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OCR for page 3479
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3479-3485, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
~ ;,
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Biochemical evolution III: Polymerization on organophilic
silica-rich surfaces, crystal chemical modeling, formation
of first cells, and geological clues
(biological evolution/silica/feldspar/zeolite/first cell walls)
JOSEPH V. SMITH*T, FREDERICK P. ARNOLD, JR.l, IAN PARSONS§, AND MARTIN R. LEE§
*Department of Geophysical Sciences and Center for Advanced Radiation Sources, 5734 South Ellis Avenue, The University of Chicago, Chicago, IL 60637;
"Advanced Research Systems, 5640 South Ellis Avenue, The University of Chicago, Chicago, IL 60637; and Department of Geology and Geophysics, University
of Edinburgh, Edinburgh EH9 3JW, United Kingdom
ABSTRACT Catalysis at organophilic silica-rich surfaces
of zeolites and feldspars might generate replicating biopoly-
mers from simple chemicals supplied by meteorites, volcanic
gases, and other geological sources. Crystal-chemical mod-
eling yielded packings for amino acids neatly encapsulated in
10-ring channels of the molecular sieve silicalite-ZSM-5-
(mutirzaiteJ. Calculation of binding and activation energies for
catalytic assembly into polymers is progressing for a chemical
composition with one catalytic Al-OH site per 25 neutral Si
tetrahedral sites. Internal channel intersections and external
terminations provide special stereochemical features suitable
for complex organic species. Polymer migration along nano/
micrometer channels of ancient weathered feldspars, plus
exploitation of phosphorus and various transition metals in
entrapped apatite and other microminerals, might have gen-
erated complexes of replicating catalytic biomolecules, leading
to primitive cellular organisms. The first cell wall might have
been an internal mineral surface, from which the cell devel-
oped a protective biological cap emerging into a nutrient-rich
"soup." Ultimately, the biological cap might have expanded
into a complete cell wall, allowing mobility and colonization of
energy-rich challenging environments. Electron microscopy
of honeycomb channels inside weathered feldspars of the Shap
granite (northwest England) has revealed modern bacteria,
perhaps indicative of Archean ones. All known early rocks
were metamorphosed too highly during geologic time to
permit simple survival of large-pore zeolites, honeycombed
feldspar, and encapsulated species. Possible microscopic clues
to the proposed mineral adsorbents/catalysts are discussed
for planning of systematic study of black cherts from weakly
metamorphosed Archaean sediments.
Introduction and Summary of Biochemical Evolution: Part
I. Darwin/Oparin/Haldane/Watson/Crick biological evolu-
tion provides a plausible framework for integrating the patchy
paleontological record with the complex biochemical zoo of
the present Earth (literature review: ref. 1~. But how could the
first replicating and energy-supplying molecules have been
assembled from simpler materials that were undoubtedly
available on the early protocontinents? Bernal preferred "life"
to begin by catalytic assembly on the surface of a mineral, but
all pre-1998 attempts using clays and other minerals to assem-
ble an integrated scheme of physicochemical processes had
significant weaknesses. Catalysis of organic compounds dis-
persed in aqueous "soup" requires a mechanism for concen-
trating the organic species next to each other on a catalytic
substrate. Biochemically significant polymers, such as polypep
PNAS is available online at www.pnas.org.
tides and RNAs, must be protected from photochemical
destruction by solar radiation and must not be overly heated.
A stable cell wall is needed to protect the first primitive
organ~sm.
Part I (1) pointed out that certain inorganic materials have
internal surfaces that are both organophilic and catalytic,
allowing efficient capture of organic species for catalytic
assembly into polymers in a protective environment. These
physicochemical features are related to the state of the art for
zeolite catalysts in the chemical industry, the observed prop-
erties of zeolite, feldspar (2), and silica minerals, and a
plausible framework for the accretion and early history of the
Earth's crust and atmosphere (1~. Various materials from the
zeolite, feldspar, and silica mineral groups were listed as
having surfaces with the capacity to adsorb organic species
preferentially over water molecules and catalyze them into
polymers. We focus here on mutinaite, a zeolite mineral
recently discovered in Antarctica, which is the natural analog
of the ZSM-5/silicalite series of synthetic microporous mate-
rials (Note: Microporous does not imply that the pores are of
micrometer size; indeed the pores in zeolites are generally less
than a nanometer across). This type of molecular sieve is based
on a tetrahedral framework containing a three-dimensional
channel system spanned by rings of 10 oxygen atoms (Fig. 1
Upper Left and Upper Right`). The silica-rich end-member of the
ZSM-5 series, silicalite, is very organophilic, and Al-substituted
synthetic relatives catalyze organic reactions at Al-OH re-
gions. Silicalite provides a useful basis for modeling adsorp-
tion/catalytic processes that would apply in principle, but not
in detail, to other materials in paper I (1~.
