These articles explore some of the scientific explanations for the origin of the first biological cell on this planet, as well as its early expansion/diversification. Not all hypotheses are included, but these are the most widely considered ones.Pick one scientific hypothesis mentioned in the readings (or elsewhere on the internet) for the origin of life and briefly give specific evidence for it that you find the most compelling. Your evidence should come from one or more of the readings; feel free to scour the internet for other sources. You can quote specific passages; but be sure to mention the paper and page number where the quote occurs. One convincing paragraph. Use the site links, attached pdf or other sources from online. http://www.livescience.com/13363-7-theories-origin… http://exploringorigins.org/index.html
origin_of_life___a_review_of_facts_and_speculation.pdf

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REVIEWS
TIBS 23 – DECEMBER 1998
The origin of life – a review of
facts and speculations
Leslie E. Orgel
Three popular hypotheses attempt to explain the origin of prebiotic molecules: synthesis in a reducing atmosphere, input in meteorites and
synthesis on metal sulfides in deep-sea vents. It is not possible to decide
which is correct. It is also unclear whether the RNA world was the first
biological world or whether some simpler world preceded it.
THE PROBLEM OF the origin of life on
the earth has much in common with a
well-constructed detective story. There
is no shortage of clues pointing to the
way in which the crime, the contamination of the pristine environment of the
early earth, was committed. On the contrary, there are far too many clues and
far too many suspects. It would be hard
to find two investigators who agree on
even the broad outline of the events that
occurred so long ago and made possible
the subsequent evolution of life in all its
variety. Here, I outline two of the main
questions and some of the conflicting
evidence that has been used in attempts
to answer them. First, however, I summarize the few areas where there is
fairly general agreement.
The earth is slightly more than 4.5 billion years old. For the first half billion
years or so after its formation, it was
impacted by objects large enough to
evaporate the oceans and sterilize the
surface1,2. Well-preserved microfossils
of organisms that have morphologies
similar to those of modern blue-green
algae, and date back about 3.5 billion
years, have been found3, and indirect
but persuasive evidence supports the
proposal that life was present 3.8 billion
years ago4. Life, therefore, originated on
or was transported to the earth at some
point within a window of a few hundred
million years that opened about four billion years ago. The majority of workers
in the field reject the hypothesis that life
was transported to the earth from somewhere else in the galaxy and take it for
granted that life began de novo on the
early earth.
L. E. Orgel is at the Salk Institute for
Biological Studies, 10010 N. Torrey Pines
Road, La Jolla, CA 92037, USA.
Email: orgel@salk.edu
The uniformity of biochemistry in all
living organisms argues strongly that all
modern organisms descend from a lastcommon ancestor (LCA). Detailed analysis of protein sequences suggests that
the LCA had a complexity comparable
to that of a simple modern bacterium
and lived 3.2–3.8 billion years ago5. If we
knew the stages by which the LCA
evolved from abiotic components present on the primitive earth, we would
have a complete account of the origin of
life. In practice, the most ambitious studies of the origins of life address much
simpler questions. Here, I discuss two of
them. What were the sources of the
small organic molecules that made up
the first self-replicating systems? How
did biological organization evolve from an
abiotic supply of small organic molecules?
Abiotic synthesis of small organic molecules.
Miller, a graduate student who was
working with Harold Urey, began the
modern era in the study of the origin of
life at a time when most people believed
that the atmosphere of the early earth
was strongly reducing. Miller6 subjected
a mixture of methane, ammonia and hydrogen to an electric discharge and led
the products into liquid water. He
showed that a substantial percentage of
the carbon in the gas mixture was incorporated into a relatively small group of
simple organic molecules and that several
of the naturally occurring amino acids
were prominent among these products.
This was a surprising result; organic
chemists would have expected a muchless-tractable product mixture. The
Urey–Miller experiments were widely accepted as a model of prebiotic synthesis
of amino acids by the action of lightning.
Miller and his co-workers went on to
study electric-discharge synthesis of
amino acids in greater detail7,8. Using
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more-powerful analytical techniques,
they identified many more amino acids –
some, but by no means all, of which
occur in living organisms. They also
showed that a major synthetic route to
the amino acids is through the Strecker
reaction – that is, from aldehydes, hydrogen cyanide and ammonia. Glycine,
for example, is formed from formaldehyde, cyanide and ammonia – all of which
can be detected among the products
formed in the electric-discharge reaction.
In the years following the Urey–Miller
experiments, the synthesis of biologically interesting molecules from products that could be obtained from a
reducing gas mixture became the principle aim of prebiotic chemistry (Fig. 1).
