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…