Evolution

Evolution is an orderly change from one condition to another. Evolution can be defined as a change in the genetic composition of a population from generation to generation.

1.0Evolutionary Biology

Evolutionary Biology is the study of history of life forms i.e. the changes in flora and fauna that have occurred over millions of years on Earth.

To understand the changes in flora and fauna that have occurred over millions of years on Earth, we must have an understanding of the context of origin of life, i.e., evolution of Earth, of stars and indeed of the universe itself.

2.0What is Universe

The universe is a huge cluster of galaxies. Galaxies contain stars and clouds of gas and dust. Considering the size of universe, Earth is indeed a speck i.e. The universe is so vast  that the stellar distances are measured in light years.

When we look at stars on a clear night sky we are, in a way, looking back in time. What we see today is an object whose emitted light started its journey millions of years back and from trillions of kilometers away and reaching our eyes now. However, when we see objects in our immediate surroundings, we see them instantly and hence in the present time. Therefore, when we see stars we apparently are peeping into the past.

The Big Bang theory attempts to explain to us the origin of the universe.

3.0Big Bang Theory- Proposed By Abbe Lemaitre

According to it, the universe originated about 20 billion years ago due to a thermonuclear explosion of a dense entity. This single huge explosion, which is unimaginable in physical terms, is called the big bang.

The universe expanded and hence, the temperature came down. Hydrogen and Helium were formed sometime later.

The gaseous clouds which were formed by the big bang condensed under gravitation and converted into many flat discs like structures called nebula, made up of atoms and small particles. Solar nebula was one of them, which formed our solar system.

The very hot central part of solar nebula became even hotter due to pressure and converted into the sun. 

Later, due to condensation of atoms and dust particles moving around the sun, planets were formed.

In the solar system of the Milky Way galaxy, Earth was supposed to have been formed about 4.5 billion years back.

4.0Primitive Earth

There was no atmosphere on early Earth but later, water vapor, methane, carbon dioxide and ammonia released from molten mass covered the surface. 

The UV rays from the sun broke up water into Hydrogen and Oxygen and the lighter H₂ escaped.

All the oxygen present at that time, combined with ammonia and methane to form water, CO₂ and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans.

The conditions on Earth were – high temperature, volcanic storms, lightening, unstable radioactive elements, UV rays, reducing atmosphere containing CH₄, NH₃, etc.

Life appeared 500 million years after the formation of Earth, i.e., almost four billion years back.