Miller-Urey Experiment
The widely reported experiment by Miller and Urey, sometimes referred to as the "Miller Experiment", was conducted in the 1950's.  At that time, under 'just so' assumptions made in order to make the chemistry work, the early earth's atmosphere was assumed to be essentially free of oxygen.  In the well known experiment, that Carl Sagan popularized as having produced the 'stuff of life', gases of the assumed early earth atmosphere were put in a closed apparatus, electrical discharge arcs were passed through the circulating gases, and products were trapped out and analyzed.  Some organic molecules resulted, including a few amino acids.  Most products were similar to tar.

However, geochemical evidence dating back at least two decades now confirms an abundance of oxygen in the early atmosphere.  Practically speaking, this alone relegates the Miller Urey experiment to an interesting chemical demonstration, but being irrelevent to chemical origin of life discussions, since the oxygen would not permit the reactions to take place and survive. 

Amino acids cannot be formed and survive in an oxygen rich environment. Hence, Miller and Urey forced their experimentally designed 'atmosphere' to contain no free oxygen, as was the common belief of their day. However, evidence from the geologic record now confirm that the early earth contained significant amounts of oxygen, and that the earlier accepted model of a reducing atmosphere (oxygen poor) was false.

The experimental design, while at the time praised as innovative, incorporated an amino acid 'trap'. The function of this 'trap' was to try and preserve any possibly created amino acids before they would be destroyed by the various chemicals in the apparatus. While successful in trapping some amino acids, this is now recognized as not being analogous to the real natural world - there are no known or even hypothesized protective traps observed in nature.

Last, the amino acids produced by the experiment, most of which were non life relevant tars, were racemic, or an approximately equal ratio of dextro- and levo- (right and left handed) molecular arrangement, called chirality. However, amino acids in living organisms are 100% left handed. Racemic mixtures of amino acids are actually toxic to life, not the 'stuff of life' as originally announced.

Evolutionary biologists and origin of life researchers now recognize the Miller Urey experiment as an interesting but largely now unimportant experiment.

Some proposed textbooks make statements like, "Miller and Urey's experiments showed that under the proposed conditions on the early Earth, small organic molecules, such as amino acids, could form."

This statement, while technically accurate, is highly misleading in that the conditions on the early Earth were NOT as modeled in the experiment. The caption should be changed to read, "Miller and Urey showed that some amino acids could be produced under certain laboratory conditions. However, geoscientists now do not believe those conditions selected for use in the laboratory are representative of the early Earth atmosphere."