Primordial Soup Evolution

The Composition of the Primordial Soup

If meteorites are used to reconstruct the composition of the soup, then 14 of the 20 amino acids used by life will be absent. Only glycine, alanine, valine, serine, aspartate and glutamate would be available in the soup. No proteins used by life today use only these 6 amino acids. While this prediction of the soup’s composition is probably the most accurate, it is an undesirable composition. So this chapter will assume a much more favorable composition.

   Life uses 20 amino acids. Seventeen of these have been synthesized in the lab under conditions that might be similar to the conditions found on earth 4 billion years ago. No single experiment has ever created more than 10 amino acids. Some amino acids are quite easy to synthesize and others are very difficult. The amino acids that are easy to synthesize invariable are the primary product of these experiments. The other amino acids occur in various concentrations depending on the conditions chosen to carry out the experiment. Three amino acids, histidine, arginine, and lysine, have not been synthesized under plausible conditions.2   Because no single experiment has generated more than 10 amino acids, if the soup’s composition is taken from the results of a single prebiotic experiment, then the composition will also be unfavorable for protein evolution. Most proteins need 18 or 19 different amino acids to function. To construct a favorable composition for protein evolution, it is either necessary to combine many different prebiotic experiments or to just assume that the absent amino acids are present. This section will take the latter approach.

   On page 87 of his book, Miller lists the results from one of the most successful prebiotic experiments.³ The yields of ten amino acids are listed in this table.

   As a reasonable starting point, assume the abundance of the amino acids in the primordial soup tracks Miller’s table. Ten amino acids are not found in Miller’s table. Seven of these have been synthesized under plausible prebiotic conditions. Assume that these seven are as abundant as threonine. Threonine is the least common amino acid listed in Miller’s table. Three amino acids have not been synthesized in the lab. Assume that these are found in the soup at 1/10 the concentration of threonine. Finally, assume that the 20 amino acids that life uses comprise 1/4 of all amino acids present in the soup. Thus, the soup ratio of biological to non-biological amino acids is similar to the ratio found in meteorites.

   All of these assumptions improve the odds that a protein will emerge in the soup. For example, one could easily assume that the ten proteins not found in Miller’s table were also absent from the soup. With this single assumption, the information and molecular knowledge found in most proteins becomes infinite. Furthermore, the assumption to exclude chemicals like aldehydes and formic acid greatly improves the likelihood for protein evolution.


   With these assumptions in place, labeling wooden blocks according to amino acid abundance yields table 5.1. The number of blocks in the second column are taken from Miller’s table. The far right column is based on what might have been given the constraints of the favorable assumptions discussed above.


Table 5.1: Wooden Blocks Used to represent Chemicals in the Soup

* Most amino acids exist in two forms. The forms are mirror images of each other. Life only uses one image. Glycine is the only amino acid that does not have a mirror image. Thus, the number reported for glycine is table 5.1 corresponds to the concentration reported in Miller’s table. The numbers associated with all other amino acids in the left column are � the value reported in Miller’s table.The Evolution of a Functional Protein in the Primordial Soup

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