by Charles Baum
ers tried their hand at other biopolymers. Leslie Orgel, research profes-
sor at the Salk Institute for Biological Studies in San Diego, succeeded
in forming a variety of proteinlike chains of amino acids up to several
dozen molecules long. Orgel and his students discovered that different
minerals preferentially select and polymerize different molecules from
a water-based solution. By combining the right mineral with the right
molecule, they could form polymers at will.
In conjunction with his experiments, Orgel also developed an el-
egant theory of “polymerization on the rocks,” in which he pointed out
both the promise and problems with mineral surfaces. Minerals such
as clays and hydroxides certainly can adsorb interesting biomolecules,
he noted, including the amino acids and nucleotides essential to life.
Furthermore, once two of these molecules are adsorbed close to each
other, they have a tendency to bond. As more and more molecules are
added to a lengthening polymer, however, the strand becomes more
and more tightly bound to the mineral surface. How, he asks, can a
polymer contribute to life if it’s stuck to the rocks?
One possible answer came from the Harvard University labora-
tory of geneticist Jack Szostak, who mixed together clays, RNA nucle-
otides, and lipids in the same experiment. Lo and behold, the clays not
only adsorbed RNA, but also hastened the formation of lipid vesicles.
In the process, RNA-decorated clay particles were incorporated into
the vesicles. This spontaneous self-assembly of RNA-containing
vesicles, though a long, long way from synthesizing life, is perhaps the
closest anyone has come to forming a cell-like entity from scratch.
MORE MINERAL MAGIC
Every interaction between a mineral and a molecule requires knowl-
edge of two different chemical entities—a crystalline solid and a tiny
carbon-based cluster of atoms. Most origin-of-life experts come from
the world of organic chemistry—the carbon-based biomolecule part.
It’s easy to get the impression that minerals are brought into origin-of-
MINERALS TO THE RESCUE
159
life experiments only when nothing else works, and then as a sort of
magic powder. Atomic-scale details of the molecule–mineral interac-
tions are usually fuzzy at best.
Gustaf Arrhenius, a senior NASA-supported researcher at the
Scripps Institution of Oceanography, in La Jolla, California, has a
somewhat different background. He knows organic chemistry, to be
sure, but he was trained in inorganic chemistry and has a deep appre-
ciation of crystalline solids and their structural idiosyncrasies. Conse-
quently, he has zeroed in on the mineral group known as double-layer
hydroxides. Like clays, the double-layer hydroxides are common in na-
ture, and they come in almost limitless compositional variants, with
magnesium, iron, chromium, nickel, calcium, aluminum, and many
other elements mixing and matching in the double-layer structure.
Additional complexity arises because small molecules—water, carbon
dioxide, or a variety of other common species—occupy the spaces be-
tween the layers. By changing their chemical contents, Arrhenius has
been able to fine-tune his double-layer hydroxides to perform specific
chemical tasks.
Unlike Ferris and Orgel, Arrhenius did not try to form long poly-
mers on the surfaces of his mineral powders; rather, he exploited the
tendency of double-layer hydroxides to soak up small organic mol-
Gustaf Arrhenius demonstrated that double-layer hydroxide minerals have the abil-
ity to attract small molecules and catalyze their reactions into larger species of biological interest (after Arrhenius et al., 1993).
160
GENESIS
ecules in the rather large spaces between the layers. Once confined and
concentrated, these small molecules tend to form larger molecular spe-
cies that are not otherwise likely to emerge from the soup. His most
interesting products so far have been sugar phosphates, in which a
sugar molecule is linked to a phosphate group—a suggestive pairing,
in that it forms the backbone of every DNA and RNA molecule.
Joseph Smith has explored yet another intriguing mineralogical
wrinkle. Smith is an authority on zeolites, a diverse family of natural
and synthetic crystals that feature latticelike frameworks of silicon, alu-
minum, and oxygen atoms. These open frameworks result in molecule-
sized channels that run the length and width of the crystals. The
tendency of some hydrocarbon molecules to enter these zeolite chan-
nels, while other molecules remain behind, lies at the heart of the
multitrillion-dollar petroleum refining business.
Instead of looking at mineral surfaces, Smith imagines prebiotic
molecules concentrating inside zeolite channels. Zeolites abound in
the weathering products of volcanic rocks; organic molecules were cer-
tainly present in the same environments. Once packed with a long col-
umn of molecules, a zeolite might promote additional reactions,
including polymerization. Smith even suggests that the first cell wall
might have been “an internal mineral surface.” Experiments have yet to
back up these ideas; it’s hard to study what’s going on inside a mineral.
But the message is clear—minerals and molecules could have inter-
acted in a lot of intriguing ways on the early Earth.
CLAY LIFE
Many researchers have looked to minerals to give life a jump-start,
whether as protective containers, organizing surfaces, or catalytic en-
gines of prebiotic chemistry. In a provocative 1966 article (and in nu-
merous subsequent publications) the Scottish origin-of-life expert
A. G. (Graham) Cairns-Smith carried these ideas even further by
speculating that fine-grained crystals of clay might themselves have
been the first life on Earth.
