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The Origin of Life
John H. McClendon,
Earth-Science Reviews, 1999, 47, 71-93.
Abstract
Microfossil finds have been firmly established at about 3.5 Ga (giga
annee=109 years), but no rocks older than about 4.0 Ga have been
demonstrated, leaving the history of the first 0.6 Ga missing. This
gap has been filled by models of the solar system. The origin of the
ocean, atmosphere, and much crustal material apparently lies in a
heavy rain of comets, subsequent to the catastrophic Moon-forming
event. The earliest microfossils are those of the Apex chert in
Australia, about 3.5 Ga old. `Prebiotic' simulations of possible
biochemistry have made some progress in recent years, but many
obstacles remain, and there is no agreement as to the course of
development. The `ribose nucleic acid (RNA) World', aboriginal `clay
genes', and catalysis on iron-sulfide precipitates are not ruled out.
The search for the `last common ancestor' has reached a point between
the Bacteria and the Archaea. It is possible that this organism may
have been a thermophile, similar to many modern hot spring organisms.
But it is likely to have been an autotroph, and a late development
after the true origin of life. Even more speculative are suggestions
about the origins of metabolic sequences, in particular the origin of
the genetic code. Since all modern organisms share this code (and many
other things), there had to be a long history of development during
the blank period of Earth history.
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This major review of the abiogenesis literature gives particular
emphasis to the geological context and also to the lengthy transition
from the first replicating cell to the last common ancestor.
The geologic evidence for the early earth having a reducing atmosphere
is well discussed. "I have found no recent authors who support the
strongly reducing atmosphere model, attractive as it is from a
chemical synthesis point of view" (p,78).
The earth is thought to have accreted from dust and planetesimals,
largely from dry materials. The water is considered to have come in
later - from comets. One view is that the ocean was in place by 4.4
Ga. McClendon says "This is a worry, because it would have diluted
any organic matter that was there, making further synthetic reactions
less probable." (p.76). Whatever the timing, there are weighty issues
concerning the volume of the Hadean ocean. This is because models of
earth history are only beginning to grow the continents - and the
earth's surface would be submerged. "Ponds might have existed on the
sides of early volcanoes, but continents did not exist, and the ocean
was apparently massive very early. Miller's experiments assumed ...
.. that the atmosphere was full of methane and ammonia, but the
discussion above again puts this in grave doubt. Since we know that
life did arise, we are obligated to find mechanisms to accumulate
enough organic matter to start life" (p.78). One could point out that
this obligation is imposed by a commitment to methodological
naturalism.
The chemical simulations are reviewed. "For prebiotic amino acids,
the situation is this: these are easy to make if you do not worry
about chirality.. .. Meteorites, in particular the Murchison, contain
much the same amino acids as Miller obtained in his spark synthesis in
an atmosphere of methane, ammonia and water (Table 1). However, these
are all close to racemic, while proteins are only made from L-amino
acids. In addition, some of the protein amino acids, such as lysine,
histidine and arginine, are not found in detectable amounts. On the
other hand, many amino acids are found which have no role in modern
proteins, such as norvaline and norleucine .. .." (p.81). Later,
McClendon draws attention to particular problems created by the
scarcity of lysine in primordial synthesis experiments - it is
essential for life as we know it.
Discussion of the different proposals is provided. As an example,
consider the "protein-first" scenario proposed by Fox.
"As for the amino acid polymers, i.e. proteins, Sidney Fox (Fox and
Dose 1977) has demonstrated the ease of polymerisation into
'proteinoids' by hot dehydration, but although some catalytic
activity was expressed, no regular structure was present. Chyba and
McDonald (1995) were particularly critical of the need for
dehydrating conditions for this and other syntheses, as unlikely on
the Hadean Earth. The only likely location would be on the sides of
volcanoes emergent from the ocean, but maybe this would be important.
"In modern proteins, the order of the monomers in the protein
determines its function. In the above proteinoids, more or less
random order is obtained, which is further degraded by the occurrence
of racemic monomers (giving an irregular chain) and the possibility
of cross-links beteen the double function amino acids: aspartic and
glutamic acids and lysine. The only way out of this dilemma is the
use of surface catalysts to determine the structure of the polymer.
