The Origin of Life, Part I; Miller's Experiment

As Richard Dawkins puts it, “the theory of evolution is about as much open to doubt as the theory that the Earth goes round the sun”. We can also be sure that all of the diverse forms of life we see around us today have arisen from some common, primitive, single original living entity. Our first problem is, then, how this living “thing” originated. In 1953, Stanley Miller conducted his famous (or infamous) experiment. At the time he was a graduate student at the University of Chicago. For decades, scientists had speculated whether the complex organic compounds characteristic of living things could have somehow been generated spontaneously on the early Earth. Spontaneous generation of organic compounds can’t happen today. This is because organic compounds are too fragile.

It is possible that, given enough time, a complex compound might just come together. If it did, however, it would immediately be taken apart. This is because today our planet is just filled with oxygen. Oxygen breaks down organic compounds. Oxygen pulls electrons out of organic compounds and turns them into inorganic compounds.

How can we even get the formation of any kind of organic compound, if as soon as anything begins to arise by chance, it is immediately taken apart? Well, this one is easy. If oxygen is bothering you, just get rid of it.

Before the Miller-Urey experiment, two scientists, Alexander Oparin and J.B.S. Haldane, independently suggested that the early Earth actually did not have much or any oxygen. Oxygen is all around us in the atmosphere, but they suggested that when the planet was formed, the first atmosphere that developed was entirely composed of just a few gases: hydrogen, methane, ammonia and water vapor. This would be as the atmospheres of the moons of other planets that have been described.

Oparin and Haldane independently suggested that the problem of spontaneous generation of organic compounds wasn’t really a big deal, because the early Earth did not have an oxidizing atmosphere. To test this hypothesis, what Miller did was set out to reproduce the conditions presumed to exist on the early Earth before life have arisen, and see if he could get the spontaneous production of organic compounds.

A Simple Experiment, Powerful Results

Miller’s experiment was set up this way: He had two flasks connected by a series of glass tubes. He had a lower flask in which he put water, and he heated this water gently with a little flame. He would cause the water to evaporate and create water vapor, which would circulate into a higher flask. In the upper flask, Miller also added a number of other gasses. He created an atmosphere similar to the one of the early Earth, consistent of hydrogen, methane, ammonia and wáter vapor.

The Experiment. Souce: Wikipedia.

Miller also exposed the gases in this upper chamber to a lot of energy by putting two electrodes that would create electrical sparks. He knew that he needed energy to create any kind of compound, certainly organic compounds.

This is actually a pretty simple experiment, and you can almost do this in your own house. All of these materials are easily available. You could replicate Miller’s experiment and results, which were spectacular. In only a couple of days, he found he could synthesize a whole range of different organic compounds, including some very complex ones, like amino-acids.

The scientific community immediately set out to replicate this. Many people replicated the experiment and it quickly became clear that depending on starting conditions, it was possible to spontaneously, without any preexisting organic molecule, produce all of the amino-acids that are normally found in living material. Most intriguingly of all, you could create nucleotides, which are the building blocks of nucleic acids, DNA and RNA.

The implication of Miller’s experiment and those that followed was that there appeared to be no trouble at all for complex organic compounds to arise spontaneously on the inorganic early Earth. This is a first stepping stone to the origin of life from non-living matter.

On the other hand, as exciting as this result was, the organic compounds that Miller created were still relatively simple compared to the stuff that we are made of. What else do we need to get something that we would call “living”? We have to take our synthesis of organic compounds even farther, beyond these organic building blocks, to get the varied extremely complex molecules that living systems are really made of. We will see how that was possible in the early Earth next time.

Return from Miller's Experiment to the Origin of Life