The “Standard” Model

• Earth is about 4.5 billion years old, the first life appeared about 3.5 billion years ago

• The first billion years (the “prebiotic era”) involved not only the cooling of the Earth (the surface probably cooled relatively quickly), but also random chemistry that resulted in what we would consider life

The “Standard” Model

• Exact accounting of the raw materials for this random chemistry is obviously not possible, but various guesses have been made

• Oparin and Haldane independently and roughly simultaneously suggested that these raw materials, along with UV rays from the sun (Sun?) and lightning, could have resulted in the formation of self-replicating molecules and/or systems

The “Standard” Model

• Since we now know that nucleic acids can serve as templates for their own copying, they seem to be good candidates for prebiotic self-replicating molecules

• Can we get nucleic acids from the prebiotic “soup” and energy (e.g., UV light) given a billion years of reaction time?

• The answer appears to be “maybe”

The Miller Experiment

• In 1953, Stanley Miller published a paper showing the results of an attempt to simulate these conditions (except the billion years part)

• Miller focused on CH4, NH3, H2O, and H2 as the critical atmospheric components

• He boiled the H2O in a closed apparatus containing the other gases, while an electric discharge was continuously generated

• This ran for a week

The Miller Experiment

The Miller Experiment

• He examined the products for the presence of amino acids

• The fact that the water was boiling, and the treatment of the samples after the experiment, made contamination by microorganisms very unlikely

• Products were run on paper chromatography and stained with ninhydrin, spots were identified by spiking samples with known amino acids and comparing chromatograms

The Miller Experiment

The Miller Experiment

• Miller identified aspartic acid, glycine, alanine (a and b), aminobutyric acid, and two spots not identified

• Note that glycine and aspartic acid are precursors for purine nucleotide biosynthesis

• Miller noted that there were probably additional products present in smaller amounts

The Miller Experiment

• So it appeared that the first test of the prebiotic soup idea resulted in known biomolecules

Miller and Urey

• In 1959 Miller and Urey published a paper considering a number of issues with regards to the origins of life

• One of the important points they noted was that Oparin originally suggested that a reducing atmosphere would have been conducive to the formation of organic molecules

Miller and Urey

• Miller’s experiment (along with subsequent ones) used a reducing atmosphere, whereas other work used an oxidizing atmosphere (such as we have today)

• The attempts to synthesize organic compounds under the oxidizing conditions pretty much failed, whereas the reducing conditions gave a variety of interesting organic compounds

Miller and Urey

• Miller and Urey thought that the change from the ancient reducing atmosphere to the oxidizing atmosphere we enjoy today was largely due to the escape of H2 from the ancient atmosphere

• This is worth noting, since such a mechanism would probably have been more gradual than the major alternative mechanism (evolution of photosynthesis)

• Whether the change was gradual or rapid could easily affect the full spectrum of possible or likely prebiotic reactions (is it possible photosynthesis predates the first living organism?)

Miller and Urey

• Miller and Urey tediously examine the equilibria of the relevant carbon and nitrogen forms to guess at components of the atmosphere on the prebiotic Earth

• They also consider the various possible sources of energy to drive the reactions (recall Miller used electricity), concluding UV light was probably the most relevant

Miller and Urey

• Subsequent to Miller’s original experiment, other groups did similar work using UV as the energy source

• These experiments produced a number of interesting organic compounds

• A more thorough analysis of the products of Miller’s original work revealed a much richer bounty of compounds than first thought

Miller and Urey

Miller and Urey

• Miller and Urey then speculate on the involvement of reactions on surfaces, such as clay or minerals

• Such possibilities remain important considerations to this day

Summary of Miller-like Experiments

Alan D. Gishlick, National Center for Science Education