Remarkably, Oro and Kimble9 were able
to synthesize adenine from hydrogen
cyanide and ammonia. Somewhat later,
Sanchez, Ferris and I10 showed that
cyanoacetylene is a major product of
the action of an electric discharge on a
mixture of methane and nitrogen and
that cyanoacetylene is a plausible
source of the pyrimidine bases uracil
and cytosine. This new information,
together with previous studies that
showed that sugars are formed readily
from formaldehyde11,12, convinced many
students of the origins of life that they
understood the first stage in the appearance of life on the earth: the formation
of a prebiotic soup of biomonomers.
As in any good detective story, however, the principle suspect, the reducing
atmosphere, has an alibi. Recent studies
have convinced most workers concerned with the atmosphere of the early
earth that it could never have been
strongly reducing13. If this is true,
Miller’s experiments, and most other
early studies of prebiotic chemistry, are
irrelevant. I believe that the dismissal of
the reducing atmosphere is premature,
because we do not completely understand the early history of the earth’s atmosphere. It is hard to believe that the
ease with which sugars, amino acids,
purines and pyrimidines are formed
under reducing-atmosphere conditions
is either a coincidence or a false clue
planted by a malicious creator.
Many of those who dismiss the possibility of a reducing atmosphere believe
that the crime was an outside job. A substantial proportion of the meteorites
that fall on the earth belong to a class
known as carbonaceous chondrites14.
These are particularly interesting because
they contain a significant amount of organic carbon and because some of the
standard amino acids and nucleic-acid
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REVIEWS
bases are present8. Could the prebiotic
soup have originated in preformed organic material brought to the earth by
meteorites and comets?
Supporters of the impact theory have
argued convincingly that sufficient organic carbon must have been present in
the meteorites and comets that reached
the surface of the early earth to have
stocked an abundant soup. However,
would this material have survived the
intense heating that accompanies the
entry of large bodies into the atmosphere and their subsequent collisions
with the surface of the earth? The results of theoretical calculations depend
strongly on assumptions made about
the composition and density of the atmosphere, the distribution of sizes of
the impacting objects, etc.15 The impact
theory is probably the most popular at
present, but nobody has proved that impacts were the most important sources
of prebiotic organic compounds.
The newest suspects are the deep-sea
vents, submarine cracks in the earth’s
surface where superheated water rich in
transition-metal ions and hydrogen sulfide mixes abruptly with cold sea water.
These vents are sites of abundant biological activity, much of it independent
of solar energy. Wächtershäuser16,17 has
proposed a scenario for the origin of life
that might fit such an environment. He
hypothesizes that the reaction between
iron(II) sulfide and hydrogen sulfide [a
reaction that yields pyrites (FeS2) and
hydrogen] could provide the free energy
necessary for reduction of carbon dioxide to molecules capable of supporting
the origin of life. He asserts that life originated on the surface of iron sulfides as
a result of such chemistry. The assumptions that complex metabolic cycles
self-organize on the surface and that the
significant products never escape from
the surface are essential parts of this
theory; in Wächtershäuser’s opinion,
there never was a prebiotic soup!
Stetter and colleagues18 have confirmed the novel suggestion that hydrogen sulfide, in the presence of iron(II)
sulfide, acts as a reducing agent. They
have reduced, for example, acetylene to
ethane, and mercaptoacetic acid to
acetic acid, but they have not reported
reduction of CO2. However, in a new
study, Wächtershäuser and co-workers19 have shown that FeS spiked with
NiS reduces carbon monoxide. Given
that carbon monoxide might well have
been present in large amounts in the
gases escaping from the vents,
Wächtershäuser’s findings could well
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TIBS 23 – DECEMBER 1998
CH2O
CH4 1 NH3 1 H2O
HCN
HC
C C
N
Ca(OH)2
Sugars, including a small
amount of ribose
Electric
discharge
Amino acids, including glycine
Aqueous
ammonia
Adenine
Cyanate
or urea
Cytosine
Figure 1
Early prebiotic syntheses of biomonomers.
prove important. If metal sulfides can be
shown to catalyze the synthesis of a sufficient variety of organic molecules from
carbon monoxide, the vent theory of the
origins of biomonomers will become
very attractive.
In summary, there are three main contending theories of the prebiotic origin
of biomonomers (not to mention several
other less-popular options). No theory
is compelling, and none can be rejected
out of hand. Perhaps it is time for a conspiracy theory; more than one of the
sources of organic molecules discussed
above may have collaborated to make
possible the origin of life.
