Origin of Life and Evolution

The origin of life on earth is understood against the history of the universe and of the earth itself. This page covers the Big Bang, the early earth, panspermia, spontaneous generation and Pasteur's experiment, the Oparin-Haldane idea of chemical evolution, Miller's experiment and Darwin's theory of natural selection, as in the NCERT Class 12 chapter Evolution. NEET often asks the gases used by Miller, the three ideas of special creation and the meaning of Darwinian fitness in questions on the origin of life.

1. What Evolutionary Biology Studies

• Evolutionary biology: the study of the history of life forms on earth.
• To follow the changes in flora and fauna over millions of years, we must know how life began.
• That, in turn, needs the story of the evolution of the earth, of the stars and of the universe itself.
• The story of the origin of life and of biodiversity is the longest of all reconstructed and conjectured stories.

2. The Universe and the Early Earth

2.1 Looking back in time

• Distances to stars are measured in light years.
• Light reaching us from a star left it millions of years ago, from trillions of kilometres away.
• So, looking at the stars, we are in a way peeping into the past.
• Objects in our immediate surroundings are seen instantly, that is, in the present time.

2.2 The universe

• The origin of life is considered a unique event in the history of the universe.
• The universe is vast; compared with it, the earth is almost only a speck.
• ★ Exam imp The universe is very old: about 13.8 billion years.
• Huge clusters of galaxies make up the universe; galaxies contain stars and clouds of gas and dust.
• Big Bang theory: explains the origin of the universe as a single, huge explosion, unimaginable in physical terms.

2.3 From the Big Bang to the first oceans

1. After the explosion, the universe expanded and its temperature fell.
2. Hydrogen and helium formed some time later.
3. The gases condensed under gravitation and formed the galaxies of the present-day universe.
4. In the solar system of the Milky Way galaxy, the earth formed about 4.5 billion years ago.
5. The early earth had no atmosphere. Water vapour, methane, carbon dioxide and ammonia released from the molten mass covered its surface.
6. UV rays from the sun split water into hydrogen and oxygen; the lighter ${\mathrm{H}}_2$ escaped.
7. Oxygen combined with ammonia and methane to form water, $\mathrm{C}{\mathrm{O}}_2$ and other compounds.
8. The ozone layer formed.
9. As the earth cooled, water vapour fell as rain, filled the depressions and formed the oceans.
10. Life appeared about 500 million years after the earth formed, that is, almost 4 billion years ago.

3. Early Ideas on the Origin of Life

3.1 Panspermia

• Some scientists believe that life came to earth from outer space.
• Early Greek thinkers held that units of life called spores were transferred to different planets, including the earth.
• ★ Exam imp Panspermia: the idea that life arrived from outer space; it is still a favourite idea of some astronomers.

3.2 Spontaneous generation

• Spontaneous generation: the old belief that life arose from decaying and rotting matter, such as straw and mud.
• It was accepted for a long time.

3.3 Pasteur's experiment

Louis Pasteur tested the idea by careful experiments:

1. He kept killed yeast in pre-sterilised flasks: no life arose.
2. He kept killed yeast in another flask open to air: new living organisms arose.
3. Conclusion: life comes only from pre-existing life.

4. Chemical Evolution and Miller's Experiment

4.1 The Oparin-Haldane proposal

• ★ Exam imp Oparin of Russia and Haldane of England proposed that the first life came from pre-existing non-living organic molecules (for example, RNA and protein).
• They held that the formation of life was preceded by chemical evolution.
• Conditions on the early earth: high temperature, volcanic storms and a reducing atmosphere containing $\mathrm{C}{\mathrm{H}}_4$, $\mathrm{N}{\mathrm{H}}_3$ and other gases.

4.2 Miller's experiment (1953)

• S.L. Miller, an American scientist, recreated these conditions on a laboratory scale in 1953.

1. ★ Exam imp A closed flask held $\mathrm{C}{\mathrm{H}}_4$, ${\mathrm{H}}_2$, $\mathrm{N}{\mathrm{H}}_3$ and water vapour at 800 °C.
2. An electric discharge (spark) was passed through the gases between two electrodes.
3. The products collected in water: amino acids had formed.

• In similar experiments, others observed the formation of sugars, nitrogen bases, pigment and fats.
• Analysis of meteorite content revealed similar compounds, so similar processes are probably occurring elsewhere in space.
• With this limited evidence, the first part of the story, chemical evolution, was more or less accepted.

5. The First Forms of Life

• We have no idea how the first self-replicating metabolic capsule of life arose.
• ★ Exam imp The first non-cellular forms of life could have originated about 3 billion years ago.
• They would have been giant molecules (RNA, protein, polysaccharides, etc.); these capsules perhaps reproduced their own molecules.
• The first cellular form of life probably did not arise until about 2000 million years ago.
• These were probably single cells, and all life forms lived in a water environment only.
• ★ Exam imp Abiogenesis: the view that the first form of life arose slowly, through evolutionary forces, from non-living molecules. Most scientists accept it.
• How these first cells evolved into today's complex biodiversity is the story of evolution.

6. Special Creation and Darwin's Theory

6.1 The theory of special creation

Conventional religious literature describes the theory of special creation. It has three connotations: