Science and Joe

The discovery of DNA’s three-dimensional double helical structure in 1953 was amazing. However, other very important scientific discoveries were also made that year, including via the Miller-Urey Experiment.

The Miller-Urey experiment, conducted by American scientists Stanley Miller and Harold C. Urey, aimed to replicate the conditions on very early Earth to understand the origin of life. Specifically, how did organic molecules that form the foundation for cellular life, including nucleic acids such as DNA, and amino acids which compose proteins, come to be produced from reactions involving gaseous molecules such as ammonia, methane, hydrogen and water vapour that were believed to have been in high abundance in the atmosphere of early Earth?

The process, termed abiogenesis, was hypothesised in the 1920s by Aleksandr Oparin and  J.B.S Haldane who postulated that organic molecules were produced spontaneously, eventually creating the primordial soup that led to the existence of cells. Miller and Urey wanted to experimentally simulate the conditions under which the synthesis of organic molecules on primordial Earth may have occurred.

Miller-Urey’s experiment was centred on the atmosphere of early Earth having very little oxygen, but a lot of hydrogen and hydrogen containing gases such as ammonia and methane. These conditions were thought to be conducive with reduction reactions which is the process where one substance donates electrons to another substance. Ammonia and methane are prominent reducing agents meaning they become oxidised (lose electrons) in the process. These REDOX reactions, assisted by an external energy source (lightning), were believed to have allowed for the seemingly spontaneous production of complex organic molecules from simple atmospheric gaseous molecules. Miller-Urey sought to examine this theory, focusing mainly on amino acids.

In the experiment, two glass flasks were connected by glass tubing. Miller and Urey filled the lower flask with water which was boiled to produce water vapour and mimicked the ocean of early Earth. The water vapour travelled through the glass tubing to the upper flask that contained hydrogen gas, methane and ammonia, mimicking the suspected atmosphere of early Earth. The gases mixed in the upper flask before Miller and Urey provided an energy source via two inserted electrodes that simulated lightning. A condenser cooled the contents of the upper flask and allowed for phase conversion from gas to liquid. The precipitating droplets of liquid travelled downwards to a trap where samples were extracted for analysis (via paper chromatography), leading to the identification of amino acids including glycine, alanine and aspartate.

The experiment was repeated in subsequent decades by Miller himself and others, many of whom changed the method including using ultraviolet light as an energy source, different gases such as carbon dioxide and nitrogen, and more advanced analytical techniques. The production of amino acids, carbohydrates, and nitrogenous bases such as adenine, was recorded.

The production of organic molecules using carbon dioxide and nitrogen encourages opposition against the reducing atmosphere hypothesis, instead suggesting that early Earth’s atmosphere may have been more neutral, with methane and ammonia perhaps existing in smaller volumes.
 
What is certain is that Miller and Urey’s experiment was a crucial tool in helping us understand how life on Earth came to be.