Self-organization
There is no general agreement about
the source of prebiotic organic molecules
on the early earth, but there are several
plausible theories, each backed by
some experimental data. The situation
with regard to the evolution of a selfreplicating system is less satisfactory;
there are at least as many suspects, but
there are virtually no experimental data.
The fairly general acceptance of the
hypothesis that there was once an RNA
world (i.e. a self-contained biological
world in which RNA molecules functioned both as genetic materials and as
enzyme-like catalysts) has changed the
direction of research into the origins of
life20. The central puzzle is now seen to
be the origin of the RNA world. Two specific, but intertwined, questions are central to the debate. Was RNA the first
genetic material or was it preceded by
one or more simpler genetic materials?
How much self-organization of reaction
sequences is possible in the absence of
a genetic material? I shall concentrate
on the first question.
The assumption that a polymer that
doubled as a genetic material and as a
source of enzyme-like catalytic activity
once existed profoundly changes the
goals of prebiotic synthesis. The central
issue becomes the synthesis of the first
genetic monomers: nucleotides or whatever preceded them. The synthesis of
amino acids, coenzymes, etc. becomes
a side issue, because there is no reason
to believe that they were ever synthesized abiotically; some or all of them
might have been introduced as direct
or indirect consequences of the enzymelike activities of RNA or its precursor(s).
Supporters of the hypothesis that RNA
was the first genetic material must explain where the nucleotides came from
and how they self-organized. Those who
believe in a simpler precursor have the
difficult task of identifying such a precursor, but they hope that explaining
monomer synthesis will then be simpler.
Returning to the idiom of the detective story, accumulating evidence suggests that RNA, a prime suspect, could
have completed the difficult task of organizing itself into a self-contained replicating system. It has proved possible to
isolate sequences that catalyze a wide
variety of organic reactions from pools of
random RNA21,22. As regards the origin of
the RNA world, the most important reactions are those in which a preformed
template-RNA strand catalyzes the synthesis of its complement from monomers
or short oligomers. Eklund and co-workers23 have isolated catalysts for the ligation of short oligonucleotides surprisingly easily, and the catalysts carry out
ligation with adequate specificity. These
molecules are the RNA equivalents of the
RNA and DNA ligases. Considerable progress has also been made in selecting
RNA equivalents of RNA polymerases24.
If the RNA world evolved de novo, it
must have depended initially on an abiotic
source of activated nucleotides. However,
oxidation–reduction, methylation, oligosaccharide synthesis, etc., supported by
nucleotide-containing coenzyme, probably became part of the chemistry of the
RNA world before the invention of protein
synthesis. Unfortunately, we cannot say
just how complex the RNA world could
have been until we know more about the
REVIEWS
TIBS 23 – DECEMBER 1998
range of reactions that can be catalyzed
by ribozymes. It seems likely that RNA
could have catalyzed most of the steps
involved in the synthesis of nucleotides25,
and possibly the coupling of redox reactions to the synthesis of phosphodiesters and peptides, but this remains to
be demonstrated experimentally.
The experiments on the selection of
ribozymes that catalyze nucleic acid
replication (discussed above) use as inputs pools of RNA molecules synthesized
by enzymes. Recently, Ferris and coworkers26,27 have made considerable progress in the assembly of RNA oligomers
from monomers, using an abundant clay
mineral, montmorillonite, as a catalyst.
The substrates that they use, nucleoside
59-phosphorimidazolides, were probably
not prebiotic molecules, but the experiments do indicate that the use of minerals
as adsorbents and catalysts could allow
the accumulation of long oligonucleotides
once suitable activated monomers are
available. We have shown that, using activated monomers, non-enzymatic copying of a wide range of oligonucleotide
sequences is possible28 and have obtained similar, but less extensive, results
for ligation of short oligomers.
An optimist could propose the
following scenario. First, activated
mononucleotides oligomerize on montmorillonite or an equivalent mineral.
Next, copying of longer templates, using
monomers or short oligomers as substrates, leads to the accumulation of a library of dsRNA molecules. Finally, an
RNA double helix, one of whose strands
has generalized RNA-polymerase activity,
dissociates; the polymerase strand copies
its complement to produce a second
polymerase molecule, which copies the
first to produce a second complement –
and so on. The RNA world could therefore have arisen from a pool of activated
nucleotides29. All that would have been
needed is a pool of activated nucleotides!
