"Similarly,
experimenters have discovered that ribonucleotides will not form oligomers or small chains
of up to ten units of nucleic acids unless done by means of anhydrous heating.
Furthermore, such heating must occur in the presence of both a nucleotide triphosphate and
cyanamide (CH2N2), a condensing agent."
"Dr.
Yardley," the prosecuting attorney intervened, "what is a condensing
agent?"
"Condensation,"
answered Professor Yardley, "involves a rearrangement of atoms in order to produce a
molecule of greater complexity, density or weight. Condensing agents assist this process.
"Some
scientists have hypothesized that exposed mineral, lava or sand surfaces, where
temperatures may have reached 100 degrees Celsius, could have served as crucibles on which
films of organic compounds that washed in from the ocean might have formed covalent bonds.
This would have taken place through condensation reactions.
"Furthermore,
these researchers theorize that once more complex hydrocarbons formed, some of these
molecules may have migrated, through one natural process or another, downward a few
centimeters below the surface. Such a micro-environment would have helped to protect the
newly-formed compounds from degradation reactions driven by light and heat."
Pausing for
a moment, Dr. Yardley, finished the remainder of the water in his glass. He replaced the
glass on the table in a way that suggested he was thinking about something else.
When he had
settled in his seat, his lips were pursed. Finally he spoke again.
"Even
if one were to suppose," he added, "that some of the starting ingredients cited
in the previous experiments were not produced in abundance through chemical reactions on
prebiotic Earth, one should remember that exogenous or extraterrestrial sources may have
helped supplement the normal, earthly complement of these compounds. Water, hydrogen
cyanide, ammonia, cyanoacetylene, and formaldehyde - all of which I mentioned earlier, are
found in interstellar dust clouds and may have found their way into meteors, comets or
dust particles and, then, subsequently, been transported to Earth.
"Some
scientists, in fact, have estimated that in the first 700 million years of the Earth's
existence as a planet, the Earth is likely to have passed through 4-5 interstellar clouds,
taking roughly 600,000 years to complete each such passage. For each year of passage, this
would have resulted in, approximately, between 1 - 10 million kilograms of material being
added to the Earth.
"This
is thought to be one or two orders of magnitude, or powers of ten, less than what has come
through interplanetary dust particles. Moreover, like these latter dust particles, only a
small percentage of this total mass would be in the form of simple carbon or hydrocarbon
molecules. Nonetheless, even a limited percentage of such astronomical figures still would
constitute a substantial amount of carbonaceous material available to the prebiotic
environment."
"Although,"
said the lawyer for the prosecution, "I'm quite certain , Professor Yardley, you
could provide the jurors with a great deal more information on laboratory experiments
which are intended to simulate the conditions on prebiotic Earth, I would like to shift
gears slightly. Earlier in your testimony, you had alluded to the importance of having
some degree of understanding of the systems of energy which, in a prebiotic environment,
would have driven many of the chemical reactions you just have been describing.
"Would
you please tell the court a little about this facet of evolutionary thought. Once again,
Professor, and I apologize for being a one-note-Norman on this matter, to whatever extent
possible, try to strike a balance between avoiding both oversimplification and too much
technical complexity."
Dr. Yardley
sighed slightly and, then, took a deep breath. He looked briefly at the table by the
witness stand, noticed that the pitcher had not much water in it, and made a few motions
to Mr. Mayfield indicating he would like the jug to be refilled.
As one of
the officers of the court went about the business of getting more water, Professor Yardley
started to speak. "There are," he began, "five or six energy possibilities
which are likely to have been available to prebiotic Earth for the purposes of bringing
about certain kinds of chemical evolution.
"The
first possibility requires no external input of energy. These involve physio-chemical
forces, such as hydrogen bonds, which, very likely, played a significant role in helping
certain molecules in the prebiotic environment to organize themselves or self-assemble
into more complex, and biologically relevant, packages.
