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ScienceWeek
SCIENCEWEEK
ScienceWeek
November 22, 2002
Vol. 6 Number 47
An Online Research Digest Published Weekly Since 1997
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In other words, apart from the known
and the unknown, what else is there?
-- Harold Pinter
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Section 1
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Thematic Issue: Origin of Life and Early Life on Earth
Introduction
1. Origin of Life on Earth: Prebiotic Synthesis
2. Continental Crust and Oceans on Ancient Earth.
3. On Carbon Monoxide in Comet Hale-Bopp.
4. On the Earliest Traces of Life on Earth.
5. On Earth's Earliest Fossils.
6. On the Evidence for Earth's Oldest Fossils.
7. On the Akilia Rocks and Earth's Earliest Life.
8. Early Earth: Carbonaceous Meteorites as a Source of Sugars
9. On the Origin of the Earth.
10. Life and the Evolution of Earth's Atmosphere.
11. On the RNA-World Hypothesis
12. Origin of Life: The Present Status of Chemical Theory (1998)
ScienceWeek Notices and Subscription Information
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Section 2
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INTRODUCTION
From the Editor: The two quoted excerpts that follow are apt
reminders that the scientific problem of the origin of life
continues to have slippery philosophical aspects. During the
first half of the 20th century, biologists thought of a living
cell as a variety of coacervate droplet filled with a rather
homogeneous protoplasm and obeying yet to be described laws of
colloidal systems. The expectation was that the advances of the
second half of the 20th century would simplify everything,
explain everything, tell us what we are, where we came from, and
where we're going. How naive we were! The advances of the second
half of the 20th century have made the living cell more
complicated, not less complicated, and the existential questions
are not only still unanswered but appear dimmer in conception.
Truly, the puzzle of life is not yet fully demarcated.
As far as the science itself is concerned, this is a complex
field in which biologists, chemists, and geophysicists focus on
similar questions with differing attitudes and methods. We do
hope this issue of ScienceWeek at least outlines some of the
various approaches that are now extant. Not everything is covered
here, and we apologize for our omissions. -- Dan Agin (Editor).
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"Normally, in investigating a phenomenon one must first define
it. Yet neither science nor philosophy can define life: it eludes
our attempts to pin it down. What is life? It is not
reproduction, because crystals reproduce themselves after a
fashion, while eunuchs do not. It is not some funny property of
carbon atoms, as we can, in principle at least, conceive of some
electronic analogue of a very simple bacterium. We cannot even
draw the line between life and non-life -- is a virus life, or is
it a naked string of chemicals? At some times viruses seem to be
living; at other times they can be stopped, inactive. We can
crystallize the genetic material, DNA, just as we crystallize
salt, yet DNA is the key of life. Many people have tried,
unsuccessfully, to define life. The political philosopher Engels,
a friend of Marx, considered it to be the mode of action of
albuminous substances, a definition which admits under the title
'life' such oddities as breaking eggs and making omelettes. The
famous English scientist, J.D. Bernal, who otherwise thought
clearly, came up with a similarly useless definition, that life
is the self-realization of atomic electron states, a definition
aimed more at denying God than at explaining life. Thermodynamics
helps us to come closer: all life shares the property of
increasing the local order by making the environment around it
more chaotic. Life is growth. It can exist only in time, creating
something that is locally special by exploiting the environment.
In effect, it mines the future to enrich the present. Perhaps
Cardinal Newman, a theologian, was closest to it, although he
thought in theological, not thermodynamic terms. He lived by the
maxim: 'Growth, the only evidence of life'. Life is sustained,
growing unbalance, or 'disequilibrium'. Even in old age, our
cells are still working, processing, sometimes growing. Chemical
balance, or equilibrium, means death. Indeed, in the progression
from the simple bacterium living in a hot pool to a man walking
on the Moon, the degree of sophistication of a living organism
can be measured by its degree of disequilibrium from a natural
environment. Life, like a Western economy, cannot stay still. It
must grow. If it stops, it dies and attains equilibrium as a
corpse. Yet there is still something elusive. How do we
distinguish between growing crystals and life? We still cannot
define life. At the center of the problem the mystery remains."
E.G. Nisbet: Living Earth: A Short History of Life and Its Home.
Harper Collins. 1991. p.30.
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"The first steps in the evolution of life need not have involved
life itself. Non-biological reactions may have produced organic
compounds from which the living cells would later be assembled.
This chemical evolution, if it ever happened, has left no trace
in the geological record, and neither have the first hesitant
steps of life itself. What happened in this critical phase is
speculative and controversial, but that is no reason to shun the
issue. The arguments depend as much on a definition of life as on
the rules of chemistry, and on reasonable although subjective
opinions regarding the environment of the early Earth, its
geochemical condition, and the available sources of energy. After
the chemical evolution there was life, and where one stops and
the other begins depends on what is and what is not life. A
definition, although of considerable interest, is not [always]
needed..." -- Tjeerd H. van Andel: New Views on an Old Planet: A
History of Global Change. 2nd Edition. Cambridge University
Press. 1994. p. 292.
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1. ORIGIN OF LIFE: PREBIOTIC SYNTHESIS
S. Miyakawa et al (Yokohama National University, JP) discuss
prebiotic synthesis, the authors making the following points:
1) Based on putative microfossil (1) and light carbon evidence in
ancient sedimentary rocks (2), and the possible sterilizing
consequences of the late heavy bombardment suggested by lunar
records (3), the ancestor of modern life is thought to have
originated approximately 3.8 billion years ago, although recent
findings have questioned some of these data (4,5). The
composition of the primitive atmosphere around this time remains
uncertain, but volcanic outgassing could have been a major source
of atmospheric gases. The oxidation state of volcanic gases would
have depended on the oxidation state of the upper mantle. If the
upper mantle had been in a reduced oxidation state, the gases
would have been composed mainly of H2, H2O, CO, and N2. At more
moderate temperatures, CO would have reacted with H2 to yield CH4
and H2O in the presence of catalysts, and N2 would have reacted
with H2 to yield NH3. The most reducing atmosphere possible would
have been composed of CH4, NH3, H2, and H2O (herein referred to
as a strongly reducing atmosphere), although it is difficult to
reach this state because of factors such as photo-decomposition.
