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Table of Contents for Evolution On Trial |
"The
Isau formation consists of high-grade metamorphic rocks which have gone through a process
of reformation under conditions of extremely high temperature and pressure. Consequently,
any direct fossil evidence which might have been contained in this rock formation would
have been destroyed.
"However,
there is some indirect evidence which has been discovered at Isau to suggest a
bacteria-like organism may have existed in Greenland some 3.85 billion years ago. This
evidence is based on an analysis of the ratios of two isotopes of carbon, C12
and C13, which were found in a hydrocarbon specimen taken from the rock
formation.
"Since
C12 tends to be used preferentially in biological processes rather than C13,
and since the ratio of C12 to C13 found in the sample of hydrocarbon
was high, some scientists have been quite excited by the implications of the findings.
They have concluded, despite possible methodological counter-indications, that these
findings on carbon isotope ratios may mean the hydrocarbon being examined was produced
some 3.8 billion years ago, during a process of photosynthesis in which an organism
converted carbon dioxide into oxygen along with various hydrocarbons compounds.
"Interestingly,
the term "Isau" is translated from the Inuit language as being equivalent to the
English phrase: "the farthest we can go". Whether this is true with respect to
the earliest evidence for life is concerned, remains to be seen.
"Be
this as it may, if the scientific interpretation of the significance of this analysis of
the Isau hydrocarbon is correct, then the earliest evidence for life has been placed just
some 750 million years from the time the Earth reached planetary size. Furthermore, if the
interpretation of the carbon isotope ratios is correct, living organisms have been located
only 200 - 400 million years from the time when the prebiotic conditions on Earth are
thought to have begun to stabilize with respect to a broad set of planetary, geological,
atmospheric and hydrological parameters considered to have an important bearing on the
issue of the origin-of-life.
"This
period of 200 - 400 million years establishes the temporal framework within which modern
evolutionary biology has attempted to delineate a plausible sequence of steps in chemical
evolution. This sequence would offer an explanatory account concerning the dynamic set of
factors considered necessary to produce a working prototype of a living organism capable,
minimally speaking, of processes of photosynthesis similar to what is suggested by the
Isau hydrocarbon.
"Conceivably,
there may have been some primitive form of life, a proto cell, which existed prior to the
emergence of the first modern prokaryotic-like microorganism. On the other hand, its
manner of cellular functioning probably would have been very different from, and,
therefore, a matter of speculation relative to, the kind of DNA-based organism which is
indicated by the earliest evidence we possess either with respect to the indirect evidence
of the Isau rock formation in Greenland or the direct evidence of the Warrawoona Group in
Australia.
"On the
basis of the available evidence, the Isua hydrocarbon and the Warrawoona
prokaryotes constitute remnants of the last ancestor which is shared or held in common by
all existing life forms. More distant or ancient ancestors, in the form of various kinds
of primitive proto cells, do not necessarily form part of the biological lineage of all
current life forms.
"As
such, these kind of proto cells would be regarded as spontaneously arising experiments in
life which, for whatever conditions of natural selection, fizzled out at some point. These
experimental failures, if you will, are to be distinguished from the appearance of the
first, sustained, experimental biological success story to emerge from the prebiotic
environment and which represents the last common ancestor of all subsequent life
forms."
"Alright,
Dr. Yardley," the prosecutor said, "you have established a general framework
within which, and through which, we can engage the more difficult issues surrounding
chemical evolution. For the benefit of the jurors, let's try to break up the themes of
chemical evolution into units which, to the degree this can be accomplished, will become a
little bit more user friendly for those of us who are relatively uninitiated in such
matters.
"Professor,
if you had to list four or five areas of discussion which you consider to be crucial to
developing some minimal appreciation of how evolutionary biologists go about explaining
the transition from prebiotic chemistry to the first life forms, what areas would you
cite?"
Hesitating
only slightly, Dr. Yardley replied: "First, one should address the ways in which more
complex hydrocarbons either evolved out of chemical reactions amongst simple hydrocarbons
or became available to the prebiotic environment through means other than chemical
reactions. Secondly, there would have to be some discussion of the systems of energy which
were helping to drive the chemical reactions in the prebiotic environment.
