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Basic
Worldview:
102
Atheism vs. Theism
Not Theories, Unsubstantiated Hypotheses 3
Prelude:
"Atheism/Theism" vs. "Science, the Bible, & Creation"
Atheism:
Introduction and Charges
Charge
1, Deduction and Induction
Charge
2, Question 1
Charge
2, Questions 2 and 3
Charge
2, Summary and Question 4
Charges
3 and 4, Definitions
Empirical
Evidence
Scientists
Acting as Mechanisms, Article 1
Scientists
Acting as Mechanisms, Article 2
Scientists
Acting as Mechanisms, Article 3
Occam's
Razor and Conclusions
Footnote
1
Footnote
2 and 3
Proof
of Life
Not
Theories, Unsubstantiated Hypotheses 1
Not
Theories, Unsubstantiated Hypotheses 2
Not
Theories, Unsubstantiated Hypotheses 3
Not
Theories, Unsubstantiated Hypotheses 4
Scientists:
Life on Earth Imported from Outer Space
Atheisms
Circle of Reasons
Is
God a White Crow?
(Continued...)
In the following series of paragraphs continue in the same
manner as the above series of paragraphs. The author continues
with this same line of unsubstantiated assumptions about early
RNA development.
12) In all likelihood, the four bases arose together with
a number of other substances similarly constructed of one
or more rings containing carbon and nitrogen.
The first nucleic acid-like molecules probably contained an
assortment of these compounds. Molecules rich in A, U, G and
C then were progressively selected and amplified, once some
rudimentary template-dependent synthetic mechanism allowing
base pairing arose. RNA, as it exists today, may thus have
been the first product of molecular selection.
A third stage in the evolution of the RNA world was the development
of RNA-dependent protein synthesis. Most likely, the chemical
machinery appeared first, as yet uninformed by genetic messages,
as a result of interactions among certain RNA molecules, the
precursors of future transfer, ribosomal and messenger RNAs,
and amino acids. Selection of the RNA molecules involved could
conceivably be explained on the basis of molecular advantages,
as just outlined. But for further evolution to take place,
something more was needed. RNA molecules no longer had to
be selected solely on the basis of what they were, but of
what they did; that is, exerting some catalytic activity,
most prominently making proteins. This implies that RNA molecules
capable of participating in protein synthesis enjoyed a selective
advantage, not because they were themselves easier to replicate
or more stable, but because the proteins they were making
favored their replication by some kind of indirect feedback
loop.
This stage signals the limit of what could have happened in
an unstructured soup. To evolve further, the system had to
be partitioned into a large number of competing primitive
cells, or protocells, capable of growing and of multiplying
by division. This partitioning could have happened earlier.
Nobody knows. But it could not have happened later. This condition
implies that protometabolism also produced the materials needed
for the assembly of the membranes surrounding the protocells.
In today's world, these materials are complex proteins and
fatty lipid molecules. They were probably simpler in the RNA
world, though more elaborate than the undifferentiated "goo"
or "scum" that is sometimes suggested.
Once the chemical machinery for protein synthesis was installed,
information could enter the system, via interactions among
certain RNA components of the machinery--the future messenger
RNAs--and other, amino acid-carrying RNA molecules--the future
transfer RNAs. Translation and the genetic code progressively
developed concurrently during this stage, which presumably
was driven by Darwinian competition among protocells endowed
with different variants of the RNA molecules involved. Any
RNA mutation that made the structures of useful proteins more
closely dependent on the structures of replicatable RNAs,
thereby increasing the replicatability of the useful proteins
themselves, conferred some evolutionary advantage on the protocell
concerned, which was allowed to compete more effectively for
available resources and to grow and multiply faster than the
others.
The RNA world entered the last stage in its evolution when
translation had become sufficiently accurate to unambiguously
link the sequences of individual proteins with the sequences
of individual RNA genes. This is the situation that exists
today (with DNA carrying the primary genetic information),
except that present-day systems are enormously more accurate
and elaborate than the first systems must have been. Most
likely, the first RNA genes were very short, no longer than
70 to 100 nucleotides (the modern gene runs several thousand
nucleotides), with the corresponding proteins (more like protein
fragments, called peptides) containing no more than 20 to
30 amino acids.
