Discovering the origins of life on Earth

Organic molecules Before life could begin the organic molecules necessary for life first had to ‘evolve’. These molecules include carbohydrates, lipids, amino acids, proteins and nucleic acids. The conditions on early Earth were much more favourable for the development of organic molecules than today’s Earth. E.g. organic molecules react very rapidly with oxygen. If oxygen had been present in the early atmosphere organic compounds would not have been able to build up to concentrations necessary for life to begin.

Theory 1 - Chemicals from outer space. Scientists believe that early Earth was much more heavily bombarded by meteorites than what it is today. Some scientists also believe that organic molecules came to Earth on these meteorites. This theory has been given credence by the discovery of organic molecules and even amino acids in certain types of meteorites. Some scientists go a step further and theorise that life actually evolved in outer space and was then seeded on to Earth through meteorites. This theory is often called Panspermia.

Theory 2 - Chemicals formed on Earth In the 1920’s the Russian biochemist Alexander Oparin and British scientist John Haldane both independently suggested that the early atmosphere contained all the necessary chemical requirements for life to form. Their hypothesis was that organic chemical evolution preceded cellular evolution. This theory was given increased acceptability by the experiment of Urey and Miller in the 1950’s. This theory is sometimes called the chemosynthetic orgin of life.

Urey and Miller Stanley Miller was a graduate student of Professor Harold Urey at the University of Chicago in the 1950’s. Urey believed in the hypothesis of Oparin and Haldane that the organic chemicals necessary for life evolved from simple chemicals during the early Earth. His student, Miller, set up an experiment to determine if this could be proven.

The Urey-Miller Experiment - method A closed system was set up with an ‘atmosphere’ chamber containing the gases methane, ammonia and hydrogen. There was also a lower chamber filled with water to simulate the oceans. An electrical discharge was introduced into the atmosphere chamber to simulate lightning and cosmic rays. See Figure 1.5.1, p. 59.

The Urey-Miller Experiment - results After letting the experiment run for one week Urey and Miller decided to analyse their results. They found amino acids and other organic chemicals in the ‘ocean’ and ‘atmosphere’ chambers. Their experiment was important because it supported Oparin and Haldane’s theory by showing it was possible for complex organic molecules such as amino acids to form under the conditions of early Earth. Subsequent experiments showed that ultraviolet light could be used instead of electricity and that nitrogenous bases could also be produced.

Questions 1)What molecules are necessary for life? 2)Draw a labelled diagram of the Urey-Miller experiment. 3)Outline the roles of Oparin, Haldane, Urey & Miller in developing our understanding of how the ingredients for life were formed. 4)Explain the different theories of panspermia and chemosynthesis. 5)Why was it necessary for organic molecules to evolve in an earlier Earth?