"Has
anyone," Mr. Tappin asked, "come up with a non-protein related way of mining the
energy of the ion gradient that exists in conjunction with the cell membrane?"
"Over
the years, a lot of different theories have been proposed in this regard," the
professor remarked. "These usually concern variation on themes involving some kind of
electron tunneling, ion migration, or proton transfer.
"So
far, however, there doesn't appear to be a plausible way of making these mechanisms
capable of working in any consistent, reliable fashion, or capable of generating the
levels of energy which would be required to maintain membrane functioning, not to mention
many other cellular processes. In addition, even if one could come up with a viable,
non-protein-related mechanism for mining energy from the membrane's ion gradient, there is
no way of either storing the energy once it reaches the interior of the cell, nor is there
any way of transferring the charge in order to chemically activate other molecules
involved in cell processes, since, as far as is known, both the storage of charge as well
as the charge-transfer processes are effected by proteins, although the energy storage
compound, itself, is often some kind of a nucleotide, rather than a protein."
"Dr.
Yardley, would you agree," inquired Mr. Tappin, "that even if one could come up
with a plausible prebiotic theory for, one, the migration of charge across the
permeability barrier of the membrane, two, the storage of charge, and, three, the transfer
of charge, all of which we will assume are capable of operating quite independently of
proteins, wouldn't one still be faced with the problem of having to explain how the
non-protein system evolved to produce the protein-based system which now helps govern
charge-migration, charge-storage and charge-transfer in the biological organisms with
which we are presently familiar?"
"Yes,"
acknowledged the professor. "I don't see how one could avoid having to address this
problem under such circumstances.
"In
fact, in my opinion, this is precisely the sort of difficulty which emerges in relation to
theories of the origin-of-life which focus on the possible role of clay minerals. The
proponents of these theories talk about the capacity of clay surfaces to carry out some of
the functions important to life, such as exhibiting a few catalytic properties that can
help bring about certain stages in the polymerization of some of the nucleotides in
nucleic acids, as well as some peptide chaining; or, providing a surface on which
concentration reactions can take place; or, offering a means to compartmentalize and
organize different metabolic pathway; as well as having the potential to store, and
replicate, certain kinds of information on crystalline patterns, somewhat reminiscent of
genetic system. However, in point of fact, even if one were to ignore all the problems and
rather severe limitations which surround such capabilities in mineral clays,like kaolin
and montmorillonite, nonetheless, these theorists have no way of explaining how life, as
we understand it, came into being.
"In
effect, they avoid the real problems surrounding origin-of-life issues by trying to define
life in another, very limited and superficial way. As a result, they tend to multiply the
theoretical problems because not only must they account for the rise of such clay mineral
photocells, these theorists also must come up with a plausible theory of transition which
accounts for the genetic takeover of these clay mineral systems by protocells which are
not based on clay minerals - unless, of course, such clay mineral protocells are not part
of our evolutionary lineage, in which case, whether the theory is right or wrong, it
really has nothing to do with life as we understand it.
"Above
and beyond the foregoing, there is a further problem concerning the viability of a clay
mineral hypothesis for the origin-of-life. Many clays, including kaolin, tend to be
extremely rare in pre-Cambrian sediments.
"This
fact does not constitute a fatal blow to these kinds of hypothesis. On the other hand,
such a fact does tend to lessen the chances of such a hypothesis being correct.
"Quite
frequently, one will find various kinds of inorganic conjectures thrown into the picture
in an attempt to augment or complement the clay mineral origin-of-life hypothesis. For
instance, relatively recently there was a conjecture by a European theorist which is based
on the manner in which iron sulfides, like pyrite, contain free energy when the iron
becomes reduced to a ferrous state.
"Using
such an observation as a launching pad, this theorist postulated that, possibly, if one
could find a way of coupling this free energy to possible reactants in a protocell-like
environment, then an important component in the formation of one or more primitive
metabolic pathways would have been established. When one added that this kind of energy
source might tend to be found in close contact with, say, clay mineral surfaces which,
among other things, were capable of bringing about concentration reactions, such a
conjecture became quite attractive to some people.
"However,"
Dr. Yardley concluded, "no plausible, dependable means has been found for accounting
how the charge-transfer, or coupling, process will take place in conjunction with
potential chemical reactants in a protocell-like environment. Therefore, the iron sulfides
conjecture remains nothing but an unrealized conjecture.
"Similarly,
some people have proposed that when the various components of nucleotides - ribose,
phosphate, and a nucleic base of one kind or another - are adsorbed onto the surface of
some clay mineral, then, perhaps, the specific character of the mineral might have brought
these components together in particular orientations. Unfortunately, for this kind of
proposal, none of the minerals which have been tested to date have exhibited the requisite
specificity to be able to generate nucleotides with the sort of structural character which
is observed in living organisms."
