"Professor
Yardley, let's assume," posited the counsel for the defense, "that I were
willing to forget all the problems which have been raised with respect to the
concentration issue. Do we have any way of knowing what proportion of the amino acids
formed in the Archean era ocean through Strecker synthesis would be the twenty varieties
of amino acid occurring in living organisms rather than the many other kinds of amino acid
that are possible - some of which have been discovered in meteorites?"
"I
imagine," answered the professor, "there are individuals with the talent to be
able to come up with some kind of thermodynamic model that would provide a set of
theoretically-driven distribution values for all the different kinds of amino acid which
might be possible. However, such a model would be affected by so many variable
considerations, conditions and forces, I'm not sure even our current supercomputers could
keep track of the problems that would arise in this kind of model.
"One
could assume less complex amino acids might tend to be somewhat disproportionately
represented in relation to more complex amino acids. On the other hand, a wide array of
localized thermodynamic conditions might arise which could run against these sort of
tendencies.
"If
temperatures in the ocean were low, say, near 0 degrees Celsius, then one would expect
thermal decomposition to be low. However, some amino acids, like alanine and glycine, have
far greater stability than do other amino acids, like serine.
"Consequently,
stability properties would have to be factored in even if the water temperature were to
remain near 0 degrees Celsius, which is unlikely. This is unlikely because within the last
twenty to thirty million years there is evidence that bottom water temperatures can vary
as much as 10 to 15 degrees as the Earth goes through various climatic transitions.
"What
variations in water temperature, top or bottom, may have been taking place across hundreds
of million years in an Archean era ocean and atmosphere are anybody's guess. Furthermore,
how the decomposition tendencies of the twenty amino acids which occur in living organisms
would stack up to the decomposition tendencies of all the other amino acids which are
possible is another issue which would have to be factored in.
"Then,
of course, one would have to work in the decomposing effect that hydrothermal vents and
active volcanoes would have on amino acids which had been formed. Since we really don't
have any idea of how prevalent either of these processes were during the Archean era, this
introduces a further unknown into any prospective model which is being constructed.
"The
effects of ultraviolet radiation in the 200 meter photic zone would have to be considered.
In addition, once hydrolysis had done its magic and helped amino acids to form, then the
newly-synthesized, more complex amino acids become even more vulnerable to the forces of
hydrolysis than is the case for the molecules which reacted together to form them.
"Furthermore,"
added the Professor, "one cannot assume the only sort of synthesis reactions going on
in the Archean era ocean are ones which lead to the formation of amino acids. Other,
non-amino acid kinds of hydrocarbon are likely to have arisen, and this means there would
have been chemical competition for available reactants, with unknown ramifications for the
rate and extent of amino acid formation, both in relation to the twenty amino acids which
are important to life forms, as well as in relation to the other varieties of amino acid
which are not important to life forms on Earth."
"Dr.
Yardley, is there," Mr. Tappin inquired, "any mechanism you know of which would
have led to the specific selection of the twenty amino acids fundamental to life forms on
Earth from among the myriad numbers and kinds of other amino acids that are likely to have
arisen in the Archean era ocean through Strecker synthesis?"
"No,"
the professor answered, "I know of no plausible theory which would explain the
selection process which we believe went on during the Archean era. It may well have been a
stochastic process, and since we don't know enough about the factors shaping that process,
we really cannot do anything but speculate why certain probability distributions might
have been thermodynamically and/or kinetically favored over other probability
distributions."
"Professor
Yardley," continued the defense counsel, "with respect to the amino acids
synthesized in the Archean era ocean through the Strecker process, would they have formed
a racemic mixture - that is, a mixture consisting of roughly equal numbers of both
left-handed and right-handed optical isomers of the various kinds of amino acid?"
"If our
laboratory experiments are any indication, "the professor replied, "then, yes,
the Archean era mixture is likely to have been racemic in character. Nevertheless, I
previously have mentioned a meteorite found in the Antarctic which contained some
exclusively right-handed amino acids, and this discovery does carry some potential
implications for what may have occurred in the Archean era ocean."
"Are
you aware, Dr. Yardley," asked the lawyer, "of any plausible account which might
explain why one might end up with a set of same-handed optical isomers rather than a
racemic mixture of amino acids?"
"Over
the years," stated the professor, "there have been a number of proposals
directed toward this problem of chirality or handedness. The only hypothesis which I have
found to be plausible is one proposed back in the 1950s.
"Essentially,
this hypothesis assumes that when sunlight passed through the atmosphere of the Archean
era, light took on a small degree of polarization. As a result, the polarized ultraviolet
component of sunlight during the Archean era may have had a preferential tendency to
degrade right-handed optical isomer forms of amino acids, leaving intact the left-handed
optical isomer forms which have been observed in the vast majority of Earth
organisms."
"Dr.
Yardley, don't most of the biologically important carbohydrate molecules tend to exhibit
right-handed optical isomer preference?" Mr. Tappin inquired.