2) The oxidation state of the upper mantle 3.8 billion years ago
is generally thought to have been near its current value. Modern
volcanic gases are composed mainly of H2O and CO2. J.C. Walker
(1985) suggested that the partial pressure of CO2 in the early
atmosphere could have been as high as 10 bars. The dominant view
in recent years has thus been that the atmosphere when life
originated was composed of CO2, N2, and H2O combined with lesser
amounts of CO, CH4, and H2.
3) In summary: Most models of the primitive atmosphere around the
time life originated suggest that the atmosphere was dominated by
carbon dioxide, largely based on the notion that the atmosphere
was derived via volcanic outgassing, and that those gases were
similar to those found in modern volcanic effluent. These models
tend to downplay the possibility of a strongly reducing
atmosphere, which had been thought to be important for prebiotic
synthesis and thus the origin of life. However, there is no
definitive geologic evidence for the oxidation state of the early
atmosphere and bioorganic compounds are not efficiently
synthesized from CO2 atmospheres. The authors report a
demonstration that a CO-CO2-N2-H2O atmosphere can give a variety
of bioorganic compounds with yields comparable to those obtained
from a strongly reducing atmosphere. Atmospheres containing
carbon monoxide might therefore have been conducive to prebiotic
synthesis and perhaps the origin of life. CO-dominant atmospheres
could have existed if the production rate of CO from impacts of
extraterrestrial materials were high or if the upper mantle had
been more reduced than today.
References (abridged):
1. Schopf, J. W. (1993) Science 260, 640-646.
2. Mojzsis, S. J. , Arrhenius, G. , McKeegan, K. D. , Harrison,
T. M. , Nutman, A. P. & Friend, C. R. L. (1996) Nature 384, 55-
59.
3. Maher, K. A. (1988) Nature 331, 612-614.
4. Schopf, J. W. , Kudryavtsev, A. B. , Agresti, D. G. , Wdowiak,
T. J. & Czaja, A. D. (2002) Nature 416, 73-76.
5. Brasier, M. D. , Green, O. R. , Jephcoat, A. P. , Kleppe, A.
K. , Van Kranendonk, M. J. , Lindsay, J. F. , Steele, A. &
Grassineau, N. V. (2002) Nature 416, 76-81.
Proc. Nat. Acad. Sci. 2002 99:14628
Related Background Brief:
IMPACT FRUSTRATION OF THE ORIGIN OF LIFE. One possible definition
for the origin of life on Earth is the time at which the interval
between devastating environmental insults by impact exceeded the
timescale for establishing self-replicating proto-organisms. A
quantitative relationship for the Hadean (pre-3,800 Myr ago) and
Early Archean (3,800 to 3,400 Myr) impact flux can be derived
from the lunar and terrestrial impact records. Also, the effects
of impact-related processes on the various environments proposed
for abiogenesis (the development of life through chemical
evolution from inorganic materials) can be estimated. Using a
range of plausible values for the timescale for abiogenesis, the
interval in time when life might first have bootstrapped itself
into existence can be found for each environment. The authors
report that if the deep marine hydrothermal setting provided a
suitable site, abiogenesis could have happened as early as 4,000
to 4,200 Myr ago, whereas at the surface of the Earth abiogenesis
could have occurred between 3,700 and 4,000 Myr. K.A. Maher and
D.J. Stevenson: Nature 1988 331:612.
Related Background Brief:
THE PREBIOLOGICAL PALEOATMOSPHERE: STABILITY AND COMPOSITION. In
the past, it was generally assumed that the early atmosphere of
the Earth contained appreciable quantities of methane (CH4) and
ammonia (NH3). This was the type of atmosphere believed to be the
most suitable environment for chemical evolution, the
nonbiological formation of complex organic molecules, the
precursors of living systems. The authors argue that
photochemical considerations indicate that a CH4-NH3 dominated
early atmosphere was probably very short-lived, if it ever
existed at all. Instead, an early atmosphere of carbon dioxide
(CO2) and nitrogen (N2) is favored by photochemical as well as
geological and geochemical considerations. Photochemical
calculations also indicate that the total oxygen column density
of the prebiological paleoatmosphere did not exceed 10^(-7) of
the present atmospheric level. J.S. Levine et al: Orig Life 1982
12:245.
Related Background Brief:
PREBIOTIC SYNTHESIS IN ATMOSPHERES CONTAINING CH4, CO, AND CO2.
I. AMINO ACIDS. The authors report a study of prebiotic synthesis
of organic compounds using a spark discharge on various simulated
primitive earth atmospheres at 25 degrees C. Methane mixtures
contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4 molar ratios from
0 to 4 and pNH3 = 0.1 torr. A similar set of experiments without
added NH3 was performed. The yields of amino acids (1.2 to 4.7%
based on the carbon) are approximately independent of the H2/CH4
ratio and whether NH3 was present, and a wide variety of amino
acids are obtained. Mixtures of H2 + CO + H2O + N2 and H2 + CO2 +
H2O + N2, with and without added NH3, all gave about 2% yields of
amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For a H2/CO2
ratio of 0, the yield of amino acids is extremely low (10^(-3)%).
Glycine is almost the only amino acid produced from CO and CO2
model atmospheres. These results show that the maximum yield is
about the same for the three carbon sources at high H2/carbon
ratios, but that CH4 is superior at low H2/carbon ratios. In
addition, CH4 gives a much greater variety of amino acids than
either CO or CO2. The authors suggest that if it is assumed that
an abundance of amino acids more complex than glycine was
required for the origin of life, then these results indicate the
requirement for CH4 in the primitive atmosphere. G. Schlesinger
and S.L. Miller: J Mol Evol 1983 19:376.
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2. ON CONTINENTAL CRUST AND OCEANS ON ANCIENT EARTH.
