As Earth cooled, the water vapor condensed into the oceans and much of the hydrogen was lost into space.
In the 1920s, Russian chemist A. I. Oparin and British scientist J. B. S. Haldane independently postulated that conditions on early Earth favored the synthesis of organic compounds from inorganic precursors.
They reasoned that this could not happen today because high levels of oxygen in the atmosphere attack chemical bonds.
A reducing environment in the early atmosphere would have promoted the joining of simple molecules to form more complex ones.
The considerable energy required to make organic molecules could be provided by lightning and the intense UV radiation that penetrated the primitive atmosphere.
Young suns emit more UV radiation. The lack of an ozone layer in the early atmosphere would have allowed this radiation to reach Earth.
Haldane suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.
In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating, in the laboratory, the conditions that had been postulated for early Earth.
They discharged sparks in an “atmosphere” of gases and water vapor.
The Miller-Urey experiments produced a variety of amino acids and other organic molecules.
Other attempts to reproduce the Miller-Urey experiment with other gas mixtures have also produced organic molecules, although in smaller quantities.
It is unclear whether the atmosphere contained enough methane and ammonia to be reducing.
There is growing evidence that the early atmosphere was made up primarily of nitrogen and carbon dioxide.
Miller-Urey-type experiments with such atmospheres have not produced organic molecules.
It is likely that small “pockets” of the early atmosphere near volcanic openings were reducing.
Alternate sites proposed for the synthesis of organic molecules include submerged volcanoes and deep-sea vents where hot water and minerals gush into the deep ocean.
These regions are rich in inorganic sulfur and iron compounds, which are important in ATP synthesis by present-day organisms.
Some of the organic compounds from which the first life on Earth arose may have come from space.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-3