The Miller-Urey experiment (or Urey-Miller experiment) was an experiment that simulated hypothetical conditions present on the early Earth and tested for the occurrence of chemical evolution (the Oparin and Haldane hypothesis stated that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors; the Miller-Urey tested this hypothesis). The experiment is considered to be the classic experiment on the origin of life. It was conducted in 1953 by Stanley L. Miller and Harold C. Urey at the University of Chicago.
Experiment and interpretation
The experiment used water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2). The chemicals were all sealed inside a sterile array of glass tubes and flasks connected together in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes. The liquid water was heated to induce evaporation, sparks were fired through the atmosphere and water vapor to simulate lightning, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle.
At the end of one week of continuous operation, Miller and Urey observed that as much as 10-15% of the carbon within the system was now in the form of organic compounds. Two percent of the carbon had formed some of the amino acids which are used to make proteins in living cells.
The molecules produced were relatively simple organic molecules, far from a complete living biochemical system, but the experiment established that natural processes could produce the building blocks of life without requiring life to synthesize them in the first place.
This experiment inspired many similar experiments in a similar vein. In 1961, Joan Oro found that amino acids could be made from hydrogen cyanide (HCN) and ammonia in a water solution. He also found that his experiment produced a large amount of the nucleotide base adenine. Experiments conducted later showed that the other RNA and DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere.
Earth's early atmosphere
There have been a number of objections to the implications derived from these experiments. It is now believed that Earth's original atmosphere did not contain as large a quantity of reducing molecules as were thought at the time:
- Originally it was thought that the primitive secondary atmosphere contained mostly NH3 and CH4. However, it is likely that most of the atmospheric carbon was CO2 with perhaps some CO and the nitrogen mostly N2. The reasons for this are (a) volcanic gas has more CO2, CO and N2 than CH4 and NH3 and (b) UV radiation destroys NH3 and CH4 so that these molecules would have been short-lived. UV light photolyses H2O to H· and ·OH radicals. These then attack methane, giving eventually CO2 and releasing H2 which would be lost into space.
- In practice gas mixtures containing CO, CO2, N2, etc. give much the same products as those containing CH4 and NH3 so long as there is no O2. The H atoms come mostly from water vapor. In fact, in order to generate aromatic amino acids under primitive earth conditions it is necessary to use less hydrogen-rich gaseous mixtures. Most of the natural amino acids, hydroxyacids, purines, pyrimidines, and sugars have been produced in variants of the Miller experiment.
More recent results may have called this into question, however. Simulations done at the University of Waterloo and University of Colorado in 2005 indicated that the early atmosphere of Earth could have contained up 40% hydrogen, implying a much more hospitable environment for the formation of prebiotic organic molecules. The escape of hydrogen from Earth's atmosphere into space may have occurred at only 1% of the rate previously believed based on revised estimates of the upper atmosphere's temperature. The authors themselves, in recognition of the gravity of their result, note: "In this new scenario, organics can be produced efficiently in the early atmosphere, leading us back to the organic-rich soup-in-the-ocean concept." and "I think this study makes the experiments by Miller and others relevant again" (statements by Prof. Owen Toon).
Also, although lightning storms are thought to have been very common in the primordial atmosphere, they are not thought to have been as common as the amount of electricity used by the Miller-Urey experiment may imply. These factors suggest that much lower concentrations of biochemicals would have been produced on Earth than was originally predicted (although the time scale would be 100 million years instead of a week). Similar experiments, both with different sources of energy and with different mixtures of gases, have resulted in amino and hydroxy acids being produced; it is likely that at least some organic compounds would have been generated on the early Earth.
However, as soon as oxygen gas is added to the mixture, no organic molecules are formed. Recent research has been seized upon by opponents of Urey-Miller hypothesis which shows the presence of uranium in sediments dated to 3.7 Ga and indicates it was transported in solution by oxygenated water (otherwise it would have precipitated out) (Rosing & Frei 2004). It is wrongly argued by some, in attempt to invalidate the hypothesis of abiogenesis, that this presence of oxygen precludes the formation of prebiotic molecules via a Miller-Urey-like scenario. However, the authors of the paper are arguing that the oxygen is evidence merely of the existence of photosynthetic organisms 3.7 Ga ago (a value about 200 Ma earlier than current values), a conclusion which would possibly have the effect of pushing back the time frame in which Miller-Urey reactions and abiogenesis could potentially have occurred, it would not preclude them in any way. Though there is somewhat controversial evidence for very small (less than 0.1%) amounts of oxygen in the atmosphere almost as old as Earth's oldest rocks the authors are not in any way arguing for the existence of a strongly oxygen containing atmosphere occurring any earlier than previously thought, and they state:"..In fact most evidence suggests that oxygenic photosynthesis was present during time periods from which there is evidence for a non-oxygenic atmosphere".
Conditions similar to those of the Miller-Urey experiments are present in other regions of the solar system, often substituting ultraviolet light for lightning as the driving force for chemical reactions. On September 28 1969, a meteorite that fell over Murchison, Victoria, Australia was found to contain over 90 different amino acids, nineteen of which are found in Earth life. Comets and other icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds (such as tholins) formed by these processes, in some cases so much so that the surfaces of these bodies are turned dark red or as black as asphalt. The early Earth was bombarded heavily by comets, possibly providing a large supply of complex organic molecules along with the water and other volatiles they contributed. (This could also imply an origin of life outside of Earth, which then migrated here. See: Panspermia)
Recent related studies
During recent years, studies have been made of the amino acid composition of "old" areas in "old" genes, defined as those that are found to be common to organisms from several widely separated species, assumed to share only the last universal ancestor (LUA) of all extant species. These studies found that these areas are enriched in those amino acids that are also most readily produced in the Miller-Urey experiment. This suggests that the original genetic code was based on a smaller number of amino acids -- only those available in prebiotic nature -- than the current one. (Brooks et al. 2002).