"For
over forty years, thanks to the monumental work of, among others, Watson and Crick,
scientists have known that the purine, nucleic base adenine in DNA and RNA pairs
spontaneously with the pyrimidine, nucleic base uracil in RNA or the pyrimidine, nucleic
base thymine in DNA. Similarly, the purine, nucleic base guanine pairs, in spontaneous
fashion, with the pyrimidine, nucleic base in both DNA and RNA.
"These
pairings are known as Watson-Crick bonds and are a specific example of hydrogen bonding.
The complementary pairs of nucleic bases which are strung along two strands of DNA or RNA
are brought together in stable configurations by these bonds and, in the process, help
lend the double helical structure to the joining of these strands with which most of us
are familiar from school and the media.
"Hydrogen
bonds occur as a result of the positive and negative, or dipolar, characteristics which
arise in compounds containing hydrogen, oxygen and nitrogen atoms arranged in the right
kind of geometrical configuration. More specifically, nitrogen and oxygen are both
relatively electronegative in nature, whereas hydrogen tends to be electropositive in
character.
"This
means oxygen and nitrogen are inclined, under certain circumstances, to draw toward their
nuclei a few of the electrons of geometrically well-placed, neighboring hydrogen atoms or
molecules. As a result, the affected hydrogen atoms of these neighboring molecules become
electropositive and, therefore, have a tendency to establish bonds with other neighboring
atoms or molecules which offer electrochemically compatible opportunities.
"These
hydrogen bonds bring a certain amount of stability to the manner in which, under certain
circumstances, atoms and molecules arrange or organize themselves. Consequently, they are
thermodynamically-favored arrangements because of their tendency to help stabilize the way
energy is distributed in a molecular configuration.
"Hydrogen
bonds are characteristic of what are referred to as polar molecules. The polar aspect of
these molecules is rooted, as indicated previously, in the process of creating
electrochemically-charged dipolar, or positive and negative, regions.
"Polar
molecules, such as water and ribonucleic acids, have very different physical and chemical
properties from non-polar molecules which do not possess such dipolar regions. Many
hydrocarbons that do not contain nitrogen and/or oxygen tend to be non-polar in nature.
"The
bottom line on all of this is that hydrogen bonding, of which Watson-Crick pairing in
complementary bases of DNA and RNA is an extremely important example, is an instance of a
spontaneous, thermodynamically favored generation of greater complexity. A chemical
reaction is said to be spontaneous if it can take place without requiring any additional
energy.
"The
reason a reaction can take place without the need of additional energy is because the
energy available to the system has a natural tendency to redistribute itself until no
further redistribution of that energy is capable of occurring in a spontaneous fashion.
This redistribution process leads to a stable configuration of energy distribution which
is why a reaction is said to be thermodynamically favored since, under most circumstances,
the thermodynamic nature of chemical reactions is to spontaneously follow whatever
pathways are available that will lead to such stability.
"Spontaneous
reactions yield energy. In other words, if one measures the potential energy of the final,
stabilized state of this kind of reaction, one will find less energy than was present at
the beginning of the reaction.
"One of
the reasons why the final state of spontaneous reactions is stable is because not all of
the energy which is being released remains in a chemically useable form. Some of the
released energy is in the form of heat which is unavailable- that is, it cannot be
harnessed to run the reaction in a reverse direction, back to the original, initial state
prior to the reaction's commencement.
"The
term 'free energy' is often used to refer to the form of energy in a given chemical system
which is available to be redistributed, if possible, in a way that allows the system to
find, if not already realized, its most stable configuration of energy. This configuration
is that point at which the available free energy reaches, through the spontaneous activity
characteristic of the system in question, its lowest level consistent with such stability.
"As I
indicated previously, in the process of yielding or releasing energy during the time
required for a spontaneous reaction to run & #145;downhill& #146; to its stable
state, there is a portion of the released energy which is transformed to a form of energy,
namely heat, other than free energy. Entropy is a measure of the amount of energy which
has been converted from its free energy form to its non-free energy form.
"Spontaneous
reactions always result in a decline of free energy. In other words, the total amount of
free energy of the products of a chemical reaction always will be less than the total free
energy of the initial reactants of the reaction.