"Yes,
that's right," the professor indicated.
"So,
wouldn't one expect," postulated the lawyer, "that the same polarized
ultraviolet component of Archean era sunlight which degraded right-handed amino acid
isomers would also degrade right-handed carbohydrate isomers? Consequently, how does one
account for the fact one finds right-handed carbohydrate isomers playing fundamental roles
in living organisms?"
"This
is a problem," Dr. Yardley admitted, "but there may have been other kinds of
selection mechanisms at work in addition to the polarized ultraviolet component of Archean
era sunlight."
"Does
anyone," challenged the defense counsel, "know what these other selection
mechanisms were that are assumed to have been operative during the Archean era?"
"Not at
this point in time," answered the professor.
"Dr.
Yardley, even if," the lawyer hypothesized, "one were to accept the
polarized-light hypothesis as the reason why left-handed amino acids were selectively
favored over right-handed amino acids as far as ultraviolet degradation is concerned, this
still leaves at least two problems. First of all, the polarized- light assumption doesn't
explain why DNA would possess a tendency to call for exclusively left-handed amino acids
to be synthesized in the cell. Secondly, one still hasn't explained how the twenty amino
acids common to life forms on Earth came to be selectively favored over the other
left-handed amino acid optical isomers which would have survived being degraded by
slightly polarized ultraviolet radiation. Would you agree with my assessment of the
situation, Dr. Yardley?"
"As far
as the second problem is concerned," stated the professor, "I would agree no
fully satisfactory account presently exists for explaining why the twenty left-handed
amino acid isomers were selected over other possible left-handed amino acid isomer
candidates. As far as the first problem described by you is concerned, something could be
said.
"Selection
forces would have favored the DNA and/or RNA system which would have arisen that relied on
the optical isomer form of amino acid that was available - in this case, the left-handed
amino acid isomer. If a DNA and/or RNA system would have arisen which depended on the
existence of a pool of right-handed amino acid isomers, then, given that polarized
ultraviolet light had selectively destroyed all, or most, of these kinds of isomer, such a
DNA/RNA system would not have been favored by the prevailing conditions of the Archean era
world. Prebiotic conditions would have favored the DNA/RNA system which called for, or
needed, left-handed amino acid isomers."
"Excuse
me, Dr. Yardley, perhaps, I don't understand the situation," said Mr. Tappin.
"Although your account or explanation makes sense in the context of having assumed
that a left-handed-amino-acid- preferring DNA/RNA system already had arisen, your account
doesn't really explain how such a left-handed-amino-acid-preferring DNA/RNA system arose
in the first place ... does it?"
"No, it
doesn't," the professor acknowledged.
"In
fact," continued the lawyer, "wouldn't one be justified in arguing that the
process of natural selection really is incapable of accounting for change over time except
in a post-facto manner? By this, I mean that although natural selection can help explain
why certain capabilities, once they arise, may have been selectively favored by existing
conditions, nevertheless, natural selection cannot explain how such capabilities arose in
the first place, can it, Professor?"
"Well,"
Dr. Yardley responded, "some theorists do speak in terms of the idea of 'evolutionary
pressure'. In other words, they believe the collective character of any given set of
conditions may, in a sense, generate a certain amount of pressure to induce the sort of
changes which would be favorably selected by those conditions."
"How
does this process of inducement work?" Mr. Tappin asked. "How does the
physical/chemical world induce a given system to change both its structural character, as
well as its way of operating, so that the system adopts a structure and set of processes
which would be selectively favored by the prevailing conditions of that physical and
chemical world?"
"Its a
very complicated issue," replied the professor. "There is a great deal of work
going on with the science of complexity, as well as chaos theory and the theory of
dissipative structures which is directed toward trying to answer questions like
this."
"Has
anyone," inquired the lawyer, "come up with a model in any of these disciplines
which has been accepted by the scientific community as a plausible account of how
prevailing physical and chemical circumstances induce a system to generate structural and
dynamic changes which are, capable of taking advantage of precisely the conditions which
prevail in the world at a given time?"
"Not
yet," responded the professor.
"Then,
Dr. Yardley, would one be doing injustice to the available evidence," Mr. Tappin
pressed, "if one were to say, at least at this point in time, that the notion of
evolutionary pressure is a totally unproven hypothesis, however convenient and desirable
an idea it may be for evolutionary theory?"
"No, I
would have to say," the professor admitted, "that no injustice would be done to
the available evidence."
"Consequently,"
summarized the defense counsel, "currently, there really is no plausible,
generally-accepted explanation of how or why DNA or RNA systems arose which showed a
preference for left-handed amino acid isomers as well as right-handed carbohydrate
isomers. Would you agree with this statement, Dr. Yardley?"
"Yes,
at the present time, what you have said is the case," agreed the professor.
"Mr.
Tappin," intervened Judge Arnsberger, "I'm going to exercise some discretion at
this juncture and propose that court be adjourned for lunch. Court will reconvene again at
2:00 p. m. this afternoon."