Nucleotides are complicated molecules. The synthesis of sugars from
formaldehyde gives a complex mixture,
in which ribose is always a minor component. The formation of a nucleoside
from a base and a sugar is not an easy
reaction and, at least for pyrimidine nucleosides, has not been achieved under
prebiotic conditions; the phosphorylation
of nucleosides tends to give a complex
mixture of products30. The inhibition
of the template-directed reactions on Dtemplates by L-substrates is a further
difficulty31. It is almost inconceivable
that nucleic acid replication could have
got started, unless there is a much
simpler mechanism for the prebiotic
synthesis of nucleotides. Eschenmoser
and his colleagues32 have had considerable success in generating ribose 2,4diphosphate in a potentially prebiotic
reaction from glycolaldehyde monophosphate and formaldehyde. Direct
prebiotic synthesis of nucleotides by
novel chemistry is therefore not hopeless. Nonetheless, it is more likely that
some organized form of chemistry preceded the RNA world. This leads us to a
discussion of genetic takeover.
Cairns-Smith33, long before the argument became popular, emphasized how
improbable it is that a molecule as high
tech as RNA could have appeared de
novo on the primitive earth. He proposed that the first form of life was a
self-replicating clay. He suggested that
the synthesis of organic molecules became part of the competitive strategy of
the clay world and that the inorganic
genome was taken over by one of its
organic creations. Cairns-Smith’s postulate of an inorganic life form has failed to
gather any experimental support. The
idea lives on in the limbo of uninvestigated hypotheses. However, CairnsSmith also contemplated the possibility
that RNA was preceded by one or more
linear organic genomes34. This idea has
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REVIEWS
taken root, but its implications have not
always been appreciated.
If RNA was not the first genetic material, biochemistry might provide no
clues to the origins of life. Presumably,
the biological world that immediately
preceded the RNA world already had
the capacity to synthesize nucleotides.
This should help us to formulate hypotheses about its chemical characteristics. However, if there were two or
more worlds before the RNA world, the
original chemistry might have left no
trace in contemporary biochemistry. In
that case, the chemistry of the origins of
life is unlikely to be discovered without
investigating in detail all the chemistry
that might have occurred on the primitive
earth – whether or not that chemistry has
any relation to biochemistry. This gloomy
prospect has not prevented discussion
of alternative genetic systems.
The only potentially informational
systems, other than nucleic acids, that
have been discovered are closely related to nucleic acids. Eschenmoser and
his colleagues35 have undertaken a systematic study of the properties of nucleic acid analogs in which ribose is replaced by another sugar or in which the
furanose form of ribose is replaced by
the pyranose form (Fig. 2b). Strikingly,
polynucleotides based on the pyranosyl
isomer of ribose (p-RNA) form Watson–
Crick-paired double helices that are more
stable than RNA, and p-RNAs are less
likely than the corresponding RNAs to
form multiple-strand competing structures35. Furthermore, the helices twist
much more gradually than those in the
standard nucleic acids, which should
make it easier to separate strands during replication. Pyranosyl RNA seems to
be an excellent choice as a genetic system; in some ways, it might be an improvement on the standard nucleic acids.
However, prebiotic synthesis of pyranosyl nucleotides is not likely to prove
much easier than synthesis of the standard isomers, although a route through
ribose 2,4-diphosphate is being explored
by Eschenmoser and his colleagues.
Peptide nucleic acid (PNA) is another
nucleic acid analog that has been studied extensively (Fig. 2c). It was synthesized by Nielsen and colleagues36 during
work on antisense RNA. PNA is an uncharged, achiral analog of RNA or DNA;
the ribose-phosphate backbone of the
nucleic acid is replaced by a backbone
held together by amide bonds. PNA
forms very stable double helices with
complementary RNA or DNA36,37. We
have shown that information can be
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TIBS 23 – DECEMBER 1998
(a)
(c)
(b)
NH
O
B
O
B
O
O
N
O
HO
P
2O
O
O2
O
O
P
O
2O
O
P
O2
O
P
HO
2O
O
O
P
O2
N
O
O
O
O
B
B
B
NH
O
O
O
NH
O
B
HO
O
O
O
O
O
B
O
P
O
B
B
N
O
O
O
O
Figure 2
DNA and potentially informational oligonucleotide analogs. (a) DNA. (b) Pyranosyl analog of
RNA. (c) Peptide nucleic acid.
transferred from PNA to RNA, and vice
versa, in template-directed reactions38,39
and that PNA–DNA chimeras form readily
on either DNA or PNA templates40. Thus,
a transition from a PNA world to an RNA
world is possible. Nonetheless, I think it
unlikely that PNA was ever important on
the early earth, because PNA monomers
cyclize when they are activated; this
would make oligomer formation very
difficult under preb …
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