S.A. Wilde et al (Curtin University of Technology, AU) discuss
ancient Earth, the authors making the following points:
1) No crustal rocks are known to have survived since the time of
the intense meteor bombardment that affected Earth(1) between its
formation approximately 4550 Myr ago and 4030 Myr, the age of the
oldest known components in the Acasta Gneiss of northwestern
Canada(2). But evidence of an even older crust is provided by
detrital zircons in metamorphosed sediments at Mt Narryer(3) and
Jack Hills(4,5) in the Narryer Gneiss Terrane, Yilgarn Craton,
Western Australia, where grains as old as 4,276 Myr have been
found.
2) The authors report, based on a detailed micro-analytical study
of Jack Hills zircons, the discovery of a detrital zircon with an
age as old as 4,404 +- 8 Myr -- about 130 million years older
than any previously identified on Earth. The authors found that
the zircon is zoned with respect to rare earth elements and
oxygen isotope ratios (18O values from 7.4 to 5.0), indicating
that it formed from an evolving magmatic source. The evolved
chemistry, high O-18 value, and micro-inclusions of SiO2 are
consistent with growth from a granitic melt(2), with an O-18
value from 8.5 to 9.5. Magmatic oxygen isotope ratios in this
range point toward the involvement of supracrustal material that
has undergone low-temperature interaction with a liquid
hydrosphere. The authors suggest this zircon thus represents the
earliest evidence for continental crust and oceans on the Earth.
3) Furthermore, the authors suggest the existence of liquid water
at 4.4 Gyr ago could have important implications for the
evolution of life. Microfossils as old as 3.5 Gyr are known.
Metasediments and carbonaceous materials with low biogenic carbon
isotope ratios are known at 3.8 Gyr ago. Zircon crystal W74/2-36
is over 500 Myr older than this organic matter, and if liquid
water was available to cause the evolved geochemistry measured by
the authors, then such water was also available for possible
biological processes. High-energy asteroid bombardment before 3.9
Gyr ago is consistent with periodic formation and destruction of
early oceans and the possibility that primitive life, if it
evolved in the oceans, was globally extinguished more than once.
References (abridged):
1. Ryder, G. Chronology of early bombardment in the inner solar
system. Geol. Soc. Am. Abstr. Progm 21, A299 (1992).
2. Bowring, S A. & Williams, I. S. Priscoan (4.00-4.03)
orthogneisses from northwestern Canada. Contrib. Mineral. Petrol.
134, 3-16 (1999).
3. Froude, D. O. et al. Ion microprobe identification of 4,100-
4,200 Myr-old terrestrial zircons. Nature 304, 616-618 (1983).
4. Compston, W. & Pidgeon, R. T. Jack Hills, evidence of more
very old detrital zircons in Western Australia. Nature 321, 766-
769 (1986).
5. Wilde, S. A. & Pidgeon, R. T. in 3rd International Archaean
Symposium (Perth), Excursion Guidebook (eds Ho, S. E., Glover, J.
E., Myers, J. S. & Muhling, J. R.) 82-95 (University of Western
Australia Extension Publication, Vol. 21, Perth, 1990).
Nature 2001 409:175
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3. IDENTIFICATION OF TWO SOURCES OF CARBON MONOXIDE IN COMET HALE
BOPP.
M.A. Disanti et al (Catholic University of America, US) discuss
the comet Hale-Bopp, the authors making the following points:
1) The composition of ices in comets may reflect that of the
molecular cloud in which the Sun formed, or it may show evidence
of chemical processing in the pre-planetary accretion disk around
the proto-Sun. As carbon monoxide (CO) is ubiquitous in molecular
clouds(1,2), its abundance with respect to water could help to
determine the degree to which pre-cometary material was
processed, although variations in CO abundance may also be
influenced by the distance from the Sun at which comets formed(3-
5). Observations have not hitherto provided an unambiguous
measure of CO in the cometary ice (native CO). Evidence for an
extended source of CO associated with comet Halley was provided
by the Giotto spacecraft, but alternative interpretations exist.
2) The appearance of comet Hale Bopp enabled studies of the
evolution of cometary activity in unprecedented detail. At large
heliocentric distances, volatile release was driven by CO
sublimation. However, gas production rates obtained at large
heliocentric distances do not provide information on the amount
of native CO relative to H2O, the dominant native ice, due to the
substantially different volatilities of these two compounds.
Closer to the Sun, water sublimation controls the release of all
species, and reliable mixing ratios for native ices can be
obtained.
3) The authors report observations of comet Hale Bopp which show
that about half of the CO in the comet comes directly from ice
stored in the nucleus. The abundance of this CO with respect to
water (12 per cent) is smaller than in quiescent regions of
molecular clouds, but is consistent with that measured in proto-
stellar envelopes, suggesting that the ices underwent some
processing before their inclusion into Hale Bopp. The remaining
CO arises in the coma, probably through thermal destruction of
more complex molecules.
References (abridged):
1. Rank, D. M., Townes, C. H. & Welch, W. J. Interstellar
molecules and dense clouds. Science 174, 1083-1101 (1971).
2. Turner, B. E. Recent progress in astrochemistry. Space Sci.
Rev. 51, 235-337 (1989).
3. Mumma, M. J. Organic volatiles in comets: Their relation to
interstellar ices and solar nebula material. Astron. Soc. Pacif.
Conf. Ser. 122, 369-396 (1997).
4. Mumma, M. J., Weissman, P. R. & Stern, S. A.in Protostars and
Planets, III(eds Levy, E. H. & Lunine, J.I.) 1177-1252 (Univ.
Arizona Press, Tucson, 1993).
5. Sandford, S. A. & Allamandola, L. J. The condensation and
vaporization behavior of H2O:CO ices and implications for
interstellar grains and cometary activity. Icarus 76, 201-224
(1988).
Nature 1999 399:662.
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4. REASSESSING THE EVIDENCE FOR THE EARLIEST TRACES OF LIFE.
