In chapter 5, the difficulties with
creating a functional protein in the primordial soup were explored. A similar analysis
will now be undertaken for RNA. Because of the scarcity of the RNA subunits (especially
ribose and cytosine), the information content of any RNA molecule that evolves in the soup
is expected to be very high.
If the soup existed, its exact composition is unknown. Nevertheless, several
generalizations are possible. Ribose and cytosine should be extremely rare (see chapter
9). Furthermore, ribose will react with any free amino acids in the soup forming an
insoluble polymer. Adenine can be synthesized in the lab, but not under plausible
conditions with high yield. Even phosphate will be scarce if inorganic salt is present in
the soup.7
While the concentration of cytosine and ribose in the soup is probably zero,
applying information theory to this situation is not productive because infinite
information, implies zero chance for success. So instead this section will make some very
favorable assumptions concerning the composition of the soup. The assumptions are not
realistic. They are made for educational purposes only.
Favorable Assumptions:
1) All molecules of phosphates, sugars and bases in the soup exist only as activated
nucleotides. That is any adenine in the soup is assumed to be attached to ribose or
another sugar. All sugars have a high energy phosphate group attached.
2) No amino acids are found in the soup. While these are easily synthesized in prebiotic
experiments, they must be excluded as they react quickly with ribose and other aldehydes,
removing ribose from solution and preventing more ribose from forming. Amino acids and
ribose cannot coexist in the soup.
3) No aldehydes exist in the soup. While these are required for the synthesis of ribose
and other sugars, they cannot be allowed to persist. Aldehydes react with the biological
bases, amino acids and sugars. These reactions will interfere with the formation of RNA.
Given this starting point, what is the probability that an RNA molecule will emerge from
the soup?
• Every time an activated nucleotide attacks a ribose, it has a 50%
chance of attacking the wrong carbon atom. This results in premature chain termination.2,3
• Half of the ribose present is the wrong isomer, this also results
in premature chain termination.2,3
• 3/4 of the bases attached to the ribose are not biological. That
is adenine, guanine, cytosine, and uracil are only used in 1/4 of the activated
nucleotides. The most common base is likely ammonia or some other simple amide.
• 3/4 of the activated nucleotides use a sugar other than ribose or
deoxyribose. This also results in premature chain termination. Given that ribose is
usually only a minor product in any prebiotic experiment that synthesizes simple sugars,
this is a very generous assumption.
Even with these most favorable assumptions that ignore all competing side
reactions, every nucleotide added to the RNA chain still contributes a minimum of 6 bits
of information (for every 64 nucleotides added to the chain, only 1 is expected to be
biologically relevant, and this corresponds to 3.32 x log (64/1) = 6 bits of information).
This is three times the value calculated for amino acids in chapter 5.
Thus, a 200 base pair random RNA sequence contains 6 x 200 = 1200 bits of
information, and as explained in chapter 5, this information can be related to a
probability because natural selection does not guide the evolution of random sequences
before self replication exists.
Thus, a 200 base pair random RNA sequence has a 1 in 21200 chance
of emerging in the primordial soup. Given that only 65 out of 15 trillion will show
ribozyme functionality, the odds are staggering - 1 chance in 3.9 x 10372
tries. Furthermore, this calculation is only for a ribozyme capable of regulating a simple
chemical reaction. The odds of a self replicating ribozyme emerging are certainly much
smaller.
In summary, the probability of creating a 200 base ribozyme is extremely
small, because so few random sequences contain the required knowledge, but given that no
200 base RNA molecules existed on the primitive earth, the odds are no longer almost zero,
but instead almost zero multiplied by zero. Finally, as noted in chapter
5, using information theory to calculate the odds has some drawbacks. Information theory
only takes into account the concentration of the various chemicals. It does not have the
ability to deal with chemical properties that may make certain reactions more probably,
and this can skew the results in favor of evolution or against it. In the case of RNA, a
very strong argument can be made that the skewing is strongly in favor of evolution. This
is because the above calculation excluded amino acids and aldehydes from the soup. Thus,
the information calculated above represents RNA that evolves in a test tube, not the real
world.
Next: Self Replication and Perpetual Motion
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