Hardly a conversation can be had about the origin of life on Earth without mention of the Miller-Urey experiment. Very little is known about the conditions on Earth during the time that life would have been forming. Harold Urey and his then graduate student Stanley Miller were amongst the first scientists to postulate about early conditions. They conducted an experiment that has been repeated in its original and altered form for over five decades. Their work has become seminal for those studying the chemistry of the origin of life on Earth.
Although scientists continue to collect new data that sheds light on the subject, there is still quite a bit of debate over the composition of early Earth's atmosphere. What does the atmosphere have to do with the origin of life, you might ask? Well, the chemical composition of the atmosphere strongly influences the types of chemical reactions occurring at the surface of the planet, and consequently impacts the conditions under which life would have originated. Based on work published in The Origin of Life by the Russian scientist Alexander Oparin in 1938, Miller suggested that life was forming during a time when Earth's atmosphere consisted of methane, ammonia, water, and hydrogen. This chemical makeup is quite different from our modern atmosphere of nitrogen, oxygen, and other gases. Miller introduced these molecules into a sealed flask, applied an electric discharge, and allowed the system to cycle for a week. What he discovered has impacted origins research for over fifty years.
The original apparatus used by Miller and Urey was quite simple compared to today's standards. It essentially consisted of two glass flasks connected by glass tubing. One flask served as the boiling flask, where gases and other molecules could accumulate in a water phase. The other flask (located above the boiling flask) served as a place where gases could accumulate and mix together. An electrical discharge, meant to simulate lightning to produce free radicals, was provided by using an induction coil.
The experimental procedure was also straightforward. Water was first added to the boiling flask. Then the apparatus was evacuated completely of air. Once the air had been removed, hydrogen gas (H2), methane (CH4) and ammonia (NH3) were pumped into the apparatus. Finally, the water in the flask was boiled and the electrical discharge was started. The entire system was allowed to run continuously for a week.
Like any good scientist, Miller took copious notes of what happened inside the apparatus during the weeklong experiment. After the first day, the water in the flask turned distinctly pink. As the week progressed, the solution inside became more and more red and also a bit cloudy. Once the experiment was complete, Miller and Urey determined that the cloudiness, or turbidity, of the solution was due to silica from the glass. The reddish color, however, resulted from organic compounds that "stuck" to the silica. Although difficult to see at first, Miller also noted yellow organic molecules.
At the end of the week, Miller collected the contents of the apparatus and tested the contents for amino acids using chromatography. Initial tests confirmed the presence of glycine, alpha-alanine, and beta-alanine and suggested that aspartic acid and alpha-amino-n-butyric acid had also been produced. This list of amino acids falls miles short of the 20 amino acids commonly used by life on Earth. However, Miller and Urey both suspected that other amino acids were also present, but in such small amounts that their detection was difficult to impossible.
The intent of Miller was not to try and produce amino acids. Rather, his intent was to explore the early conditions on Earth and what the naturally occurring results would be. What he discovered was that, although the conditions he proposed are not optimum, organic molecule synthesis could have been a natural consequence in Earth's history. More importantly, Miller and Urey went on to explore amino acid synthesis by developing more efficient apparition and altering the initial atmospheric conditions in the simulated environment.
Scientists studying the atmosphere of early Earth now believe that the primary atmospheric constituents were different from those first proposed by Oparin and later tested by Urey. James Casting at Pennsylvania State University has suggested that the atmosphere on Earth just after the succession of heavy bombardment would have been dominated by carbon dioxide and nitrogen and contained small amounts of carbon monoxide, hydrogen gas, and reduced sulfur gases. Now instead of a simple set of glass flasks connected by tubes and sealed, scientists use complex computer models and mathematical equations to simulate the conditions of early Earth. Unless we develop a time machine, we will never know exactly what the planet was like. But through good observations and critical analysis by all scientists in the field, we will definitely arrive at feasible theories about the beginnings of Earth. The Miller-Urey experiment traveled only the first tentative steps along the road from simple molecules to a cell. But it showed that some of life's core ingredients can form quickly and naturally, and that concentrating chemicals and adding energy can lead to a progression from simplicity to complexity.