Miller Urey Experiment

Many consider discussions on how life originated on earth to be purely hypothetical, but in 1952 two American chemists--Harold C. Urey and Stanley Miller--set out to test the time's most prominent 'origin of life on earth' theory. Here, we will learn about the Miller-Urey experiment!

• First, we will look at the definition of the Miller-Urey experiment.
• Then, we will talk about the results of the Miller-Urey experiment.
• After, we will explore the significance of the Miller-Urey experiment.

The Definition of the Miller-Urey Experiment

Let's start by looking at the definition of the Miller-Urey experiment.

The Miller-Urey experiment was an experiment that tested the Oparin-Haldane Hypothesis which was, at the time, a highly regarded theory for the evolution of life on earth through chemical evolution.

What was the Oparin-Haldane Hypothesis?

The Oparin-Haldane Hypothesis suggested life emerged from a series of step by step reactions between inorganic matter driven by a large energy input. These reactions initially produced the 'building blocks' of life (e.g., amino acids and nucleotides), then more and more complex molecules until primitive life forms arose.

Miller and Urey set out to demonstrate that organic molecules could be produced from the simple inorganic molecules present in the primordial soup as the Oparin-Haldane Hypothesis proposed.

Figure 1. Harold Urey performing an experiment.

The Oparin-Haldane Hypothesis--note that this point is important--described life emerging in the oceans and under methane-rich reducing atmospheric conditions. So, these were the conditions that Miller and Urey attempted to mimic.

Miller and Urey attempted to recreate the reducing primordial atmospheric conditions laid out by Oparin and Haldane (Figure 2) by combining four gases in an enclosed environment:

1. Water vapor

2. Methane

3. Ammonia

4. Molecular hydrogen

The pair of scientists then stimulated their faux atmosphere with electrical pules to simulate energy provided by lightning, UV rays or hydrothermal vents and left the experiment running to see if the building blocks for life would form.

Miller-Urey Experiment Results

After running for a week, the liquid simulating the ocean inside their apparatus turned a brownish-black color.

Miller and Urey's analysis of the solution showed complex stepwise chemical reactions had occurred forming simple organic molecules, including amino acids - proving organic molecules could form under the conditions laid out in the Oparin-Haldane hypothesis.

Before these findings, scientists had thought the building blocks of life like amino acids could only be produced by life, inside an organism.

With this, the Miller-Urey Experiment produced the first evidence that organic molecules could be spontaneously produced from only inorganic molecules, suggesting Oparin's primordial soup could have existed at some point in Earth's ancient history.

The Miller-Urey experiment did not, however, fully back up the Oparin-Haldane hypothesis as it only tested the initial stages of chemical evolution, and didn't dive deeper into the role of coacervates and membrane formation.

Miller-Urey Experiment Debunked

The Miller-Urey experiment was modelled on, and recreated conditions laid out under the Oparin-Haldane Hypothesis. Primarily recreating the reducing atmospheric conditions the previous pair stipulated was crucial for the formation of early life.

Though recent geochemical analysis of the earth's primordial atmosphere paints a different picture...

Scientists now think the earth's primordial atmosphere was composed mainly of carbon dioxide and nitrogen: an atmospheric makeup very different from the heavy ammonia and methane atmosphere that Miller and Urey recreated.

The Miller-Urey Experiment Undergoes Further Testing

In 1983, Miller attempted to recreate his experiment using the updated mixture of gases - but failed to produce much more than a few amino acids.

More recently American chemists have again repeated the famous Miller-Urey Experiment using the more accurate gaseous mixtures.

Whilst their experiments returned similarly poor amino acid turn out, they noticed nitrates forming in the product. These nitrates were able to break down amino acids as quickly as they formed, yet in the conditions of primordial earth iron and carbonate minerals would have reacted with these nitrates before they had the chance to do so.

Adding these crucial chemicals to the mix produces a solution that, whilst not as complex as the initial findings of the Miller-Urey Experiment, is abundant in amino acids.

These findings have renewed hope that continued experiments will further pin down likely hypotheses, scenarios, and conditions for the origin of life on earth.

Debunking the Miller-Urey Experiment: Chemicals from Space

Whilst the Miller-Urey Experiment proved organic matter can be produced from inorganic matter alone, some scientists are not convinced this is strong enough evidence for the origin of life through chemical evolution alone. The Miller-Urey Experiment failed to produce all the building blocks needed for life - some complex nucleotides have yet to be produced even in subsequent experiments.

The competition's answer to how these more complex building blocks came about is: matter from space. Many scientists believe these complex nucleotides could have been brought to earth through meteorite collisions, and from there evolved into the life that occupies our planet today. However, it is important to note this is just one of the many origin of life theories.

Miller-Urey Experiment Conclusion

The Miller-Urey Experiment was a test tube earth experiment, recreating the reducing primordial atmospheric conditions thought to have been present during the origin of life on earth.

The Miller Urey experiment set out to provide evidence for the Oparin-Haldane hypothesis and has provided evidence for the occurrence of the first simple steps of chemical evolution. Giving validity to Darwin's puddle and Oparin's primordial soup theories.

Perhaps more importantly, however, is the field of pre-biotic chemical experiments which followed. Thanks to Miller and Urey we now know more than previously thought possible about potential ways life could have originated.

The significance of the Miller-Urey Experiment

Before Miller and Urey performed their famous experiments, ideas such as Darwin's puddle of chemistry and life and Oparin's primordial soup were nothing more than speculation.

Miller and Urey devised a way to put some ideas about the origin of life to the test. Their experiment has also prompted a wide variety of research and similar experiments showing similar chemical evolution under a wide range of conditions and subject to different energy sources.

The main component of all living organisms is organic compounds. Organic compounds are complex molecules with carbon at the center. Prior to the findings of the Miller-Urey Experiment it was thought these complex biotic chemicals could only be produced by life forms.

The Miller-Urey Experiment, however, was a pivotal moment in the history of research into the origin of life on earth - as Miller and Urey provided the first evidence that organic molecules could come from inorganic molecules. With their experiments, a whole new field of chemistry, known as pre-biotic chemistry was born.