Biochemical Evolution


Chapter 4 - Biochemical Evolution

Did life arise spontaneously from non-living matter? This question can be answered "no" with reasonable certainty.

Every living creature contains at least one blueprint which furnishes instructions for making all the creature's biochemicals. This blueprint is a long chain of chemical units called nucleotides. The chain is neither ordered nor random but is, instead, complex; information is stored by means of a linear sequence. In contrast to an ordered sequence, the algorithm required to specify a complex sequence is as long as the sequence itself. In a living creature, a complex polynucleotide is never constructed from nucleotide building blocks without the assistance of another kind of chemical called an informed enzyme. The informed enzyme is a long chain of chemical units called amino acids. This chain is also complex and also stores information by means of a linear sequence. In a living creature, an informed enzyme is never constructed from amino acid building blocks without a complex polynucleotide to provide the code. Thus, we are faced with the catch-22 of life's origin. The complex polynucleotide, which is the blueprint for all the biochemicals of a living creature, cannot be constructed without an informed enzyme; but the informed enzyme cannot be constructed without a complex polynucleotide to code for it! If life developed spontaneously in a primordial soup of nucleotides, amino acids and nutrients, then either the first complex polynucleotide or the first informed enzyme was formed without the assistance of a pre-existing complex chemical entity. The probability of such an event is virtually zero.

By way of illustration, consider a racemic mixture of all 20 L-amino bio-acids and the corresponding 19 optical isomers. What is the probability that these 39 different kinds of amino acids will assemble themselves into a particular kind of primitive complex amino acid chain called cytochrome-c? This particular chain has 101 amino acid sites. If each of the 39 types of amino acid has the same probability of being incorporated into the chain, then the number of sequences which can be formed is simply 39 raised to the 101 power or 4.98 x 10160. Given a more than generous estimate of the number of synonymous amino acid residues for each site in the cytochrome c chain, the number of cytochrome c sequences which can be formed has been estimated to be 1.2 x 1064. Therefore, the probability of producing a cytochrome c chain by random chance is 2.4 x 10-97. If the primordial soup contained 1044 amino acid molecules (a gross overestimate) which combined, broke-up, and recombined in groups of 101 every second for a billion years, the probability of finding one molecule of cytochrome-c during that period is (2.4 x 10-97)(3.15 x 1058) or 7.56 x 10-39. This is the probability that a sequence of honest coin tosses will produce 126 heads in a row. Keep in mind that cytochrome-c is only a primitive protein which contains nowhere near the information found in an informed enzyme.

The cytochrome-c illustration was based on the assumption that amino acid chains were formed in a prebiotic soup by a specific chemical process: random formation and destruction of full length chains each second for a billion years. If the illustration had been based on path independent thermodynamic concepts, the computed probability would have been virtually the same.

Random chance has been abandoned as an acceptable model for the coding of any macromolecule essential to a living system, except in introductory texts and popularizations. Subsequent to the failure of random chance as a viable mechanism, various remedial approaches have been proposed.

One approach is to insinuate chemical bonding preferences of nucleotides or amino acids cause the spontaneous formation of complex sequences. Unfortunately for those who would prefer to view first life as the inevitable consequence of nature's laws at work over vast spans of time, the concept of "directed chance" or "biochemical predestination" has neither experimental nor theoretical support.

Mineral catalysis is also being suggested as a potentially important factor in prebiotic evolution. Unfortunately, mineral catalysis can only assist in polymerizing short, random chains of polymers from selected biomonomers. It cannot account for the configurational-entropy-work required to polymerize the macromolecules of life. Other mechanisms based on non-equilibrium thermodynamics and on the thermal synthesis of "proteinoids" have also failed to explain the spontaneous generation of complexity.

Several mechanisms can provide the chemical and thermal-entropy-work necessary to form random polypeptides. In no case, however, can any scientifically sound mechanism provide the additional configurational-entropy-work necessary to convert random polypeptides into proteins. The problems that beset protein synthesis apply with even greater force to DNA synthesis.

A conviction that complex amino acid or nucleotide chains appeared spontaneously in the primitive milieu on earth is based more on faith than science. Abandonment of this conviction would leave few options for those who prefer to reject God as the designer of complex macromolecules and choose to believe the universe is teeming with spontaneously generated life. Nevertheless, the scientific answer to the question asked at the beginning of this chapter appears to be "no." (8,9)