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Chemical Evolution: The Problem Of Improbable Proteins

by Richard Peachey

Living cells employ proteins to perform a variety of important functions, including catalysis, structure, storage, communication, movement, transport, gatekeeping, and defense. To construct proteins from amino acid "building blocks," cells use very complex procedures. But evolutionists suggest that these key molecules originally came into being through naturalistic, unguided processes during chemical evolution.

Let's attempt a calculation of the likelihood that a single protein could be formed 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

1. The "primordial soup" would surely have contained other organic compounds besides amino acids — such as amino compounds with no acid group, organic acids with no amino group, ketones, aldehydes, and sugars. Let us generously estimate the chance that a particular unit within the chain would be an amino acid (rather than one of the other possible molecules) as 1/3. The odds of the entire chain consisting of only amino acids would then be (1/3)^100.

2. Cells select from a list of just 20 specific amino acids when they make proteins. But many other varieties of amino acids also exist. For example, over 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 20/70. The probability of the whole protein being composed of allowable amino acids would then be (20/70)^100.

3. Most amino acids exist in two alternate versions ("isomers"), referred to as left- and right-handed (or L- and D-). Natural processes would have formed the same quantity of each isomer in the primordial soup. But cells make proteins only from L-amino acids. Each amino acid added to a growing protein would therefore have only 1/2 chance of being an L-isomer. Actually, since the simplest amino acid glycine exists in one form only, the probability can be more accurately stated as 21/40, assuming the primordial soup contains equal concentrations of all the types of amino acid used by cells. For the whole protein, then, the chance of all amino acids being correct isomers (that is, either glycine, or L-isomers of other amino acids) is (21/40)^100.

4. Amino acids in proteins are joined together by a specific linkage called an alpha-peptide bond. In such a bond, the alpha-amino group of one amino acid is linked by dehydration synthesis2 to the alpha-acid group of the other amino acid — that is, the reacting functional groups must be the ones attached to the central carbon (alpha-carbon) of each 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 alpha-carbon is about ten. We can therefore approximate the chance of forming a correct bond between amino acids as 20/30. The total number of such bonds in our protein will be 99, so the probability of all the linkages being alpha-peptide bonds is (20/30)^99.

5. Protein function requires the use of specific amino acids in at least some portion of the protein. In some cases, a mutation resulting in a change of even one amino acid in the active site of a protein 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 one of 4 amino acids will be acceptable in such locations, the probability of obtaining a functional protein will be (4/20)^5.

Combining these five separate probabilities related to the chance formation of one rather short protein molecule, we estimate the overall probability as:

(1/3)^100 x (20/70)^100 x (21/40)^100 x (20/30)^99 x (4/20)^5 = 9.31 x 10-152

Now, it has been determined that a minimal functioning cell (an obligate parasite) must have no fewer than 265 protein-coding genes.3 Assuming that each gene codes for just a single protein (now known to be quite a generous assumption), 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:

(9.31 x 10-152)^265 = 10-40023 (or 1 chance in 1040023)

If we ponder that (a) the whole universe is estimated to contain only about 1080 subatomic particles; (b) fewer than 1018 seconds are thought to have elapsed since the universe began; and (c) mathematicians reckon that any event with a probability of less than 10-50 is effectively impossible . . . then the above calculation easily destroys any hope whatsoever of a random, chance chemical origin of life! (Interestingly, the noted astronomer Fred Hoyle arrived at a similar figure of 10-40000 through a different route.4)

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

Apart from the probability issues discussed above, there is the further difficulty 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.”5 For the formation of a dipeptide (the bonding together of just two amino acids) by dehydration synthesis, a typical Keq (equilibrium constant, the ratio of product to reactant concentrations) is about 0.007 at 25°C. Allowing a very generous initial concentration of 1 mole per liter for all amino acids in the primordial soup, the equilibrium concentration of 100-unit amino acid chains would then be:

0.007^99 = 10-214 mole per liter

Based on these thermodynamic considerations alone, therefore, we can be confident that no such molecule will ever form by naturalistic unguided processes!

Footnotes

1. For calculations of my points 3, 4, and 5 using less generous assumptions, see Stephen C. Meyer, "Word Games." In William A. Dembski and James M. Kushiner (editors). 2001. Signs of Intelligence: Understanding Intelligent Design. Grand Rapids: Brazos Press (Baker). pp. 109ff.

2. Dehydration synthesis here entails the removal of hydrogen (H) from the amino group and hydroxyl (OH) from the acid group, producing water (HOH) and allowing a chemical bond to form between the two amino acids.

3. 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 286:2165-2169.

4. Fred Hoyle. 1981. Evolution from Space. New York: Touchstone. p. 24.

5. George Wald. Scientific American, Aug. 1954, p. 44.


For Further Reading

Mike Riddle, "Can Natural Processes Explain the Origin of Life?" <http://www.answersingenesis.org/articles/nab2/natural-processes-origin-of-life>