M.A. van Zuilen et al (Scripps Institute of Oceanography, US)
discuss the earliest traces of life, the authors making the
following points:
1) The isotopic composition of graphite is commonly used as a
biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed
terrestrial rocks. Earlier studies on isotopic characteristics of
graphite occurring in rocks of the approximately 3.8-Gyr-old Isua
supracrustal belt (ISB) in southern West Greenland have suggested
the presence of a vast microbial ecosystem in the early
Archean(1-4). This interpretation, however, has to be approached
with extreme care(5).
2) Metasedimentary units, including detrital conglomerates,
turbidites, chemically precipitated banded iron formations (BIFs)
and metacherts, as well as pillow structures of mafic rocks,
indicate the accumulation of ISB rocks in an aquatic environment.
Carbonate-rich Isua rocks have also originally been interpreted
as being part of the sedimentary succession. However, recent
studies have disproved this interpretation. Metacarbonate
deposits and calc-silicate rocks bear signs of having formed by
metasomatic reactions during fluid infiltration across contacts
between igneous ultramafic rocks and their host rocks. It is now
widely accepted that most, if not all, carbonate in Isua is
metasomatic and not sedimentary in origin.
3) The authors demonstrate that graphite occurs abundantly in
secondary carbonate veins in the ISB that are formed at depth in
the crust by injection of hot fluids reacting with older crustal
rocks (metasomatism). During these reactions, graphite forms from
the disproportionation of Fe(II)-bearing carbonates at high
temperature. These metasomatic rocks, which clearly lack
biological relevance, were earlier thought to be of sedimentary
origin and their graphite association provided the basis for
inferences about early life(1-4). The authors suggest the new
observations thus call for a reassessment of previously presented
evidence for ancient traces of life in the highly metamorphosed
Early Archaean rock record.
References (abridged):
1. Mojzsis, S. J., Arrhenius, G. et al. Evidence for life on
Earth before 3,800 million years ago. Nature 384, 55-59 (1996).
2. Schidlowski, M., Appel, P. W. U., Eichmann, R. & Junge, C. E.
Carbon isotope geochemistry of the 3.7. 10-9 yr-old Isua
sediments, West Greenland: implications for the Archaean carbon
and oxygen cycles. Geochim. Cosmochim. Acta 43, 189-199 (1979).
3. Schidlowski, M. A 3,800-million-year isotopic record of life
from carbon in sedimentary rocks. Nature 333, 313-318 (1988).
4. Schidlowski, M. Carbon isotopes as biogeochemical recorders of
life over 3.8 Ga of Earth history: evolution of a concept.
Precambr. Res. 106, 117-134 (2001).
5. Hayes, J. M. The earliest memories of life on Earth. Nature
384, 21-22 (1996).
Nature 2002 418:627
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5. LASER RAMAN IMAGERY OF EARTH'S EARLIEST FOSSILS.
J.W. Schopf et al (University of California Los Angeles, US)
discuss Earth's earliest fossils, the authors making the
following points:
1) Unlike the familiar Phanerozoic history of life, evolution
during the earlier and much longer Precambrian segment of
geological time centered on prokaryotic microbes(1). Because such
microorganisms are minute, are preserved incompletely in
geological materials, and have simple morphologies that can be
mimicked by nonbiological mineral microstructures, discriminating
between true microbial fossils and microscopic pseudofossil
"lookalikes" can be difficult(2,3). Thus, valid identification of
fossil microbes, which is essential to understanding the
prokaryote-dominated, Precambrian 85% of life's history, can
require more than traditional palaeontology that is focused on
morphology.
2) Over the past few decades, the rules for accepting ancient
microfossil-like objects as bona fide Precambrian fossils have
become well established. Such objects should be demonstrably
biogenic, and indigenous to and syngenetic with the formation of
rocks of known provenance and well-defined Precambrian
age(2,4,5). Of these criteria, the most difficult to satisfy has
proved to be biogenicity, in particular for the notably few
fossil-like objects reported from Archaean (> 2,500 million years
(Myr) old) deposits(2,4) -- the putative biological origin of
which has been beset by controversy(2). This difficulty has been
obviated in part by analyses of the isotopic composition of
carbon in coexisting inorganic (carbonate) minerals and in whole-
rock acid-resistant carbonaceous residues (kerogens), which have
been used to trace the isotopic signature of biological
(photoautotrophic) carbon fixation to at least 3,500 Myr ago. But
because investigations of such bulk kerogen samples yield only an
average value of the materials analyzed, they cannot provide
information about the biogenicity of individual minute objects
that are claimed to be fossil.
3) By combining optically discernible morphology with analyses of
chemical composition, laser Raman spectroscopic imagery of
individual microscopic fossils provides a means by which to
address the problem of valid identification of fossil microbes.
The authors report an application of this technique to
exceptionally ancient fossil microbe-like objects, including the
oldest such specimens reported from the geological record. The
authors demonstrate that the results obtained substantiate the
biological origin of the earliest cellular fossils known.
References (abridged):
1. Schopf, J. W. in The Proterozoic Biosphere, A
Multidisciplinary Study (eds Schopf, J. W. & Klein, C.) 587-593
(Cambridge Univ. Press, New York, 1992).
2. Schopf, J. W. & Walter, M. R. in Earth's Earliest Biosphere,
Its Origin and Evolution (ed. Schopf, J. W.) 214-239 (Princeton
Univ. Press, Princeton, 1983).
3. Mendelson, C. V. & Schopf, J. W. in The Proterozoic Biosphere,
A Multidisciplinary Study (eds Schopf, J. W. & Klein, C.) 865-951
(Cambridge Univ. Press, New York, 1992).
4. Schopf, J. W. in The Proterozoic Biosphere, A
Multidisciplinary Study (eds Schopf, J. W. & Klein, C.) 25-39
(Cambridge Univ. Press, New York, 1992).
5. Schopf, J. W. Microfossils of the Early Archean Apex chert:
new evidence of the antiquity of life. Science 260, 640-646
(1993).
