Theories of the origin of life
Early ideas said life came from a divine power or from non-living matter. Pasteur proved life comes only from life, and Oparin and Haldane explained chemical evolution.
Theories at a glance
| Theory | What it says | Who or what supports it |
|---|---|---|
| Special creation | A divine power created all living things at one time, in the form we see now | Religious belief; there is no scientific proof |
| Spontaneous generation (abiogenesis) | Life arose suddenly from non-living things, such as frogs from mud and maggots from rotting meat | Supported by Aristotle; disproved by Redi and Pasteur |
| Biogenesis | Life comes only from pre-existing life | Francesco Redi (covered meat), Louis Pasteur (swan-neck flask) |
| Cosmozoic theory (panspermia) | Spores or seeds of life reached Earth from outer space | Does not explain how life began in the first place |
| Chemical evolution (modern theory) | Life formed slowly from simple chemicals on the early Earth | Oparin and Haldane; supported by the Miller and Urey experiment |
Abiogenesis vs biogenesis
Pasteur's test: boiled broth in a swan-neck flask stayed clear, because the curved neck trapped dust and germs. When the neck was broken, germs entered and the broth turned cloudy.
Chemical evolution: from gases to the first cell
This modern theory was given by **A. I. Oparin** (1924) and **J. B. S. Haldane** (1929). It took millions of years.
- 1The early Earth was very hot. Its air had methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour, but no free oxygen.a reducing atmosphere
- 2Lightning, ultraviolet (UV) rays and volcanic heat gave the energy for these gases to react.
- 3Simple organic molecules formed: amino acids, sugars, fatty acids and nitrogen bases.
- 4Rain washed them into the warm seas. The seas became a primordial soup (a hot, dilute soup of chemicals).
- 5Small molecules joined into large ones: proteins and nucleic acids (DNA and RNA).
- 6Groups of these molecules got wrapped in a thin layer. These droplets are called coacervates.
- 7★Over time some droplets could grow and copy themselves. They became the first simple cells, living without oxygen and feeding on the soup.
★ key step
The Miller and Urey experiment (1953)
**Stanley Miller** and **Harold Urey** built a closed glass apparatus that copied the early Earth.
- 1A small flask of water was boiled to give water vapour.copies the early sea
- 2A large flask held methane, ammonia and hydrogen with the vapour.copies the early air
- 3Two electrodes passed electric sparks through the gases.copies lightning
- 4A condenser cooled the gases into a liquid. The liquid collected in a U-tube trap.copies rain
- 5★After about a week, the liquid contained amino acids such as glycine and alanine.organic molecules formed from inorganic gases
★ key step
How to draw the Miller and Urey apparatus
- Draw a small round flask at the bottom left with water in it. Draw a flame under it. Label it boiling water (water vapour).
- From its neck, draw a tube rising to a large round flask at the top right. Label it CH₄, NH₃, H₂ + water vapour.
- Draw two rods entering the large flask with a zigzag spark between them. Label electrodes and electric spark (lightning).
- From the large flask, draw a tube going down through a jacket. Label it condenser, with cold water in and cold water out.
- Below the condenser, draw a U-shaped tube holding liquid. Label it trap: water with amino acids.
- Join the trap back to the small flask so the loop is closed. Write the title: Miller and Urey experiment.
Boiled milk kept covered stays fresh longer, because germs from the air cannot get in. This is the same idea Pasteur used to prove that life comes only from life.
2 marks: “What is abiogenesis? Who disproved it?” Abiogenesis (spontaneous generation) is the idea that life arose from non-living matter. Louis Pasteur disproved it with his swan-neck flask experiment.
4 marks: “Explain any four theories of the origin of life.” Special creation (a divine power; no scientific proof); spontaneous generation (life from non-living matter; disproved by Redi and Pasteur); biogenesis (life only from life; swan-neck flask); panspermia (spores from space); chemical evolution (Oparin and Haldane).
7 marks: “Explain the theory of chemical evolution and the experiment that supports it.” Oparin and Haldane; reducing atmosphere; energy from lightning and UV; organic molecules, primordial soup, proteins and nucleic acids, coacervates, first cells; Miller and Urey experiment with a labelled diagram.
Swan neck, no germs, no growth: life comes only from life.