Nonexperts in computer modeling of crystal structures
might note the conventions in Fig. 1 Upper Left and Upper
Right. Three-dimensional imaging must be idealized and trun-
cated. Fig. 1 Upper Left displays 10 oxygen atoms as spheres
half the conventional atomic radius of 1.4 A. All other atoms
are shown merely by the intersection of spokes. Each tetrahe-
drally coordinated (T) atom lies at the intersection of four
yellow spokes and each O atom at the intersection of two
maroon spokes. Fig. 1 Upper Right shows all the O atoms as
half-size spheres, and the Si and Al types, respectively, of T
atoms as yellow and pink spheres joined by thin grey spokes.
Only four 10-rings are shown lying in the wall of the channel,
and you, the reader, must imagine the channel extending up
and down to the surface of the crystal where some adjustment
of chemical bonding is needed. Fig. 1 Upper Right is deliber-
ately tilted slightly with respect to Fig. 1 Upper Left. The
lTo whom reprint requests should be addressed. e-mail: smith@geol.
uchicago.edu.
3479
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3480 Colloquium Paper: Smith et al.
Proc. Natl. Acad. Sci. USA 96 (1999J
FIG. 1. Computer graphics of part of the atomic framework of silicalite/ZSM-5 with amino acids encapsulated in energetically favored positions.
(Upper Left) Tetrahedral framework of silicalite/ZSM-5 showing a 10-ring channel down the y-axis. Most of the figure consists of spokes linking
atom positions. One 10-ring of O atoms is shown by spheres displayed at half the formal atomic radii. See text for explanation: oxygen atoms, maroon
spheres and intersection of maroon spokes; tetrahedral (T) atoms, intersection of yellow spokes. Only four 10-rings are shown, whereas a perfect
crystal would have an infinite number defining the channel. Also shown are the tilted five-rings. Ten-ring channels present in the plane of the paper
are difficult to see without rotation on the video display. (Upper Right) Four glycine molecules in the zwitterion configuration encapsulated in
silicalite/ZSM-5. Glycine consists of a central C atom bonded to two H. one carboxyl COO-, and one amine NH3+. The cluster of three molecules
near the middle of the near-vertical channel has been optimized to interact mutually by way of hydrogen bonding and to be suspended by van der
Waals bonding from the O atoms of the 10-ring channel. The fourth molecule is oriented along a horizontal 10-ring channel. All the framework
O atoms are represented by half-size maroon spheres. The tetrahedrally coordinated atoms are represented by small spheres differentiated by color:
So, yellow, Al, pink. The glycine molecule is represented by a stick model with conventional color code: O. red; C, grey; N. blue; H. white. The
orientation of the channel system is rotated slightly from that in UpperLeft. (LowerLeft) Three glycine molecules within a 10-ring channel of silicalite,
viewed down the y- axis. Coloring as in Upper Left; silicalite/ZSM-5 framework shown as tetrahedra (Si, Al) and balls (O); glycine shown as tubes.
Note the alignment of the amino acids parallel to the channel and restricted lateral positions within the channel. (Lower Right) Two of the glycine
molecules from Lower Left, viewed along the z-axis. Note the 'head-to-tail' alignment of the carboxylate group of an amino acid with the amino
group of the next amino acid. Once again, the positional constraints on the amino acids in the channel, as well as their parallel alignment with the
channel, are emphasized.
conventions for amino acids are given in the Fig. 1 Upper Right
legend.
