Origin Of Life: Twentieth Century Landmarks

The Miller-Urey Experiment

In 1952, Harold Urey tried to calculate the chemical constituents of the atmosphere of the early Earth. He based his calculations on the (then) widely held view that the early atmosphere was reducing, and concluded that the main constituents were methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water (H₂O). He suggested that his student, Stanley Miller, should do an experiment attempting to synthesise organic compounds in such an atmosphere.

Miller carried out an experiment in 1953 in which he passed a continuous spark discharge at 60,000 Volts through a flask containing the gases identified by Urey, along with water. Miller found that after a week, most of the ammonia and much of the methane had been consumed. The main gaseous products were carbon monoxide (CO) and nitrogen (N₂). In addition, there was an accumulation of dark material in the water. Few of the specific constituents of this could not be identified, but it was clear that the material included a large range of organic polymers.

Analysis of the aqueous solution showed that the following had also been synthesised:-

• 25 amino acids (the main ones being glycine, alanine and aspartic acid)
• Several fatty acids
• Hydroxy acids
• Amide products.

The Miller-Urey experiment was immediately recognised as an important breakthrough in the study of the origin of life. It was received as confirmation that several of the key molecules of life could have been synthesised on the primitive Earth in the kind of conditions envisaged by Oparin and Haldane. These molecules would then have been able to take part in 'prebiotic' chemical processes, leading to the origin of life.

Since the Miller-Urey experiment, a great deal of effort has been spent investigating prebiotic chemistry. It has become apparent that organising simple molecules into assemblies capable of reproducing and evolving is a far greater task than was generally realised during the excitement that followed the experiment. In addition, the view that the early atmosphere was highly reducing was challenged towards the end of the twentieth century, and is no longer the concensus view.

Although the significance of specific details of the Miller-Urey for the origin of life may now be in question, it began the new scientific discipline of prebiotic chemistry, and has been enormously influential in the development of ideas about the origin of life.