"Consequently,
in the process of spontaneously seeking out a stable state of redistributed energy - that
is, a state of lowest possible free energy- free energy is lost. The entropy, the amount
of energy in a non-free form, tends to increase.
"Spontaneous
chemical reactions in which energy is released to the environment are known as 'exergonic'
reactions. Chemical reactions in which energy needs to be acquired from the environment
are known as 'endergonic' reactions.
"One
can use the released energy of spontaneous, 'downhill', exergonic reactions to drive
'uphill', non-spontaneous, endergonic reactions. This is referred to as a 'coupled
reaction'.
"Non-free
forms of energy are generated during both the downhill and the uphill portions of these
coupled reactions. Consequently, the total amount of entropy will be increased during the
process.
"As
long as one has downhill reactions to sponge off, then uphill reactions are possible.
However, in order to keep a sequence of coupled reactions going, one becomes engaged in a
constant process of borrowing from Paul to pay Peter who has borrowed from Mary in order
to pay George, and so on.
"Non-spontaneous
reactions always are in need of arranging a loan of energy from the spontaneous energy
generators of the world in order to be able to activate the free energy potential of the
non-spontaneous system. When there are no downhill reactions available from which an
uphill system can borrow, things come to a sort of dynamic halt known as equilibrium in
which its uphill, non-spontaneous character does not change, despite the fact activity
still is going on within the system.
"There
is a minimum amount of free energy which has to be borrowed by, or introduced into, an
uphill, non-spontaneous system in order to bring about a chemical reaction. This minimum
amount of energy is known as the free energy of activation or the activation energy.
"One of
the major issues of evolutionary theory is to provide plausible accounts of how
spontaneous, downhill generations of energy were coupled with non-spontaneous, uphill
systems of molecules to generate arrangements of hydrocarbons of increasing
complexity. Spontaneous chemical reactions which are thermodynamically favored will take
one only so far.
"Therefore,
while phenomena such as hydrogen bonding and Watson-Crick pairing are important ways of
introducing additional organization into a system without having to borrow additional
energy, much more is needed to be able to account for the gradual transition, or
evolution, from simple hydrocarbons to the emergence of living systems. Many, if not most,
of the chemical reactions which are needed to account for how life arose from a prebiotic
environment are of the uphill, non-spontaneous variety rather than the downhill,
spontaneous kind, and this means, as suggested earlier, the need to find coupling
mechanisms of one sort or another.
"There
are a fair number of coupling candidates which would have been readily available under
prebiotic conditions. I'll list the candidates first, and, then I'll explore a few of
these possibilities.
"First,
although not necessarily the most important, are electrical discharges. In a prebiotic
environment, these would be manifested through lightening.
"A
second candidate would be ultraviolet radiation. Various molecules are capable of
absorbing different dimensions of the ultraviolet portion of the spectrum of
electromagnetic radiation. When a molecule absorbs ultraviolet light of the right
wavelength, the energy of the light can be utilized to help drive certain kinds of
chemical reactions involving such a molecule.
"A
third possibility for a source of energy capable of driving some non-spontaneous, uphill
reactions would be ionizing radiation. Gamma radiation, together with so-called cosmic
rays, would be examples of this kind of candidate.
"Heat
would be a forth coupling candidate. For instance, a surface which had been heated to high
temperatures- either by sunlight, or by a nearby volcano, or by a hydrothermal vent- such
a heated surface might have provided an environment which helped bring about condensation
reactions and the forging of various kinds of covalent bonds among molecules lying about
on that surface.
"Another
possibility involves the energy associated with shock waves. Such waves, for instance,
accompany lightening discharges, but are distinct from the electrical energy of those
discharges.
"In
addition, shock waves occur when meteors traverse the Earth's atmosphere. Such waves also
are generated when there is an airburst of, say, a carbonaceous chondrite in our
atmosphere.
"Tremendous
amounts of energy are released under these circumstances. This could be coupled with, and
utilized by, various uphill systems."