Introduction to New Unpublished Studies. This third part
integrates the current state of research on biochemical evolu
OCR for page 3481
Colloquium Paper: Smith et al.
tion that will be presented at the Colloquium on Geology,
Mineralogy, and Human Welfare. tThe second part on weath-
ered honeycombed feldspars and associated bacteria in a
modern granite was under preparation as this third part was
being completed and will be published (57) in the regular part
of the Proceedings before this paper. Its key contents are given
briefly in this paper and are illustrated in Fig. 3.] We begin with
crystal-chemical modeling of amino acids inside 10-ring chan-
nels of the chosen zeolite, silicalite (Figs. 1 Upper Right, Lower
Left, and Lower Right and 2 Upper and Lower). The channel
walls are electrically neutral except for arbitrary replacement
of 4% of the silicon-oxygen tetrahedra by aluminum-oxygen-
hydroxyl catalytic centers. This 1 in 25 replacement yields nice
graphics and has no particular scientific significance. More-
over, this ratio can be varied to increase or decrease the spacing
of catalytic centers along the 10-ring channels and to vary the
electrical forces on adsorbed molecules and the repeat dis-
tances of the polymers generated by catalytic condensation.
We end with new ideas for generating primitive protocells
inside the honeycombed weathered surfaces of feldspars tPart
II (57~. Fig. 3 shows scanning-electron micrographs of the
crystallographically controlled channels in feldspars from the
Shap granite, northwest England, together with associated
modern bacteria, as models for speculation on the develop-
ment of the first primitive cells. Now to details.
Preliminary Simulations of Encapsulation of Amino Acids
in Silicalite/ZSM-5 and Catalytic Generation of Biopolymers.
The crystal-chemical reviews in refs. 1 and 2 are updated by
papers in the following areas:
· crystal chemistry of high-silica materials: Fourier Transform-
Raman studies of single-component and binary adsorption
in silicalite-1 (3~; vapor adsorption in thin silicalite-1 films
studied by spectroscopic ellipsometry (4~; adsorption/de-
sorption of n-alkanes on silicalite crystals (5~; adsorption
equilibria of C1 to C4 alkanes, CO2, and SF6 on silicalite (6~;
adsorption of linear and branched alkanes in zeolite sili-
calite-1 (7~; combined quantum mechanical/molecular me-
chanics ab Ratio modeling, demonstrating that the most
stable Br0nsted sites occur in high-silica zeolites (8~; simu-
lation of adsorption and diffusion of hydrocarbons in sili-
calite, demonstrating that a linear hydrocarbon moves more
freely than a branched one, whose CH group becomes
locked at a channel intersection (9~; adsorption isotherms of
linear alkanes in ferrierite-smaller ones, C1-C5, fill the
entire pore system, whereas C6 and C7 fit only in a 10-ring
channel unless forced into an eight-ring channel by pressure
(10, 11~; nuclear magnetic resonance of ~H in water ad-
sorbed on silicalite (12~; heterogeneity of Br0nsted acid sites
in Al-substituted faujasites (13~; nuclear magnetic resonance
of 170 in silica, albite glasses, and stilbite (14, 15~; simulation
of alkane adsorption in aluminophosphate-5, and calorim-
etry of alkane absorption in high-silica zeolites (16, 17~;
hydrophobic properties of all-silica beta zeolite (18~; struc-
tural location of sorbed p-nitroaniline in silicalite/MFI
molecular sieves from x-ray powder diffraction and 29Si
Magic Angle Spinning-NMR (19~; nature, structure, and
composition of hydrocarbon species obtained by oligomer-
ization of ethylene on acidic H-ZSM-5 molecular sieve (20~:
su~face chemistry of various minerals: coordination models
for simple surfaces of oxide and silicate minerals (214; the
role of intragranular microtextures and microstructures in
chemical and mechanical weathering, direct comparisons of
experimentally and naturally weathered alkali feldspars
(22~;
synthesis: RNA-catalyzed nucleotide synthesis of a pyrimi-
dine (23~; conversion of amino acids into peptides at 373 K
and pH 7-10 on (Ni, Fe)S surfaces (24), synthesis of
glycylglycine dipeptide in the presence of kaolin clay and
zeolites of Linde Type A, faujasite, and beta types (25~;
Proc. Natl. A cad. Sc~. USA 96 (1999) 3481
thermodynamic calculation of amino acid synthesis in hot
water and application to hydrothermal vents (black smok
ers) on ocean floor (26~; polymerization of various amino
acids on hydroxylamine and illite mica, with increasing
adsorption affinity of oligomers longer than 7-mer (27-29~.]
