Urey-Miller Experiment – A Dead End?

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Among the many issues surrounding the origin of life on Earth that must be solved is the origin of small molecules needed to build more complex molecules (like proteins or DNA) necessary to living systems. The first to really attack this problem experimentally with success were Stanley Miller and Harold Urey back in the 1950s.  At that time, Miller was a graduate student of Urey’s at the University of Chicago.  They were operating under the fundamental assumption that the early Earth’s atmosphere was reducing – meaning the atmosphere was full of hydrogen (H2) and lacking in oxygen (O2).  In his ground-breaking experiment, Miller simulated lightning (using a spark-discharge) in such a reducing atmosphere, composed of hydrogen, methane (CH4), and ammonia (NH3), then directed the products towards some water.  To his (and the rest of the scientific community’s) surprise, amongst the products produced were some of the essential amino acids for life (amino acids are the building blocks for proteins, one of the three necessary biomolecules for life along with RNA and DNA).  This was a momentous result!  For many years thereafter, scientists performed further spark-discharge experiments (simulating lightning) searching for – and finding – other necessary building blocks for life.

But, is a reducing atmosphere plausible?  Most now think no.  Rather, the early Earth atmosphere is thought to have been controlled by volcanic outgassing more similar to today’s volcanic emissions – resulting in an atmosphere composed primarily of  carbon dioxide with some nitrogen and water, but with only small amounts of hydrogen.  Unfortunately, this “neutral” atmosphere, similar to the current composition of the atmospheres of Mars and Venus, is essentially a dead-end for spark discharge experiments, with essentially no useful molecules being produced.

So – is the Urey-Miller experiment useless in terms of the production of the first building blocks for life?  Not necessarily according to OB Toon and coworkers at the University of Colorado – they are revamping the reducing atmosphere feasibility by arguing that although the hydrogen levels would have been lower coming from the early Earth volcanoes, the escape rate of hydrogen to space would have been slower, retaining a significant concentration of hydrogen in the atmosphere (see their article published in Science in 2005 for the real science, http://www.sciencemag.org/content/308/5724/1014.abstract).  This would re-validate the Urey-Miller experiments!

So, what is the conclusion??  The short answer is that there still is no consensus within the scientific community as to whether or not spark discharge was a feasible way to make the building blocks needed for life on early Earth.  It is difficult to determine the exact composition of the early atmosphere, and thus scientists are still working on the problem.

What if spark discharge experiments are a dead end?  Is there no hope for the production of these necessary molecules on early Earth?  Should we just give up?  Of course not – there are two other plausible theories on the origin of these molecules: synthesis in hydrothermal vents (spots on the ocean floor where heated water from volcanic activity spews out) and transport from space to Earth by meteors and/or comets (there were many MANY more impacts from these on early Earth).

So, going back to the hype surrounding all origin of life theories, many strictly naturalistic origin of life proponents still maintain the validity of the original Urey-Miller Experiment in public settings (like the Museum of Natural History in Washington D.C., at least as of a few years ago when I last visited…), not communicating the challenges that this original experiment now face within the scientific community.  On the other hand, critics of naturalistic origins completely discount spark-discharge experiments in the origin of life, thereby claiming that there are NO plausible naturalistic routes to the production of simple molecules needed in larger biomolecules (like DNA).  In reality, the scientific community has presented results which lie somewhere in between these two extreme positions.  It is important that both sides of this argument recognize that scientists haven’t quite figured this one out, so taking a strong stance on either side of the fence is probably a little premature…

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3 thoughts on “Urey-Miller Experiment – A Dead End?

  1. The near neutral theory on the atmosphere became popular a few decades back, but I think the tide may be turning back.

    – It doesn’t predict why the oxygenation of the atmosphere took so long.

    I know the oxygenation record is itself arguable, but the primary tension seems to come from those who observe no global cover of certain minerals which observation others can predict from ocean/land differences. (It’s another kettle of fish, I’m not going into detail here.)

    – The source of Earth water was long unknown. The last year it has turned out by reassessing Apollo Moon minerals and assessing martian meteorites that Earth-Moon and Mars likely had the same initial mantle water composition.

    Finetuning impactors to predict this would be difficult I think. Fortunately we don’t have to. Yet another recent reassessment of planetary disk formation in modern modeling predicts the same relatively low water content for the terrestrials.

    Then the initial water was bound up in the mantle as dissociated hydrogen (notes the martian meteorite geoscientists). This was later released by volcanism to form an initial CO2/H2 atmosphere with an increasing surface ocean from H2O formation as O2 is liberated from the CO2 atmosphere and from oxygenated minerals during plate tectonics.

    Loosing the atmospheric hydrogen excess to space over time would predict some of the delay time before atmosphere oxygenation as it acted to scavenge the oxygen.

    – The early cold sun is no longer much of a problem in a more mature atmosphere with little CO2, because AGW has showed that the early Kasting models are 1-3 orders of magnitude of due to uncertainties on the CO2 greenhouse effect. (I have the ref – somewhere.)

    As you say, this is rather inconsequential to chemical evolution as such, for example hydrothermal vents can set up a local reductive environment. (It does affect the organic production from impactors however, most of that comes from the impact heat and again a reductive atmosphere is much more productive.)

    But it is good to know that there is a complicated background with a lot of uncertainties as of yet.

    • Thanks for the great addition to my rather specific article. The nature of the early atmosphere (reducing, oxidizing, neutral) is of great interest to many origin of life scientists, as the atmosphere composition will not only effect the production of more complex molecules in spark-discharge experiments, but will also directly affect the pH of the early ocean. The early ocean is then the environment in which many of the next steps for life’s emergence must occur, and thus determining the early atmosphere composition has profound effects on the relevance of other origin of life experiments performed.

      Interestingly, when I was at a conference earlier this year, the data presented illustrated that there really is no consensus amongst experts as to what the composition of the atmosphere was – with most still resorting back to the more recently popular neutral atmosphere. So, as you say, there certainly are MANY uncertainties which still need to be resolved.

  2. Pingback: Great new SciAm blog post on Stanley Miller and the origin of life | OriginsSkeptic

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