Nature 2002 416:73
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6. QUESTIONING THE EVIDENCE FOR EARTH'S OLDEST FOSSILS
M.D. Brasier et al (University of Oxford, UK) discuss the
evidence for Earth's oldest fossils, the authors making the
following points:
1) Structures resembling remarkably preserved bacterial and
cyanobacterial microfossils from 3,465-million-year-old Apex
cherts of the Warrawoona Group in Western Australia(1-4)
currently provide the oldest morphological evidence for life on
Earth and have been taken to support an early beginning for
oxygen-producing photosynthesis(5). Eleven species of filamentous
prokaryote, distinguished by shape and geometry, have been put
forward as meeting the criteria required of authentic Archaean
microfossils(1-5), and contrast with other microfossils dismissed
as either unreliable or unreproducible(1,3). These structures are
nearly a billion years older than putative cyanobacterial
biomarkers, genomic arguments for cyanobacteria, an oxygenic
atmosphere, and any comparably diverse suite of microfossils(5).
2) The Apex microfossils were reported to occur in rounded grains
of chert (microcrystalline silica) transported a great distance
before redeposition in a bedded grainstone conglomerate(2,3,5),
in a setting compared with a wave-washed beach or the mouth of a
stream or river(5). However, recent field mapping and sampling
reveals that the fossiliferous chert at Chinaman Creek is not
part of the bedded succession. It is a breccia that infills one
of a series of chert veins that cross-cuts pillow basalts and
feeds up into, and is continuous with, overlying stratiform
cherts. Textural and scanning electron microscope energy
dispersive X-ray (SEM EDX) studies indicate a suite of
hydrothermally associated minerals plus the hydrothermal
alteration of adjacent pillow basalts but not overlying
komatiitic basalts. A hydrothermal origin is inferred for this
vein and closely associated stratiform cherts, much like that
inferred for some barite-chert beds of the 3,490 million year
(Myr) Dresser Formation at nearby North Pole.
3) In summary: The authors report new research on the type and
re-collected material, involving mapping, optical and electron
microscopy, digital image analysis, micro-Raman spectroscopy and
other geochemical techniques. The authors reinterpret the
purported microfossil-like structure as secondary artefacts
formed from amorphous graphite within multiple generations of
metalliferous hydrothermal vein chert and volcanic glass.
Although there is no support for primary biological morphology, a
Fischer Tropsch-type synthesis of carbon compounds and carbon
isotopic fractionation is inferred for one of the oldest known
hydrothermal systems on Earth.
References (abridged):
1. Schopf, J. W. & Packer, B. M. Early Archean (3.3 billion to
3.5 billion-year-old) microfossils from Warrawoona Group,
Australia. Science 237, 70-73 (1987).
2. Schopf, J. W. in The Proterozoic Biosphere: a
Multidisciplinary Study (eds Schopf, J. W. & Klein, C.) 25-39
(Cambridge University Press, Cambridge, 1992).
3. Schopf, J. W. Microfossils of the Early Archean Apex Chert:
new evidence of the antiquity of life. Science 260, 640-646
(1993).
4. Schopf, J. W. in Early Life on Earth (ed. Bengtson, S.) 193-
206 (Columbia University Press, New York, 1994).
5. Schopf, J. W. The Cradle of Life (Princeton Univ. Press, New
York, 1999).
Nature 2002 416:76
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7. METASOMATIC ORIGIN OF QUARTZ-PYROXENE ROCK, AKILIA, GREENLAND,
AND IMPLICATIONS FOR EARTH'S EARLIEST LIFE.
C.M. Fedo and M.J. Whitehouse (George Washington University, US)
discuss Earth's earliest life, the authors making the following
points:
1) On the island of Akilia, outer Goth†bsfjord, southwest
Greenland, a sequence of lithologies that has been interpreted as
mafic volcanic rocks intercalated with silicious sedimentary
rocks chemically precipitated from seawater [banded iron
formation (BIF)] contains carbon isotopic signatures that have
been interpreted as perhaps the oldest known life on Earth [>3850
million years ago (Ma)] (1-3), overlapping in age with
potentially planet-sterilizing asteroid impacts (4,5).
2) There are two main lithologic groups on Akilia: (i) ~3650 to
(?) >3850 million-year-old composite, banded, tonalitic AmŒtsoq
gneisses, and (ii) coarse-grained mafic/ultramafic rocks, termed
the Akilia association, which are generally assumed to be older.
On Akilia, mafic and ultramafic rocks host a distinct, banded
quartz-pyroxene lithology that has been interpreted as a BIF (1-
3). All rocks have experienced polyphase regional metamorphism,
including a granulite facies event [temperature (T) > ~600øC,
pressure (P) > 8 kbar] at ~3600 Ma, and an upper amphibolite
facies event at ~2700 Ma, which have imposed a penetrative
tectonic fabric in such a way that all lithologies are strongly
schistose (or banded) and have been transposed into parallelism.
3) The authors present new geologic, petrologic, and geochemical
evidence that favors a metasomatized ultramafic igneous origin
for rocks previously considered to be BIFs, and the authors
suggest that it is highly improbable that the rocks hosted life
at the time of their formation.
References (abridged):
1. S. J. Mojzsis, et al., Nature 384, 55 (1996).
2. A. P. Nutman, S. J. Mojzsis, C. R. L. Friend, Geochim.
Cosmochim. Acta. 61, 2475 (1997).
3. S. J. Mojzsis and T. M. Harrison, Geol. Soc. Am. Today 10, 1
(2000).