"Thirdly,
at some point, one would have to talk about the formation of proteins through the linking
together of amino acids by means of peptide bonds. This would be of great importance
because of the many different roles which proteins have in biological organisms,
including: hormonal functioning; muscular contractions; the variability of morphology or
structural form among species; electron transport in both photosynthesis and respiration;
antibody activity in the immune system; and, the transport of nutrients, ions and so on
across the membrane barrier.
"Quite
obviously, one also would have to explore the processes surrounding the formation of
nucleic acids, especially, of course, deoxyribonucleic acid or DNA and ribonucleic acid,
RNA. Both of these molecules have fundamental roles to play in the processes of
replication, transcription, translation and energy-coupling reactions which are central to
the continued existence of both individual organisms as well as a given species.
"Finally,
one would have to discuss the role which lipid formation plays in, among other things, the
structure and function of cell membranes. Biological membranes help regulate the passage
of compounds into and out of the cell, and, in doing so, provide a relatively protected,
enclosed environment in which various vital chemical reactions can take place under much
more favorable conditions than may be prevailing in the medium that is surrounding the
cell's exterior.
"In
view of the limited time available to us, Dr. Yardley, I am hoping you will be able to
summarize some of the research evidence concerning the different areas you have just
mentioned which scientists believe helps establish a compelling case in support of the
modern theory of evolution. In fact, Professor, maybe the easiest way to proceed is to
allow our discussion to unfold in accordance with the sequence of topics you have listed.
"Consequently,
if you will, Dr. Yardley, begin with the first theme you cited as being important to the
foundations of modern evolutionary theory. This concerned, I believe, the generation and
availability of complex hydrocarbons in the prebiotic environment."
"There
are," the professor said, "two broad approaches to explaining the existence of
complex hydrocarbons in the prebiotic environment. One approach focuses on the chemical
reactions and dynamics which are likely to have occurred on the Earth in prebiotic times.
"The
other approach, which is not necessarily in conflict with, or in opposition to, the first
approach, gives emphasis to the possibility that various hydrocarbons, both simple and
complex, may have been transported to Earth through carbonaceous chondrite meteors, comets
and interplanetary dust particles. I'll start with this second approach.
"The
term chondrite is derived from the millimeter-sized structures, known as chondrules, that
can be found distributed throughout the interior matrix of a meteor, along with other
kinds of stony minerals. The origin of these chondrules still has not been determined,
although they are believed to come from the aggregates of silica minerals which were
generated through the melting and fusion occurring in the solar nebula during the early
stages of the evolution of our solar system.
"Approximately
5-6% of these stony, chondrite meteorites also contain different amounts of carbon
compounds. For obvious reasons, this subset of stony meteorites is referred to as
carbonaceous chondrites.
"Usually
speaking, carbonaceous chondrite meteorites contain up to several percent, by mass, of
carbon materials, of one sort or another. Moreover, some of these compounds include
complex hydrocarbons.
"For
example, the Murchison meteorite which fell in Australia in 1969 has been studied quite
extensively. Six of the twenty amino acids found in Earth organisms were discovered in
that meteorite.
"There
also were at least twelve other kinds of amino acid compounds found in the meteorite.
Although, as far as we know, these other varieties of amino acid do not occur in
biological organisms on Earth, their presence is considered significant because it
suggests, under the right prebiotic conditions, many different species of complex amino
acids are capable of being formed.
"Some
people have disputed the Murchison findings, claiming that the amino acids discovered in
the meteorite were there as a result of contamination by organic matter from Earth. While
most researchers do not accept such claims, there is a small aura of controversy lingering
about the Murchison meteorite.
"This
charge of contamination, however, cannot be leveled at the findings of another study
involving two meteorites which have been discovered in Antarctica. These meteorites had
been buried in the frozen depths of Antarctica's ice for some 200,000 years.
"Many
varieties of amino acid were found in those two meteorites. A little less than half of
these amino acid compounds were quite different from the ones which are found in living
organisms on Earth."
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