Figs. 1 Upper Right, Lower Left, and Lower Right and 2 Upper
and Lower illustrate the current state of chemical modeling of
amino acids encapsulated in a silicalite containing one Al
substitution for 25 tetrahedral Si. Simulations were carried out
using the Sorption module within the MSI/Cerius 2 program
system (issued by Pharmacopeia, Princeton, NJ). The Consis-
tent Valence Force Field was used for all atoms.
Figs. 1 Lower Left and Lower Right and 2 Upper illustrate the
results of packing simulations based on Monte Carlo tech-
niques for glycine and histidine molecules encapsulated within
the 10-ring channels of silicalite. It is possible at low pressure
to pack 28 glycine molecules per unit cell or eight histidine. Fig.
1 Lower Left, and its rotated version in Fig. 1 Lower Right,
demonstrate how the restriction of lateral motion by the
channel system, coupled with charge effects at the amino and
carboxyl ends of the amino acids, assists in orienting them
correctly for the production of polypeptides. It can also be seen
from these illustrations that one of the chief difficulties of the
standard model for the formation of life, that of achieving
sufficient concentration of reactants while excluding or min-
imizing environmental degradation, is overcome. Not only are
the growing biopolymers protected from outside interference
and concentrated in the channels, but the limited degrees of
freedom in molecular movements assist in orienting them
optimally for polymerization.
Simple molecular mechanics simulations within the SPAR-
TAN computer package (Wavefunction, Irvine, CA) of various
model complexes are shown in Fig. 2 Upper and Lower. Fig. 2
Upper (stereoview) illustrates an adenine hydrogen bound to
the hydroxyl site of the 10-ring channel, with the carboxyl end
of a glycine residue bound to the amino group of the adenine.
This complex is correctly oriented for protonation of the
hydroxyl group of the amino acid, followed by the elimination
of water and the formation of an amide bond between the base
and the amino acid. Such a reaction would be facilitated by a
second metal site in the region, and provides one possibility for
a precursor to the autocatalytic biopolymers of the 'pre-RNA'
world. Electrostatic potential calculations on the system, using
the PM3 semiempirical Hamiltonian within the MOPAC mo-
lecular orbital package, indicate that this orientation is favor-
able for the proposed reaction.
Fig. 2 Lower illustrates one possibility of the bonding of an
amino acid to the hydroxyl site of a zeolite-type material. It is
obvious that the nitrogen functionalities of the histidine ring
could form hydrogen bonds. Furthermore, it is also possible to
bind the hydroxyl group of the carboxyl functionality to the
acid site within the zeolite framework, then dehydrate and
form a covalent bond between the surface and the amino acid.
This process is analogous to the functionalized glass or plastic
beads used in commercial DNA or protein synthesis, where the
polymer chain grows away from the supported terminus.
Reaction with hydronium ion, presuming mildly acidic media,
would enable cleavage of the chain and would release the
peptide into solution. In passing, it should also be noted that
the amino terminus of the amino acid, which is facing away
from the viewer, is oriented along the axis of the channel and
hence, in analogy to Fig. 1 Lower Right, is oriented optimally
for further reaction.
Speculations on Biochemical Evolution Currently Under
Evaluation. These illustrative results give confidence for spec-
ulations that a microporous aluminum-substituted silica ma-
terial with mainly hydrophobic channels and widely spaced
Al-OH catalytic centers might act as a sausage machine for
production of biopolymers that became assembled into pro
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3482 Colloquium Paper: Smith et al.
Proc. Natl. Acad. Sci. USA 96 (1999J
FIG. 2. Computer graphics of part of the atomic framework of silicalite/ZSM-5 with amino acids encapsulated in energetically favored positions.