4. C. F. Chyba, Geochim. Cosmochim. Acta. 57, 3351 (1993).
5. K. A. Maher and D. J. Stevenson, Nature 331, 612 (1988).
Science 2002 296:1448
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8. EARLY EARTH: CARBONACEOUS METEORITES AS A SOURCE OF SUGARS
"Stony" meteorites (aerolites) are meteorites formed solely of
rock-forming silicates, and chondrites are a type of stony
meteorite consisting of an agglomeration of millimeter-sized
globules (chondrules) that are thought to be unchanged since the
original condensation out of the nebula from which the Sun and
Solar System formed. A "carbonaceous chondrite" is a chondritic
meteorite that contains a relatively large amount of carbon, with
a resultant dark appearance. The "Murchison meteorite" is a
carbonaceous chondrite that fell in 1969 near Murchison,
Australia. The Murray meteorite, which fell to Earth in Kentucky
1950, is similar to the Murchison meteorite in its organic
content
G. Cooper et al (NASA Ames Research Center, US) discuss
meteorites and early Earth, the authors making the following
points:
1) The much studied Murchison meteorite is generally used as the
standard reference for organic compounds in extraterrestrial
material. Amino acids and other organic compounds important in
contemporary biochemistry are thought to have been delivered to
the early Earth by asteroids and comets, and such compounds may
have played a role in the origin of life on Earth.
2) Polyhydroxylated compounds (polyols) such as sugars, sugar
alcohols, and sugar acids are vital to all known lifeforms ---
they are components of nucleic acids, cell membranes, and also
act as energy sources. But there has hitherto been no conclusive
evidence for the existence of polyols in meteorites, leaving a
gap in our understanding of the origins of biologically important
organic compounds on Earth.
3) The authors report that a variety of polyols are present in,
and indigenous to, the Murchison and Murray meteorites in amounts
comparable to amino acids. Analysis of water extracts indicate
that extraterrestrial processes, including photolysis and
formaldehyde chemistry, could account for the observed compounds.
The authors conclude that polyols were present on the early Earth
and therefore at least available for incorporation into the first
forms of life.
Nature 2001 414:879
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9. ON THE ORIGIN OF THE EARTH
"The Earth was not constructed with a delicate hand. It was
hammered into shape slowly, by the brute force of a meteor
bombardment that lasted hundreds of millions of years. The soils,
the seas, and our primitive microbial ancestors emerged in the
midst of apparent chaos and catastrophe. The process began
billions of years ago as our entire solar system was congealing
from a swirling cloud of hot gases and nuclear ashes left behind
by exploded stars. Some of the objects colliding with the Earth
at this time were planetesimals -- objects as big as small
planets. The kinetic energy released by these impacts literally
shook the Earth to its core and melted much of the rocky crust
and interior. Some chunks of the planetesimals and meteors became
permanently embedded in the Earth, while other pieces were sent
hurtling off into space like giant shrapnel. The mass of the
primordial Earth accumulated slowly, like a globe that grows as a
sculptor slaps on clay, one handful at a time. With greater size,
Earth increased in its gravitational force, attracting even more
of the wandering debris of space.
"It is hard to come up with a specific date of birth for our
planet, given its gradual development. Basing their calculations
on the "radioactive clock" -- measurements of the level of
radioactive decay of certain elements found within the Earth's
crust, such as uranium and lead -- most geologists place the
Earth's age at about four and a half billion years. The Earth
went through horrendous growing pains during its first billion
years. Just as the frequency of meteor impacts began to decline,
violent volcanic eruptions began to spring up around the globe as
the planet's hot interior "degassed." When the Earth's surface
temperature finally began to cool, the massive volume of water
vapor in the atmosphere condensed and poured down from the
heavens in fierce rainstorms of truly biblical proportions. The
torrential rains lasted millions of years, creating our oceans --
the hydrosphere as we know it -- in the process.
"The original igneous and metamorphic rocks on the Earth's
surface, left behind by volcanic eruptions and upliftings from
the mantle layer below, were washed by the relentless rains, and
their minerals flowed into the oceans. This was an essential
first step in the formation of primitive soils that would
eventually support a vibrant plant and animal life. These
primitive soils lacked organic matter but contained sand, silt,
and clay minerals in various proportions.
"Clays are unique among the mineral components of soil. They are
chemically reactive, microscopic, crystal-like structures that
form out of saturated solutions of silicate and metal oxides.
Sand and silt, in contrast, are large, chemically inert particles
formed by the simple weathering and pulverization of rock. Some
clays are crystallized deep within the Earth's mantle layer, at
high temperature and pressure, and then brought to the surface by
the churning motions of the Earth. This process is driven by
radioactive heating deep within Earth's mantle and is part of the
same plate tectonic geological cycle that gradually moves the
continental crusts."
David W. Wolfe: Tales from the Underground: A Natural History of
Subterranean Life. Perseus Publishing, Cambridge (MA) 2001, p.17.
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10. LIFE AND THE EVOLUTION OF EARTH'S ATMOSPHERE
J.F. Kasting and J.L. Siefert (Pennsylvania State University, US)
discuss life on Earth, the authors making the following points:
1) Microorganisms are important for many reasons, not the least
of which is their responsibility, direct or indirect, for the
production of nearly all of the oxygen we breathe. Oxygen is
produced during photosynthesis by a reaction that can be written
as CO(sub2) + H(sub2)O --> CH(sub2)O + O(sub2). Here, "CH(sub2)O"
is a geochemist's shorthand for more complex forms of organic
matter. Most photosynthesis on land is carried out by higher
plants, not microorganisms; but terrestrial photosynthesis has
little effect on atmospheric oxygen because it is nearly balanced
by the reverse processes of respiration and decay. By contrast,
marine photosynthesis is a net source of oxygen because a small
fraction (approximately 0.1%) of the organic matter synthesized
in the oceans is buried in sediments. This small leak in the
marine organic carbon cycle is responsible for most of our
atmospheric oxygen.
