The essentials
Ideas about the origin of life have changed over time. Early thinkers believed in special creation. The theory of panspermia suggested that units of life called spores were transferred to Earth from other planets. Another old idea, spontaneous generation, held that life arose from decaying and rotting matter such as straw and mud. Louis Pasteur showed that life comes only from pre-existing life: in pre-sterilised flasks, killed yeast did not give rise to new life, while a flask open to air did.
Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (such as RNA and protein), and that the formation of life was preceded by chemical evolution, the formation of diverse organic molecules from inorganic constituents. Conditions on the early Earth included high temperature, volcanic storms and a reducing atmosphere containing CH₄, NH₃ and other gases.
In 1953, S.L. Miller, an American scientist, created similar conditions on a laboratory scale. He created electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800 °C and observed the formation of amino acids. Others observed the formation of sugars, nitrogen bases, pigments and fats. Analysis of meteorite content also revealed similar processes occurring elsewhere in space.
Key points
- Earth is believed to have formed about 4.5 billion years ago; there was no atmosphere at first.
- The first non-cellular forms of life could have originated about 3 billion years ago, possibly as giant molecules like RNA, proteins and polysaccharides.
- The first cellular forms did not originate until about 2000 million years ago; they were probably single cells in water.
Common mistakes
- Miller's experiment produced amino acids, not living cells. It supports chemical evolution, not spontaneous generation.