(suppers) Stereopair of 10-ring channel of silicalite, showing a hydrogen-bound adenine-glycine complex itself hydrogen-bound to an acid site of the
framework. Some framework atoms removed for clarity. This stereopair illustrates the ability of the zeolitic material both to accommodate large
Copolymer precursors and to provide sites at which reactions may occur. (Some students initially have difficulty viewing stereopairs. If you have
this problem, try putting a bright spot at the red atoms at the extreme top right and bottom left. Your brain should then be able to make your eyes
swivel to achieve stereo with the left eye seeing the left dot. This contrasts with the cross-eye technique used in some biological modeling.) iLower)
Histidine and water molecules in silicalite. Histidine was chosen for modeling because its imidazole ring can switch electronic states readily to catalyze
OCR for page 3483
Colloquium Paper: Smith et al.
tocells in protected honeycombs in weathered feldspars. Var-
ious new matters are being evaluated currently from the
viewpoint of physical and chemical processes and are modeled
in detail for later publication:
- First, channel intersections may prove important for stere-
ochemical control of larger functional groups, especially at
the end of a biopolymer. The intersection of an internal
channel with the outer surface should be even more impor-
tant and indeed might be considered as an anchor for a
polymer that projects outwards into a 'soup.'
· Second, after outward migration from an internal channel,
the first biopolymers would begin to coil up like a snake and
in certain places, such as a tapered tube in a honeycombed
feldspar approximately 5 to 100 nanometers across, would
begin to interact closely with the aluminosilicate surface.
· Third, various biopolymers of different types might begin to
interact and begin the evolution toward a protobacterium.
· Particularly important would be the first generations of
persistent energy-generating species containing phosphorus
and electron-transferable transition metals. Very important
is that K-rich feldspars from granites contain micrometer
inclusions of the calcium phosphate-hydroxide/halide
mineral apatite and transition-metal oxides, including ilmen-
ite, spinet, and hematite, which might well be the primary
reservoirs of these key elements.
- At some stage, a protocell lining inside an aluminosilicate
tube might develop a bilipid lining that would extend into a
cap, ultimately allowing detachment from the silicate and
free motion through a soup. Again, one might imagine a
sausage machine popping off a free cell as the remaining
protocell reconstituted itself ready for generation of the next
free cell. Schematic graphics are being envisaged along with
ideas for chemical bonding schemes.
- All these processes would involve subtle effects related to
diurnal and annual temperature cycles and wet/dry cycling
driven both by solar radiation and lunar tides that would
change the spatial distribution of chemical forces across
mineral surfaces. Ideas are not developed enough so far to
warrant further description here.
Mineralogical Observations and Speculations: Electron
Microscopy of Honeycombed Weathered Feldspars Bacteria
and Protocells. Turning now to mineralogical information, Fig.
3 contains four scanning-electron micrographs of the crystal-
lographically controlled honeycomb weathering on a modern
surface of a K-feldspar from the Shap granite (30~. Particularly
important are the micrometer-scale sausage shapes inter-
preted as electron scattering from bacteria, somewhat
shrunken from interaction with the electron beam. Many have
no particular orientation with respect to the feldspar, but the
bacterium in Lower Right is interpreted as sitting neatly in a
crevice. The near correspondence between the segmentation
of the proposed bacterium and the spacing of the feldspar
honeycomb is intriguing. Perhaps it may ultimately be possible
to quantify the original chemical linkages between the inor-
ganic substrate and the unshrunken bacterium and to use them
for modeling the above ideas.
Coupling the catalytic production of polymers at the nano-
meter scale with bacteria at the micrometer scale is plausible
in the geologic context, but requires many flights of imagina-
tion and a lot of faith. On the present Earth, volcanic glass
transforms to zeolites in continental basins and ocean floors;
the zeolites become metamorphosed to feldspars; and the
whole mineralogical assemblage becomes converted over geo
Proc. Natl. Acad. Sci. USA 96 (1999J 3483
logic time into granitic metamorphic rocks. Hence, we are
comfortable in proposing that zeolites and feldspars would
have coexisted on the early Earth, for which only the resultant
granitic metamorphic rocks have been seen so far. Hence we
can suggest for discussion purposes that a zeolite/silica/
altered feldspar sausage machine fed a range of biological
polymers into feldspar honeycombs. As discussed above, in-
termingling of polymers, generation of P-bearing energy-
transporting species from apatite, and hydrogen-bond cou-
pling between organic species and silica-rich walls, would have
generated primitive protocells. To conclude the evolution into
the first organisms, a cap between the dangerous outer regime
of 'soup' and the inner protected world might have expanded
to completely enclose the protocell so that it could swim into
the future. Part IV, under preparation, will show graphics
illustrating the scientific factors underlying these flights of
fancy about cell formation.