2) Although higher plants (e.g., kelp) are found in the oceans,
most marine photosynthesis is performed by single-celled
organisms. The most abundant of these are eukaryotic algae, such
as diatoms and coccolithophorids. Roughly 99% of primary
production can be attributed to such organisms(1). Prokaryotic
bacteria are also important for another reason. Though they make
up only approximately 1% of marine biomass, cyanobacteria (or
blue-green algae) are the main organisms responsible for fixing
nitrogen(1). This capability is quite remarkable because the
enzyme responsible for reducing N(sub2), nitrogenase, is poisoned
by oxygen. Thus, cyanobacteria have had to evolve complex
mechanisms for protecting their nitrogenase. Some, such as the
filamentous Anabaena spp., do so by fixing nitrogen only in
specialized cells called heterocysts. Other cyanobacteria fix
nitrogen at night and photosynthesize by day. Still others, such
as Trichodesmium spp. (very abundant in tropical waters), fix
nitrogen in the morning and photosynthesize in the afternoon(2).
Such specificity shows that these are highly evolved pieces of
biological machinery.
3) In summary: Harvesting light to produce energy and oxygen
(photosynthesis) is the signature of all land plants. This
ability was co-opted from a precocious and ancient form of life
known as cyanobacteria. Today these bacteria, as well as
microscopic algae, supply oxygen to the atmosphere and churn out
fixed nitrogen in Earth's vast oceans. Microorganisms may also
have played a major role in atmosphere evolution before the rise
of oxygen. Under the more dim light of a young sun cooler than
today's, certain groups of anaerobic bacteria may have been
pumping out large amounts of methane, thereby keeping the early
climate warm and inviting. The evolution of Earth's atmosphere is
linked tightly to the evolution of its biota.(3-5)
References (abridged):
1. T. Tyrell, Nature 400, 525 (1999)
2. I. Berman-Frank et al., Science 294, 1534 (2001)
3. H. D. Holland, in Early Life on Earth, S. Bengtson, Ed.
(Columbia Univ. Press, New York, 1994), pp. 237-244
4. J. Farquhar, H. Bao, M. Thiemans, Science 289, 756 (2000)
5. L. Margulis, Symbiosis in Cell Evolution: Microbial
Communities in the Archean and Proterozoic Eons (Freeman, San
Francisco, ed. 2, 1993), chap. 7, pp. 327-343
Science 2002 296:1066
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11. RNA WORLD: RNA-CATALYZED RNA POLYMERIZATION
W.K. Johnston et al (Massachusetts Institute of Technology, US)
discuss RNA-catalyzed RNA polymerization:
The "RNA world hypothesis" states that early life forms lacked
protein enzymes and depended instead on enzymes composed of RNA.
Much of the appeal of this hypothesis arises from the realization
that RNA-enzymes (ribozymes) would have been far easier to
duplicate than proteinaceous enzymes. Whereas coded protein
replication requires numerous macromolecular components
(including messenger RNAs, transfer RNAs, the ribosome, etc.),
replication of a ribozyme requires only a single macromolecular
activity: an RNA-dependent RNA polymerase that synthesizes first
a complement, and then a copy of the ribozyme. If this RNA
polymerase were itself a ribozyme, then a simple ensemble of
molecules might be capable of self-replication and eventually, in
the course of evolution, give rise to the protein-nucleic acid
world of contemporary biology. Finding a ribozyme that can
efficiently catalyze general RNA polymerization would support the
idea of the RNA world and would provide a key component for the
laboratory synthesis of minimal life forms based on RNA. The
authors report the generation of an RNA molecule that catalyzes
the type of polymerization needed for RNA replication. The
ribozyme uses nucleotide triphosphates and the coding information
of an RNA template to extend RNA primer by the successive
addition of up to 14 nucleotides -- more than a complete turn of
an RNA helix.
Science 2001 292:1319
Related Background:
ORIGIN OF LIFE: DIFFERENTIAL ADSORPTION OF NUCLEIC ACID BASES
The idea that the adsorption of organic substances onto inorganic
surfaces might have been involved in the origin of life on Earth
was discussed 50 years ago by the crystallographer J.D. Bernal
(1901-1971). More recently, in 1982, the chemist A.G. Cairns-
Smith suggested that life developed from crystals, with life
originating from replication of clay crystals. In the Cairns-
Smith model, life began through the influence of natural
selection on the growth of inorganic crystals, with clay
structures the first carriers of genetic information. Replication
occurred by the accidental detachment of layers in the clay
lattice, the layers serving as nuclei for the growth of new
daughter molecules. Later, organic chemicals were incorporated
into the structure of the replicating crystallites, and
competition favored those systems that were more adaptable
because they used organic molecules to carry out their functions.
Nucleic acids (RNA and DNA) then evolved, taking over as the
basic information repository making up the genes of the organism,
and under the pressure of natural selection, the original clay-
mineral component was dispensed with entirely. In general, the
term "RNA world hypothesis" refers to the concept that RNA
nucleotide sequences with catalytic and self-replicating
capabilities predated catalytic protein systems in prebiological
epochs. It is believed, however, that the nucleotides
constituting RNA were scarce on early Earth, so that RNA-based
life must have acquired the ability to synthesize RNA nucleotides
from simpler and more readily available precursors such as sugars
and nucleic acid bases. Apparently plausible prebiotic synthesis
routes have been proposed for sugars, sugar phosphates, and the
four RNA bases, but the coupling of these molecules into
nucleotides, specifically pyrimidine nucleotides, poses a
challenge to the RNA world hypothesis.
S.J. Sowerby et al (4 authors at 2 installations, SE DE) present
a study of the adsorption of nucleic acid bases on crystalline
graphite, the authors making the following points:
1) The authors point out that the purine and pyrimidine bases,
the coding elements of nucleic acids, are products of supposed
prebiotic chemistries that invoked cyanide and have been
synthesized in reactions that also yield amino acids. The
apparent prebiotic availability of these compounds supports the
RNA World hypothesis for the origin of life on Earth, which
proposes that the first living system consisted of one or more
polymers of catalytic RNA capable of self-replication that
subsequently evolved the ability to encode more versatile peptide
catalysts. RNA can act as both information carrier and catalyst,
and in the laboratory, RNA can be coerced into various catalytic
functions through directed Darwinian evolution (directed natural
selection).
