Chemical Evolution: The Problem Of Improbable Proteins

Let’s attempt a calculation of the likelihood of the formation of a single protein by random chemical reactions within a hypothesized “primordial soup” on the primitive Earth. For a relatively short protein of just 100 amino acids, we can calculate a separate probability in relation to each of five specific difficulties.

1. Cells select from a list of just 20 specific amino acids to make proteins. But many other varieties of amino acids also exist. For example, some 70 different amino acids have been discovered in meteorites, which astronomers suggest could have delivered extraterrestrial organic compounds to the primordial soup. Using that figure, the chance of a given amino acid in the chain being an allowable one would be ; the probability of the whole protein being composed of allowable amino acids would be .
   

2. Most amino acids exist in equal amounts of two alternate forms (“isomers”), referred to as left- and right-handed (or L- and D-). But proteins are made from only L-amino acids. Each amino acid added to the growing protein has only chance of being an L-isomer. (Actually, since glycine exists in one form only, the probability can be stated more correctly as , assuming equal concentrations of all the types of amino acid.) For the whole protein, then, the chance of all amino acids being proper isomers is .
   

3. Amino acids in proteins are joined together by a specific linkage called an a-peptide bond. The a-amino group of one amino acid is linked by dehydration synthesis to the a-acid group of the other amino acid — that is, the reacting functional groups must be the ones attached to the central carbon (a-carbon) of the amino acid. But some amino acids have more than one acid group or amino group (or polar nitrogen acting like an amino group). Among the standard twenty amino acids, the total number of such groups not attached to an a-carbon is about ten. We can therefore approximate the chance of forming a correct bond between amino acids as . The total number of such bonds will be 99, so the probability of all bonds being the correct a-peptide bond is .
   

4. The “primordial soup” must certainly contain other organic compounds in addition to amino acids, including amino compounds with no acid group, organic acids with no amino group, and many others. Let us generously estimate the chance that every compound within the chain would be an amino acid (and not some other molecule) as . The probability of the entire chain consisting only of amino acids would then be .
   

5. Protein function requires that only certain amino acids are usable in at least some portion of the protein. In some cases, a mutation that changes even one amino acid in the active site of an enzyme will destroy the function of that enzyme. Supposing, rather generously, that only 5 amino acid sites out of 100 in our protein are critical, and that any of 4 amino acids will be acceptable in such locations, the probability of obtaining a functional protein will be .
   

Combining all of these independent probabilities related to the forming of one short protein molecule by random chance processes, we estimate the overall probability as

=

Now, it has recently been determined that a minimal functioning cell (an obligate parasite) must have no fewer than 265 protein-coding genes (Clyde A. Hutchison III, Scott N. Peterson, Steven R. Gill, Robin T. Cline, Owen White, Claire M. Fraser, Hamilton O. Smith, J. Craig Venter. 1999 [Dec 10]. “Global Transposon Mutagenesis and a Minimal Mycoplasma Genome.” Science, Vol. 286, pp. 2165-2169). Assuming that each gene codes for a single protein, this implies that the cell requires at least 265 proteins to operate as a “living cell.” In that case, considering proteins only, the probability of a complete living cell coming into being through random, chance processes would be

If we ponder that the whole universe is estimated to consist of only about subatomic particles, that fewer than seconds have elapsed since the alleged “Big Bang,” and that mathematicians reckon that an event with a probability of less than is effectively impossible, the above calculation easily destroys any hope whatsoever of a random, chance chemical origin of life! (It’s interesting that the noted astronomer Fred Hoyle arrived at a similar figure of through a different route [1981. Evolution from Space. New York: Touchstone. p. 24].)

Of course, many biomolecules other than proteins are required for a fully functioning cell to be able to metabolize nutrients and reproduce itself. The above calculations have considered only proteins, and have not looked at carbohydrates (needed by cells as nutrients), lipids (constituents of cell membranes), or nucleic acids (extremely complex information molecules such as DNA and RNA, found in all cells).

A further difficulty is that random chemical reactions are more likely to lead to the destruction of any partly-formed protein than to its continuing increase in size! As stated by evolutionary biologist and Nobel laureate George Wald: “In the vast majority of the processes in which we are interested the point of equilibrium lies far over toward the side of dissolution. That is to say, spontaneous dissolution is much more probable, and hence proceeds much more rapidly, than spontaneous synthesis” (Scientific American, Aug 1954, pp. 44, 49-50).