Miller and Urey experiment

Scheme of Miller's experiment.

The Miller and Urey experiment represents the beginning of experimental abiogenesis and the first verification that organic molecules can be formed from inorganic substances under simple suitable environmental conditions. It was carried out in 1953 by Stanley Miller and Harold Clayton Urey at the University of Chicago. The experiment was key to supporting the primordial soup theory in the origin of life by Aleksandr Oparin and John Burdon Sanderson Haldane.

According to this experiment, the synthesis of organic compounds, such as amino acids, should have been easy on early Earth. Other researchers –following this procedure and varying the type and amounts of reacting substances- have produced some simple components of nucleic acids and even ATP^{[citation needed]}. This experience opened a new branch of biology, exobiology. Since then, new knowledge about DNA and RNA, prebiotic conditions on other planets, and the announcement of possible bacterial fossils found in meteorites from Mars (such as ALH 84001), have renewed the question of the origin of life.

The Experiment

The experiment used water (H₂O), methane (CH₄), ammonia (NH₃) and hydrogen (H₂). These chemicals were introduced into a sterile sealed assembly of tubes and glass containers connected together in a closed circuit. One of the containers was filled with water, with ammonia, methane and hydrogen, and another contained a pair of electrodes. The water was heated so that it evaporated, and in another container the electrodes emitted electrical discharges, which were traversed by the water vapor and the gases coming from the evaporation flask, and which simulated the atmosphere of the primitive Earth. The gases were then cooled so that the gases condensed and the droplets returned to the first container, which was heated again in a continuous cycle, thus creating different organic compounds. The cycle was stopped after one week and the solution was analyzed by paper chromatography. Aleksandr Oparin knew that the Earth lacked oxygen before life. The evidence is that when rocks with iron from that primitive age are extracted, the iron is not in the oxide form but in its metallic form.

Before life appeared on Earth, there were simple, inorganic molecules like water, methane, or ammonia. But due to the factors that occurred on Earth at that time (lightning, constant impacts of meteorites, volcanic eruptions, etc.) inorganic substances combined giving rise to amino acids such as glycine, alpha alanine, beta alanine while spots on the chromatogram for aspartic acid and alpha-aminobutyric acid were more diffuse. The pre-existing inorganic molecules became organic when there was an input of energy. The complex substances were grouped in droplets called coacervates that accumulated in the primitive seas until they gave rise to molecules capable of reproducing. This process occurred over billions of years. A part of these first living beings, millions of years later, (see Evolutionary history of plants) were the ones that transformed the large amounts of carbon dioxide into oxygen after another million years.

Stanley Miller tried to test this theory with a simple apparatus mixing water vapor, methane, ammonia, and hydrogen. These gases were thought to be the ones that existed in Earth's atmosphere at the time. To simulate the existing natural environment then he used heat, natural electrical discharges (energy inputs) he used electrodes and ultraviolet radiation. With this experiment, he simulated prebiotic conditions and with the input of energy from the electrodes, he managed to obtain amino acids, urea, various organic acids, and other organic components, but he never managed to obtain living matter, only some of its components.

The story of the experiment

In 1953 Stanley L. Miller, a student at the University of California, proposed to his director Harold Urey to carry out an experiment to counter the hypothesis of Aleksandr Oparin and J. B. S. Haldane, according to which in the conditions of the primitive Earth they had produced chemical reactions that led to the formation of organic compounds from inorganic ones, which later gave rise to the first forms of life. Urey thought that the results would be inconclusive but finally accepted Miller's proposal; They designed an apparatus in which they simulated some conditions of the atmosphere of the primitive Earth.

Description

The experiment basically consisted of subjecting a mixture of methane, ammonia, hydrogen, carbon dioxide, nitrogen, and water to 60,000-volt electrical discharges, heat, and ultraviolet light. As a result, the formation of a series of organic molecules was observed, among which acetic acid stands out, and the amino acids glycine, alanine, glutamic acid and aspartic acid, used by cells as the basic building blocks to synthesize their proteins.

The gaseous mixture was introduced into the apparatus, the water was kept boiling and later the condensation was carried out; the substances were held through the apparatus while two electrodes produced continuous electrical discharges in two vessels.

After the mixture had circulated through the apparatus, samples were extracted by means of a stopcock for analysis. In these were found, as mentioned, several amino acids, a carbohydrate and some other organic compounds. The experiment has been repeated on multiple occasions, obtaining various organic compounds. However, proteins have not yet been obtained.

In 2008, other researchers found the apparatus Miller used in his early experiments and analyzed the remaining material using more sensitive modern techniques. The experiments had included the simulation of other environments, not published at the time, such as gases released in volcanic eruptions. The subsequent analysis found 20 amino acids that are the components of the proteins and 6 components of the nucleotide acids that appear in the nucleus of the cells and are elemental substances to form life, achieving with this to provide solid evidence that supports the evolutionary development of the life on earth.

Chemistry of the experiment

The first phase of the reactions between the gas mixture of the experiment originates hydrogen cyanide (HCN), formaldehyde (CH₂O) and other intermediate active compounds such as acetylene, cyanoacetylene, etc:

CO₂ → CO + O (atomic oxygen)

CH₄ + 2O → CH₂O + H₂O

CO + NH₃ → HCN + H₂O

CH₄ + NH₃ → HCN + 3H₂ (BMA process)

Formaldehyde, ammonia, and HCN can then undergo a reaction called Strecker synthesis to form amino acids or other biomolecules:

CH₂O + HCN + NH₃ → NH₂-CH₂-CN + H₂O

NH₂-CH₂-CN + 2H₂O → NH₃ + NH₂-CH₂-COOH (glycine)

In addition, water and formaldehyde can respond due to the Butlerov reaction to produce various sugars such as ribose.

Impact

This experiment, along with considerable geological, biological, and chemical evidence, helps support the theory that the first life formed spontaneously through chemical reactions. However, there are still scientists who are not convinced. The British astrophysicist Fred Hoyle – contrary to the Big Bang theory and defender of a stationary universe, at the time – compared the supposed possibility that life appeared on Earth as a result of chemical reactions with the probability “of a tornado passing on a scrap heap to create a Boeing 747 out of the materials found there." The consensus among biologists is that Hoyle's statistical interpretation is flawed, and they refer to this argument as Hoyle's fallacy.