Origin of life

Ideas about where life came from

Pasteur: life comes only from life

Louis Pasteur boiled a broth of killed yeast in flasks. In pre-sterilised flasks kept closed, no life appeared. In a flask open to air, new microbes grew. So life did not arise on its own from dead matter; it came from pre-existing life carried in the air. This ended the theory of spontaneous generation.

From a hot Earth to the first cells

1. 1
   The Big Bang formed the universe, which your textbook gives as almost 13.8 billion years old. Galaxies and the solar system formed later.
2. 2
   The Earth formed about 4.5 billion years ago. At first it had no atmosphere.
3. 3
   Water vapour, methane, carbon dioxide and ammonia came out of the molten mass. UV rays split water into hydrogen and oxygen; the light hydrogen escaped.

   Oxygen combined with ammonia and methane to form water and CO₂; an ozone layer formed
4. 4
   As the Earth cooled, water vapour fell as rain and filled hollows to form oceans.
5. 5★
   With high temperature, lightning and a reducing atmosphere, simple molecules joined to form organic molecules such as amino acids and sugars.

   This is chemical evolution
6. 6
   Non-cellular life (giant molecules such as RNA and proteins, perhaps inside capsules) arose about 3 billion years ago.
7. 7
   The first cellular life, probably single cells living in water, arose about 2 billion years ago.

★ key step

Miller's experiment (1953)

**S.L. Miller**, an American scientist, tested the Oparin-Haldane idea in the laboratory.

1. 1
   He took a closed flask with CH₄, H₂, NH₃ and water vapour, copying the early reducing atmosphere.
2. 2
   He kept the mixture at 800 °C and passed electric sparks through it, copying lightning.
3. 3
   A condenser cooled the gases so that liquid collected below, like rain into the sea.
4. 4★
   After about a week the liquid contained amino acids.
5. 5
   Similar experiments by others gave sugars, nitrogen bases, pigments and fats. Meteorites also contain such compounds, so the same process may happen elsewhere in space.

★ key step

How to draw Miller's apparatus

• Draw a large round spark chamber at the top with two electrodes going in; label it with CH₄, NH₃, H₂ and water vapour.
• Draw a tube leading down to a condenser (a jacket with cold water in and out).
• Below the condenser, draw a U-shaped trap where liquid collects; label water with organic compounds.
• Draw a small flask of boiling water with a heat source, joined back to the spark chamber by a tube.
• Add arrows to show the gases moving round the loop, and label electric discharge at the electrodes.