How valid was the Miller Urey Experiment?
There have been a number of objections to the implications derived from these experiments. The following are extracts from Wikipedia:
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.
Off the Scientific Press
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 to 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. One of the authors, Prof. Owen Toon notes: "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... I think this study makes the experiments by Miller and others relevant again." Outgassing calculations using a chondritic model for the early earth, (Washington University, September 2005) complement the Waterloo/Colorado results in re-establishing the importance of the Miller-Urey experiment.
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 an 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".
http://biology.clc.uc.edu/courses/bio106/origins.htm (requires Netscape to do interactive parts)