Conclusion. We conclude with matters of specific geological
import. From the humanistic viewpoint, it would be extremely
significant if the early forms of life had left behind some
physicochemical evidence of their existence. The current
carbon-isotope evidence of bulk samples is indicative of some
kind of early biological evolution, but has no particular import
for the atomic-level ideas presented above. A review of the
geological evidence indicates that at least most and perhaps all
of the early Archean rocks have been metamorphosed to a high
enough level that all volcanic rocks have recrystallized. There
can be little doubt that volcanoes would have been pumping
out ash containing crystals of K-feldspar and silica minerals.
By analogy with modern conditions, much of the ash would
have been converted into zeolite beds, and there might well
have been zoned beds of zeolite minerals interacting with salty
lakes sloshed by tides and impacts. Some K-feldspar and zeolite
crystals would have been exposed to an acidic rain, and
honeycombed and grooved faces should have occurred (1, 2~.
Primitive molecules would certainly have been available dis-
persed in 'soups', as envisaged by many writers (1~.
Here are some ideas for testing whether minerals produced
by metamorphic recrystallization of earlier igneous origin
might have retained some specific signature indicative of subtle
biological processes involving feldspar, zeolite and silica min-
erals:
.
- Ancient cherts (silica-hydroxyl-rich aggregates) range in
color at least from black to brown, red, and orange-yellow.
At least some of the color variation must result from
transition metals, especially Fe and Mn, at different redox
states. Might some carbonaceous species have survived in
the black cherts? If so, would careful analysis reveal organic
breakdown products specific to primary biocatalytic precur-
sors?
· Because early organisms would have needed P and various
transition metals, would their absence or low abundance in
the metamorphosed siliceous rocks be indicative of early
biological scavenging by organisms that escaped into the
'soup'?
- Particularly challenging, because of the possibility of com-
plete failure, would be a hunt for x-ray diffraction evidence
of surviving Si-rich molecular sieves. Silicalite and other
large-pore zeolites have strong low-angle diffractions that
would stand out in low-background patterns obtained with
synchrotron x-rays, even at a concentration below 1%.
Since the review in ref. 1, the following geological/biological
publications have been added:
the making and breaking of bonds, as well as to provide several potential sites for binding and reactivity. The histidine molecules are shown by
a ball-and-stick arrangement and are colored as in Upper. The water molecules are represented by a bent bicolor rod with two white ends representing
H, and the red center represents 0. The framework is represented by tetrahedra whose shared vertices are at O positions. A further reason for
choosing histidine is its prevalence as a metal-binding site in modern proteins, undoubtedly an important function in the prebiotic world.
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3484 Colloquium Paper: Smith et al.
Proc. Natl. Acad. Sci. USA 96 (1999J
-
~ . :~;:, = ~ ~
FIG. 3. Four scanning-electron micrographs of weathered feldspar from the Shap granite. (~Upper Left) Resin cast of honeycomb texture. The
cast is somewhat flexible, so that some of the etched dislocations appear to be curved, although they were almost straight in the original feldspar,
which has been dissolved away in HF. (~Upper Rights A near-planar surface close to bar601 with a trace of etched dislocations running horizontally
and vertically across the image with ellipsoidal bacteria, some in strings like sausages hanging in a U.K. butcher's shop. (rower Left) More deeply
weathered surface showing occasional traces of etched dislocations, with sausage-shaped bacteria. (Lower Right) Detail of honeycomb on the 001
surface of a feldspar honeycomb. The holes are etch pits formed on paired outcrops of dislocations that formed on exsolution lamellae. The
bacterium, although perhaps partly shrunken by the instrument vacuum, is segmented on a scale remarkably similar to the spacing of the etch pits.