" (p.82)
Of greatest value is the delineation of what must be present for
anything to be called the first replicating cell. (Failure to do this
has led some, Fox in particular, to make bold claims about making
proto-cells in the laboratory). These points are summarised as
McClendon draws things to a conclusion: "Following the assembly of the
most primitive cell, i.e., one with a membrane and a genetic/protein
synthetic mechanism, Darwinian evolution could begin, gradually
developing into the last common ancestor." (p.84).
In the discussion of the development to the last common ancestor,
McClendon makes this observation: "One principle must be emphasised.
That is, no complex system could develop instantaneously. There had
to be a period of sequential development". (p..87). This is, of
course, fundamental to Darwinism - leaving this approach vulnerable to
the kind of analysis given by Mike Behe.
Some extracts from the conclusions:
"The biological fossil record goes back as far as we can reasonably
expect it. Perhaps new discoveries will extend the record into the
Hadean, but that seems unlikely. Unfortunately, this history
apparently begins with metabolically advanced organisms which emit
elemental oxygen. Thus, much of the origin of life is still hidden
from us. Perhaps exploration of Mars will give us more clues."
(p.88). (I had not before appreciated this aspect of the search for
life on Mars. There is really a sense of desperation here - as the
Earth is not releasing its secrets!).
"The synthetic organic chemistry of 'prebiotic' compounds has come to
an impasse. Although many monomers have been shown to occur in
simulations, nucleic acid-type polymers resist our efforts. Proteins
of regular structure are likewise missing." (p.89).
Regarding the RNA world: "Nevertheless, the gap between simple
monomers and RNA remains" (p.89). (I like to see advocates of
naturalism using the word "gap"!).
It is an interesting and well-written review. I am sure McClendon can
be challenged on some points, but his essay is a fair and thorough
review of the abiogenesis literature.
This article can be downloaded for free from the web in PDF format.
Follow links from: http://www.elsevier.com/locate/earscirev/
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By way of a conclusion, I would like to interact a little with some
words written by Howard Van Till in "Three views on Creation and
Evolution", Moreland, J.P. and Reynolds, J.M. (eds), Zondervan
Publishing House, Grand Rapids, 1999.
Van Till is very strong on the view that "Informed scientific
judgment is best done by people whose professional training and
experience is in the natural sciences" (p.193). When there are
deep differences between scientists, it would be best to suspend
judgment. But "if it is a matter of a small number of persons
contesting a judgment held by the vast majority of scientists, I
would consider the majority position far more likely to be correct."
(p.193)
On pages 184-185, Van Till has a list of the elements that
constitute "creation's formational economy". This summarises his
perception of the "fully gifted" creation. Among the set is this
entry:
".. .. biologically important molecules have the capabilities for
self-organization into complex molecular assemblies. (In the
judgment of the vast majority of scientists, some of these molecular
systems achieved the attributes of living systems in the course of
the earth's formational history.)" (p.185)
This point is essential to Van Till's position that creation's
formational economy was sufficiently robust to organize and transform
itself from elemental forms of matter into the full array of physical
structures and life-forms that we are aware of.
The point I wish to draw attention to is this: what has influenced
the "vast majority of scientists" to say that abiogenesis occurred?
Has the data emerging from science been so compelling that few could
resist drawing this conclusion? I would suggest from McClendon's
review (which I think is very fair to the researchers involved) that
this conclusion DOES NOT emerge from the research.On the contrary,
the research has come to "an impasse". It has NEVER provided
scientists with any well-grounded evidence that abiogenesis has
occurred.
So what are we to make of Van Till's confidence in the judgment of
scientists? Why do these people take this view? Could it be that
these scientists have imbibed a philosophy which drives their
thinking? When McClendon writes: "Since we know that life did arise,
we are obligated to find mechanisms to accumulate enough organic
matter to start life" - is this obligation driven by the quest for
truth or is it driven by a philosophical commitment to "mechanism"?
I hope my point is clear. We have discussed it enough times before
on this list. I offer these thoughts as a further contribution on
this general theme.
Best regards,
David J. Tyler.