It is during this stage that protein enzymes must have made
their first appearance, emerging one by one as a result of
some RNA gene mutation and endowing the mutant protocell with
the ability to carry out a new chemical reaction or to improve
an existing reaction. The improvements would enable the protocell
to grow and multiply more efficiently than other protocells
in which the mutations had not appeared. This type of Darwinian
selection must have taken place a great many times in succession
to allow enzyme-dependent metabolism to progressively replace
protometabolism.
The appearance of DNA signaled a further refinement in the
cell's information-processing system, although the date of
this development cannot be fixed precisely. It is not even
clear whether DNA appeared during the RNA world or later.
Certainly, as the genetic systems became more complex, there
were greater advantages to storing the genetic information
in a separate molecule. The chemical mutations required to
derive DNA from RNA are fairly trivial. And it is conceivable
that an RNA-replicating enzyme could have been co-opted to
transfer information from RNA to DNA. If this happened during
the RNA world, it probably did so near the end, after most
of the RNA-dependent machineries had been installed.
What can we conclude from this scenario, which, though purely
hypothetical, depicts in logical succession the events that
must have taken place if we accept the RNA-world hypothesis?
And what, if anything, can we infer about the protometabolism
that must have preceded it? I can see three properties.
First, protometabolism involved a stable set of reactions
capable not only of generating the RNA world, but also of
sustaining it for the obviously long time it took for the
development of RNA replication, protein synthesis and translation,
as well as the inauguration of enzymes and metabolism.
Second, protometabolism involved a complex set of reactions
capable of building RNA molecules and their constituents,
proteins, membrane components and possibly a variety of coenzymes,
often mentioned as parts of the catalytic armamentarium of
the RNA world.
Finally, protometabolism must have been congruent with present-day
metabolism; that is, it must have followed pathways similar
to those of present-day metabolism, even if it did not use
exactly the same materials or reactions. Many abiotic-chemistry
experts disagree with this view, which, however, I see as
enforced by the sequential manner in which the enzyme catalysts
of metabolism must have arisen and been adopted. In order
to be useful and confer a selective advantage to the mutant
protocell involved, each new enzyme must have found one or
more substances on which to act and an outlet for its product
or products. In other words, the reaction it catalyzed must
have fitted into the protometabolic network. To be sure, as
more enzymes were added and started to build their own network,
new pathways could have developed, but only as extensions
of what was initially a congruent network.
It may well be, then, that clues to the nature of that early
protometabolism exist within modern metabolism. Several proposals
of this kind have been made. Mine centers around the bond
between sulfur and a carbon-containing entity called an acyl
group, which yields a compound called a thioester. I view
the thioester bond as primeval in the development of life.
- American Scientist article
NOTE: It is worth noting in this long series of paragraphs
the repeated use of phrases such as "likelihood," "probable,"
and "may have," to describe speculative events. Near the middle
of the above section, the author even admits that his proposed
scenario is "purely hypothetical." It is significant that
this unsubstantiated, purely hypothetical scenario includes
the development of nucleotides, diversified forms of RNA,
DNA, proteins, and a whole host of new enzymes, and finally
proto-cells themselves. In other words, the explanation of
how these very critical molecules came about is purely a matter
of speculation with no empirical evidence to substantiate
those hypothetical explanations.
13) I have tried here to review some of the facts and ideas
that are being considered to account for the early stages
in the spontaneous emergence of life on earth. How much of
the hypothetical mechanisms considered will stand the test
of time is not known. But one affirmation can safely be made,
regardless of the actual nature of the processes that generated
life. These processes must have been highly deterministic.
In other words, these processes were inevitable under the
conditions that existed on the prebiotic earth. Furthermore,
these processes are bound to occur similarly wherever and
whenever similar conditions obtain. - American Scientist
article
NOTE: Here again at the end the author simultaneously
refers to this series of speculative events as both "facts
and ideas" as well as "hypothetical machinery." He is not
even confident this speculation "will stand the test of time."
14) All of which leads me to conclude that life is an obligatory
manifestation of matter, bound to arise where conditions are
appropriate. - American Scientist article
15) Life is a cosmic imperative. The universe is awash
with life. - American Scientist article
NOTE: The author has already admitted that this scenario
is "purely hypothetical." And now he admits that his conclusion
that "life is an obligatory manifestation of matter" is based
entirely on these unsubstantiated speculations. His conclusion
is based upon pure speculation, not evidence.
Sources
http://www.americanscientist.org/articles/95articles/cdeduve.html
September-October 1995
The Beginnings of Life on Earth
by Christian de Duve
http://www.discover.com/archive/index.html
First Cell
By Carl Zimmer
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