Evolution of Earth

27. Consider the following statements:
1. The Earth’s magnetic field has reversed every few hundred thousand years.
2. When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.
3. When living organisms originated, they modified the early atmosphere of the Earth.
Which of the statements given above is/are correct?
(a) 1 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Ans: (c) 1 and 3 only
Explanation:-

• Statement  1:  Reversal  of  Earth‘s  magnetic  field  is  called  Geomagnetic  reversal.  It  occurs  every  few million  years. It is a change in a planet's magnetic field such that the positions of magnetic north and magnetic  south  are  interchanged,  while  geographic  north  and  geographic  south  remain  the  same.Thus, this statement is correct.
• Statement 2: Early atmosphere has an abundance of hydrogen sulphide and Carbon dioxide.  There were  smaller  proportions  of  water  vapour,  ammonia  and  methane.  The  present  composition  got finalized very late  – around 600 million years ago (Cambrian Period). Vulcanism and the very evolution of  life  played  important  roles  in  changing  the  atmospheric  composition.  Thus,  this  statement  is incorrect.
• Statement 3: Respiration and microbial action by living organisms modified the early atmosphere. Thus, this option is correct.
• Option C is the most appropriate.

The Earth’s Magnetic field has reversed every few hundred thousand years. This has been proved through Sea-Floor spreading.

When Earth was created there was no oxygen in the atmosphere. Oxygen makes up about one-fifth the volume of Earth’¬s atmosphere today and is a central element of life as we know it. But that wasn’¬t always the case. Oxygen, although always present in compounds in Earth’¬s interior, atmosphere, and oceans, did not begin to accumulate in the atmosphere as oxygen gas (O2) until well into the planet’¬s history.
Carbon dioxide, water vapor, and methane played an important role in Earth’¬s subsequent development.
By 2.7 billion years ago, a new kind of life had established itself: photosynthetic microbes called cyanobacteria, which were capable of using the Sun’¬s energy to convert carbon dioxide and water into food with oxygen gas as a waste product. They lived in shallow seas, protected from full exposure to the Sun’¬s harmful radiation.
These organisms became so abundant that by 2.4 billion years ago the free oxygen they produced began to accumulate in the atmosphere.