Evolution

Notes

Chapter 6: Evolution

1. Evolutionary Biology

• Evolutionary Biology: Study of the history of life forms on Earth.
• Evolution: It refers to the changes in flora and fauna that have occurred over millions of years on Earth.
• According to Darwin, evolution occurs with modification or variation.
• Unit of Evolution: Population.
• Raw material of evolution is variation.

2. Origin of the Universe

• When we look at stars on a clear night sky, we are actually looking back in time.
• Stellar distances are measured in light-years.
• What we see today may have started its journey millions of years earlier.
• When we see objects in space, we are actually peeping into the past.
• Origin of Universe: About 20 billion years ago – Big Bang Theory.
• Origin of Earth: About 4.5 billion years ago.
• Origin of Life: About 4.0 billion years ago, approximately 500 million years after the formation of Earth.

3. Big Bang Theory

The Big Bang Theory explains the origin of the universe.

• Initially, there was a single huge, dense entity.
• It contained matter in a highly condensed state.
• A huge explosion, called the Big Bang, occurred.
• The universe expanded and galaxies were formed.
• The Solar System and Earth were formed later.

4. Formation of Earth and Primitive Atmosphere

During the early stages, Earth had very high temperature and a primitive atmosphere.

• The primitive atmosphere contained gases such as methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour.
• Free oxygen was absent in the primitive atmosphere.
• The primitive atmosphere was reducing in nature.

5. Theories Explaining the Origin of Life

Different theories were proposed to explain the origin of life.

5.1 Theory of Special Creation

• According to this theory, life was created by a supernatural or divine power.

5.2 Theory of Panspermia or Cosmozoic Theory

• Some scientists believed that life came from outside the Earth.
• Early Greek thinkers believed that units of life called spores were transferred to different planets, including Earth.
• Panspermia is still considered as one of the ideas proposed for the origin of life.

5.3 Theory of Spontaneous Generation (Abiogenesis)

• According to this theory, life originated spontaneously from non-living matter.
• For example, it was believed that worms could arise from mud and flies from decaying matter.