2) The authors point out that the adsorption of organic molecules
has long been considered a relevant prebiotic process. The purine
and pyrimidine bases adsorb spontaneously from aqueous media onto
inorganic solids and have been observed on the surfaces of
graphite, MoS(sub2), crystalline gold, and clays. Scanning probe
microscopy studies have shown that the nucleotide bases are
planar-arranged on these surfaces like jigsaw puzzle pieces, and
are stabilized by van der Waals interactions with the underlying
surface and by hydrogen bonds between adjacent molecules, a
configuration originally postulated in 1987 on the basis of
thermodynamic measurements made at the mercury-water interface.
The hydrogen bonds between the bases are drawn from a discrete
set of possible interactions, including those of Watson-Crick
pairing found in nucleic acids. The monolayers can be likened to
nucleic acid molecules because the sugar-phosphate scaffold also
supports the arrangement of bases. Each position in the scaffold
can be mapped to the next by a simple translation operation,
which resembles a major property of the underlying
crystal.
3) The authors report they have determined the equilibrium
adsorption isotherms for the nucleic acid purine and pyrimidine
bases dissolved in water on the surface of crystalline graphite.
The markedly different adsorption behavior of the bases comprises
an elution series: guanine > adenine > hypoxanthine > thymine >
cytosine > uracil. The authors propose that such differential
properties were relevant to the prebiotic chemistry of the bases,
and may have influenced the composition of the primordial genetic
architecture.
Proc. Nat. Acad. Sci. 2001 98:820
Related Background:
ORIGIN OF LIFE: PRODUCTION OF PEPTIDES ON INORGANIC SURFACES
Huber and Wachterschauser (Technische Universitat Munchen, DE)
report that in experiments modeling volcanic or hydrothermal
settings, amino acids were converted into their peptides by use
of coprecipitated (Ni,Fe)S and CO in conjunction with H(sub2)S
(or CH(sub3)SH) as a catalyst and condensation agent at 100
degrees centigrade and pH 7 to 10 under anaerobic aqueous
conditions. The amino acids involved in the experiments were
phenylalanine, tyrosine, and glycine. The authors suggest their
results demonstrate that amino acids can be activated under
geochemically relevant conditions, and that the results support a
thermophilic origin of life with a primordial surface metabolism
on transition metal sulfide minerals. They further suggest that a
continuously recycling library of peptides was generated on the
surfaces of a library of (Fe,Ni)S structures, and that the
results raise the possibility that CO and Ni had a much greater
role in the primordial metabolism than in any of the known extant
metabolisms. They point out that all known extant organisms are
found in habitats with low activities of CO and Ni, and they
suggest this could explain why organisms resorted to the
formation of CO from CO(sub2) and to the elimination of nickel
from many enzymes.
Science 1998 281:670
Related Background:
BIOCHEMICAL EVOLUTION: POLYMERIZATION ON MINERAL SURFACES
J. Smith (University of Chicago, US) proposes a conceptual
framework for consideration of the origins of replicating
biopolymers. Although extended Darwinian natural selection
coupled with Mendel-Watson-Crick genetic inheritance/mutation
provides a plausible framework for integrating the patchy
paleontological record with the increasingly complex biochemical
zoo of the present Earth, the actual chemical beginning of "life"
still poses major challenges. How could the first replicating and
energy-supplying molecules have been assembled from simpler
materials that were undoubtedly available on the early proto-
continents? Catalysis at mineral surfaces might generate
replicating biopolymers from simple chemicals supplied by
meteorites, volcanic gases, and photochemical gas reactions. But
many ideas are implausible in detail because the proposed mineral
surfaces strongly prefer water and other ionic species to organic
ones. The molecular sieve silicalite (Union Carbide; = Al-free
Mobil ZSM-5 zeolite) has a 3-dimensional 10-ring channel system
whose electrically neutral silicon-oxide surface strongly adsorbs
organic species over water, and the ZSM-5 type zeolite mutinaite
has recently been found in Antarctica. The author proposes that
zeolites with similar structures may have existed on the surface
of Earth during its life-origin phase, and that polymer migration
along weathered silicic surfaces of micrometer-wide channels of
feldspars might have led to assembly of replicating catalytic
biomolecules and perhaps primitive cellular organisms. The author
suggests that weakly metamorphosed Archaean rocks might retain
microscopic clues to the proposed mineral adsorbent/catalysts,
and that other frameworks are also possible, including ones with
levo/dextro one-dimensional channels.
Proc. Nat. Acad. Sci. 1998 95:3370
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12. ORIGIN OF LIFE: THE PRESENT STATUS OF CHEMICAL THEORY
The essential question involved in the origin of what we call
life is how can order arise from disorder? At the present time,
this question is approached on two fronts: 1) study of the
principal features of self-organizing systems, systems in which
order does arise from disorder, systems in which order is indeed
demanded from disorder on thermodynamic grounds; and 2) study of
the detailed chemistry of such systems, the chemistry of
organization and the chemistry of components. In the case of
components, it is essential that appropriate self-organizing
components exist in the first place if they are to become self-
organized, and such candidate components are thus the focus of
much chemical research in this area. In 1953, the chemist Stanley
Miller reported what soon became a famous experiment. To water
under a gas mixture of methane, ammonia, and hydrogen, he added
an electrical discharge. After one week of continuous electrical
discharge, he found that a number of important biological
molecules, including amino acids, had been formed. Miller
proposed his experiment as a model for the conditions under which
the essential compounds necessary for life originated . The
Miller experiment was a watershed, and it began a new era of
experimentation and analysis of possible primordial components.
Coupled with this, were the new important discoveries by
astrophysicists of the presence of organic molecules in the
interstellar medium and in meteorites. In a review of origin of
life theories, P. Radetsky (Univ. of California Santa Cruz, US)
points out that the Miller theory is no longer the consensus
theory, that contemporary geologists believe the primordial
atmosphere consisted primarily of carbon dioxide and nitrogen,
which are less reactive than the gases in the Miller experiment,
and that the field is currently embroiled in controversy fueled
for the most part by an absence of hard fact.
Earth February 1998
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