What the famous Miller-Urey experiment went wrong
Science at the start of the 20th century was undergoing many simultaneous revolutions. Radiological dating has counted the years of Earth’s existence in billions, and eons of sediment have demonstrated its geological evolution. The biological theory of evolution had become accepted, but mysteries remained as to its mechanism of selection and the molecular biology of genetics. The vestiges of life go back a long, long time, beginning with simple organisms. These ideas reached their climax with the question of abiogenesis: could the first life have sprung from non-living matter?
In 1952, a graduate student named Stanley Miller, just 22, designed an experiment to test whether the amino acids that make up proteins could be created under the conditions believed to exist on primordial Earth. Together with his advisor Harold Urey, winner of the Nobel Prize, he carried out the experiment, which is now recounted repeatedly in textbooks around the world.
The experiment mixed water and simple gases – methane, ammonia, and hydrogen – and shocked them with an artificial lightning bolt in a sealed glass device. Within a few days, a thick colored substance accumulated at the bottom of the device. This rubbish contained five of the basic molecules common to living creatures. Reviewing this experiment over the years, Miller claimed to have found up to 11 amino acids. Subsequent work varying the electric spark, gases, and the device itself created another dozen. After Miller’s death in 2007, the remains of his original experiments were re-examined by his former student. There may have been as many as 20-25 amino acids created even in this early original experiment.
The Miller-Urey experiment is a bold example of testing a complex hypothesis. It is also a lesson to draw more than the most careful and limited conclusions.
Has anyone thought about glassware?
In the years following the original work, several limitations dampened the enthusiasm generated by its result. Simple amino acids did not combine to form more complex proteins or anything resembling primitive life. In addition, the exact makeup of young Earth did not match Miller’s conditions. And small details of the setup seem to have affected the results. A new study published last month in Scientific reports investigating one of those haunting details. He finds that the precise composition of the apparatus housing the experiment is crucial for the formation of amino acids.
The highly alkaline chemical broth dissolves a small amount of the borosilicate glass reactor vessel used in the original and subsequent experiments. Pieces of dissolved silica permeate the liquid, probably creating and catalyzing reactions. Eroded walls of glass can also stimulate the catalysis of various reactions. This increases the total production of amino acids and allows the formation of certain chemicals which are not created when the experiment is repeated in a Teflon apparatus. But, by carrying out the experiment in a Teflon apparatus deliberately contaminated with borosilicate, some of the lost amino acid production was recovered.
Complex questions require carefully designed experiments
The Miller-Urey experiment was based on a complicated system. Over the years, many variables have changed, such as the concentration and composition of gases. In order to demonstrate which may be plausible – that is, if biomolecules can be created from inorganic materials – it has been a stunning success. But there was no good control. We now see that this could have been a pretty big mistake.
Part of the art of science is guessing which of the myriad complexities matters and which does not. Which variables can be taken into account or understood without testing, and which ones can be intelligently evaded by the experimental design? It is a border between hard science and intuitive art. It is certainly not obvious that the glass plays a role in the result, but it apparently does.
A safer and more careful form of science is to conduct an experiment that varies from person to person. and only one variable at a time. It is a slow and laborious process. It can be extremely difficult to test complex hypotheses such as, “Could life evolve from non-life on early Earth?” The authors of the new work performed such a single-variable test. They performed the entire Miller-Urey experiment several times, varying only the presence of silicate glass. Tests carried out in a glass vessel produced one set of results, while those using a Teflon apparatus produced another.
Systematically going through each potential variable, one at a time, might be called “brute force.” But here too there is art in deciding which variable among many possibilities to test and in which way. In this case, we learned that glass silicates played an important role in the Miller-Urey experiment. Perhaps this means that the silicate rock formations on early Earth were necessary to produce life. May be.