primarily methane, ammonia, carbon dioxide and hydrogen.
• Water vapour filled the atmosphere but there was no free oxygen.
• It was thus a reducing atmosphere on primitive earth and no life existed.
• Biological evolution– from the simple organisms to complex organisms
  • As earth cooled, water vapour condensed to form liquid water.
  • Rains poured to form water bodies on earth.
  • The molecules of life were formed in the water.
  • From the molecules of the life evolved bacteria, the earliest and simplest organisms.
  • The oldest fossils of bacteria which were the first living organisms on earth have been found in rocks that are 3-5 billion years old.
  • For almost two billion years, different kinds of bacteria lived on earth.
  • One of these evolved a green pigment called chlorophyll.
  • These chlorophyll-containing bacteria used carbon dioxide and water and released oxygen through photosynthesis and started accumulating in the atmosphere.
  • Continued photosynthesis by such bacteria progressively accumulated oxygen in the atmosphere.
  • Thus the atmosphere gradually transformed from reducing to oxidizing.
  • At one point of time oxygen content in the atmosphere become 21%.
  • Such changes served as a big trigger for biological evolution to begin and progress and this led to the invasion of land by living organism.
  • As time passed, protists evolved from bacteria.
  • Both bacteria and protists are unicellular.
  • Then came multicellular organisms, the fungi followed by plants and animals.
  • Today the diversity of living organisms is comprised of five kingdoms of life. Monera, Prototictista, Fungi, Plantae and Animalia.