"In
conjunction with the previous discussion of membrane activity and functions," Mr.
Tappin specified, "isn't it the case that various classes of pigments may be involved
with the processes of photosynthesis which take place in, and about, the thylakoid
membranes in photosynthetic bacteria and blue-green algae, as well as the chloroplasts of
plants?"
"That's
right," answered Dr. Yardley.
"What
role does porphyrin play in all of this?" the defense lawyer asked.
"Porphyrins,"
explained the professor, "are one of a group of pigments which are widely distributed
among different kinds of organisms. They are derived from a porphin molecule that is a
ring structure made up of four pyrrole nuclei (C4H4NH) linked
together by carbon atoms.
"The
nitrogen atom in porphins often tends to form very strong and stable bonds with metallic
ions such as magnesium or iron. This kind of bonded group is referred to as a chelate.
"Chlorophyll,
which is present in all photosynthetic organisms, consists of a porphin group with a
magnesium ion at its center. In addition, different kinds of chlorophyll have various
kinds of side chains attached to them.
"Generally
speaking, pigments are divided into two broad classes known as accessory and principle
pigments. Accessory pigments tend to gather light energy and pass it onto the principle
pigment which, for the most part, is either chlorophyll 'a' or one of the forms of
chlorophyll occurring in certain bacteria.
"There
are, however, other classes of non-chlorophyll pigments such as carotenoid and phycobilin.
These other classes of pigments tend to have accessory, rather than principle, roles in
photosynthetic systems."
"Professor
Yardley, to the best of your knowledge," inquired the lawyer, "is there any
plausible prebiotic pathway of synthesis which might give rise to the Porphyrins that are
at the heart of the chlorophyll contained in all photosynthetic organisms?"
"None
is known at the present time," replied the professor. "Nonetheless, as I
indicated in previous testimony, on occasion, pigment-like molecules have been found in
the organic residue of some carbonaceous chondrites."
"Even
if," Mr. Tappin postulated, "we were to assume these pigment-like molecules had
a full capacity to accept and transfer light energy, and even if we were to assume these
extraterrestrial pigments were in plentiful supply and did not get degraded through
photolysis and so on, and even if one were to assume that, somehow, these pigment-like
molecules were to find their way into a protocell system, wouldn't one still be faced with
the problems of explaining how porphin-containing chlorophyll came into existence and how
these pigment-like molecules became coordinated with chlorophyll molecules in various
kinds of photosynthetic systems?"
"Yes,"
the professor conceded, "one still would be left with having to account for such
things."
"Furthermore,
Dr. Yardley, in the photosynthetic systems with which we currently are familiar, doesn't
the transfer of energy charge from accessory to principle pigments take place by means of
an electron transport system made up of a series of protein enzymes, and, therefore, even
if one were to accept the idea of an extraterrestrial pigment-like molecule playing a role
in the formation of early photocells, wouldn't one still need to account for the rise of
the requisite support system of enzymes which had the ability to serve as a specific
transport mechanism in relation to the movement of electrons to their final acceptor
destination in the protocell?"
"Yes,"
the professor acknowledged, "these sorts of phenomena would remain as problems to be
explained even if the assumptions which you have cited there are also chemosynthetic
autotrophic organisms which derive their carbon and energy in a quite different manner
from photosynthetic autotrophic organisms. Conceivably, these chemosynthetic autotrophs,
and not photosynthetic autotrophs, were the first photocells to exhibit the properties of
life."
"If I
understand what you are saying, Dr. Yardley, wouldn& #146;t evolutionary biology now
have two problems to solve rather than one?" suggested the defense counselor.
"The origin of two different kinds of autotrophs would have to be accounted for - one
which is chemosynthetic in nature and one that is photosynthetic in nature. Is this the
case?"
"It
is," stated the professor, "unless one of the two systems was the prototype from
which the other eventually was derived through an evolutionary process?"
"If
this were the case, wouldn't one still be faced with two problems?" Mr. Tappin
challenged. "The first problem would be to provide a plausible explanation for either
photosynthetic or chemosynthetic autotrophs, depending on which one an individual
considered to have arisen initially. The second problem would be to provide a plausible
explanation for the sort of transitional steps which would have permitted a very different
kind of autotrophic system to have been derived from the first autotrophic system. Isn't
this the situation, Professor, with which evolutionary biology would be, and is,
faced?"
"Yes, I
suppose it would be, and I suppose it is," Dr. Yardley responded.
"Mr.
Tappin," stated Judge Arnsberger, "once more, I must interrupt your
cross-examination. The dinner hour is at hand, and I feel we all could use a break from
these deliberations.
"Please
remember, all of my previous instructions to the jury remain in effect. These court
proceedings will be adjourned until 7:30 p.m. this evening."