Details of the feldspar weathering are given in Part II (57~.
· stardust and meteorites: Photochemical evolution of inter-
stellar/precometary organic material (31~; silica-rich micro-
meter objects in a carbonaceous chondrite (32~;
· planetary impact processes: Survival of amino acid in large
comet impacts (33~;
· early geologic events on Earth: Nitrogen fixation by volcanic
lightning (34~; redox state of upper-mantle peridotites under
the ancient cratons, and possible equilibrium of diamonds
with methane-nitrogen-rich fluids (35~; new revised Pb-ages
of Greenland gneisses at 3.65-3.70 instead of earlier 3.85
gigayear-before-present (36~; interpretation of geologic ev-
idence in favor of plate-tectonic processes in the Archean
era (374; interpretation of Archean magmatism and defor-
mation in nonplate tectonics terms (38~; details of Precam-
brian elastic sedimentation that partly match and partly
differ from recent processes (39~; evidence from mature
quartz arenites in various Archean shields of stable conti-
nental crust containing quartz-rich granitoid rocks (40~;
microbiological evidence for Fe(III) reduction to Fe(II) on
early Earth, and support for earlier idea that Fe(III) was a
more likely electron acceptor than S in microbial metabo-
lism (41), birth of the Earth's atmosphere, and the behavior
and fate of its major elements (42~;
· bacteria, cell walls, various matters: Text on bacterial bio-
geochemistry, with final chapter on origins and evolution of
biogeochemical cycles/prebiotic Earth and mineral cycles/
theoretical perspectives on the origins of life (Oparin-
Haldane theory, Cairns-Smith ideas on clays and life, pyrite,
and the origins of life, "thioester world") (43~; intracellular
bacteria in protozoa (44~; plant cell wall proteins (45); gene
molecular sequences of Archea and details of thermophiles
and cold-dwelling types (46~; hydrogen consumption by
methanogens on the early Earth (47); genome sequences
from a dozen bacteria and a yeast fit with a three-kingdom
world (48); 'Eukaryotes are suggested to have arisen through
symbiotic association of an anaerobic strictly hydrogen-
dependent strictly autotrophic archaebacterium (the host)
with a eubacterium (the symbiont) that was able to respire,
but generated molecular hydrogen as a waste product of
anaerobic heterotrophic metabolism,' (49~; bacteria in sed-
iments (50~;
· chert: The following papers about the siliceous nodules
known as chert and about related siliceous materials should
be useful in thinking about how to characterize ancient
chert: Evidence of volcanic origin of chert in the Permo-
Triassic Sydney Basin (51~; growth of chalcedony by assem-
bly of short linear polymers with silica monomers (52~;
growth fault control of ~3.5 Gybp Early Archaean cherts,
barite mounds, and chert-barite veins, North Pole Dome,
Eastern Pilbara, Western Australia, carbonaceous aggre
OCR for page 3485
Colloquium Paper: Smith et al.
gates in grey chert (53, 54~; transformation of black to white
chert (55~; classic Rhynie chert locality with evidence for a
low-energy lacustrine environment with periodic desiccation
on exposed mud flats (56~.
To conclude: From the viewpoint of geology, mineralogy,
and human welfare, it is quite obvious that major questions on
biochemical evolution remain unanswered, but might become
accessible to quantitative study with the new analytical tools
developed over the past few decades. Surface biogeochemistry
is a subject whose time has come.
F.P.A. thanks Advanced Research Systems for computer facilities.
I.P. and M.R.L. thank the U.K. National Environment Research
Council for a grant. J.V.S. thanks many scientists at UOP for allowing
him to participate in their pioneering work on organophilic silicic
molecular sieves and wishes to acknowledge the pioneering indepen
dent parallel studies by scientists from Mobil Corporation on the
ZSM-5 zeolite series.
1. Smith, J. V. (1998) Proc. Natl. Acad. Sci. USA 95, 3370-3375.
2. Smith, J. V. (1998) Proc. Natl. Acad. Sci. USA 95, 3366-3369.
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Representative terms from entire chapter:
amino acid