Virtually all other authorities assume that life emerged by a con-
tinuous chemical process of increasingly complex carbon chemistry.
Today’s biochemistry is thus a direct, albeit highly evolved, successor
of ancient carbon geochemistry. Cairns-Smith disagrees. “I believe the
unity of biochemistry is of no direct help,” he says. “Evolution did not
MINERALS TO THE RESCUE
161
start with the organic molecules that have now become universal to
life: indeed I doubt whether the first organisms . . . had any organic
molecules in them at all.” He points to five decades of failed attempts
to synthesize key macromolecules, such as RNA and proteins, in any
plausible prebiotic scenario. In his view, nucleic acids and proteins are
too complicated to build without help from the inorganic world.
Cairns-Smith’s ideas at first seem fanciful, but he is a persuasive
scientist, writer, and lecturer. He speaks in a soft Scottish accent, en-
gaging his audience with a lucid, low-key delivery. He’s not pushy with
his ideas, but he has the knack of drawing you into his way of thinking.
His writings, both for academic and general audiences, eschew jargon
and rely on simple, vivid
illustrations. Seven Clues to the Origin of Life, his popular book on the clay-life hypothesis, reads like a mystery novel,
complete with dialog, false leads, and quotations from Sherlock
Holmes. Whether you’re persuaded or not, you have to admire Cairns-
Smith’s style.
The crux of the argument rests on a simple analogy. Cairns-Smith
likens the origin of life to the construction of a stone archway, with its
carefully fitted blocks and crucial central keystone that locks the whole
structure in place. But an arch cannot be built simply by piling one
stone atop another. “The answer,” he says, “is with a scaffolding of some
kind.” A simple support structure facilitates the construction and can
then be removed. “I think this must have been the way our amazingly
‘arched’ biochemistry was built in the first place,” he wrote in a Scien-
tific American article in 1985. “The parts that now lean together surely
used to lean on something else—something low tech.” That something,
he suggests, was a clay mineral.
Like Bernal before him, Cairns-Smith points to the ubiquity of
clays, their negative surface charge, their affinity for organic molecules,
and the immense reactive surface area their fine-grained layers pro-
vide. As he was quick to point out in a Geophysical Lab seminar in the
summer of 2003, “I’m an organic chemist, not a clay mineralogist—
though I often talk like one.” Then he embellishes the point with a
surprising twist that reveals his mineralogical sophistication. Clay crys-
tals grow, and their mutable layered structure can carry and pass on a
kind of “genetic information,” in the form of a reproducible sequence
of crystal defects or variable chemical compositions. Given their abil-
ity to grow and reproduce, might not clays be thought of as living?
Even the most chauvinistic of mineralogists tend to balk at Cairns-
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GENESIS
Smith’s concept of “clay life” and “crystal genes,” but he does have an
intriguing point. Genetic information in modern life is carried by DNA
and RNA molecules, which, as noted, are each made up of sequences
of only four different nucleotides—in effect, a four-letter genetic al-
phabet. Any conceivable message can be conveyed with such an alpha-
bet and an appropriate code.
In much the same way, clay minerals commonly display periodic
structural or compositional variations that might be used to convey
information. For example, the clay structure features strong sheetlike
layers of atoms that stack one on top of another. Each of these struc-
tural layers can adopt one of three different orientations relative to its
neighbors—at 0, 120, or 240 degrees. The sequence of layer orienta-
tions is analogous to a three-letter alphabet. What’s more, layers of
different thickness and chemical character sometimes interleave with
each other, adding dramatically to the information-carrying potential
of clays.
Periodic variations can also occur within any given layer; that is,
each layer can display a feature called “twinning,” by which a given
layer can incorporate small regions in all three different orientations.
What’s more, many clays boast complex chemical compositions, with
varying mixtures of aluminum, magnesium, iron, and other elements.
These atoms reside in predictable repeating sites, each one surrounded
by an octahedron of six oxygen atoms. Move along the surface of the
clay particle and you’ll find an octahedral site every ten-billionths of
an inch or so, as regular as clockwork. But these regimented sites in the
clay structure often contain a more-or-less random sequence of alumi-
num, magnesium, and other elements, as well as vacant sites and other
defects. Cairns-Smith argues that such a random distribution of atoms
at the exposed mineral surface can also act as a kind of genetic se-
quence, each element representing a different letter in this peculiar al-
phabet. “In two-dimensional crystal genes, information would be held
as a pattern on one face of the crystal,” he posits.
So what? How can a random sequence of layer orientations or a
pattern of elements at the clay surface be regarded as a living entity?
How can it possibly reproduce itself? Cairns-Smith proposes two in-
triguing, though unproved, possibilities. First, he speculates, clay crys-
tals might grow in such a way as to copy any given layer, stacking
sequence, or element pattern over and over again, and then flake apart
in what amounts to an act of self-replication. In this scenario, clay par-
MINERALS TO THE RESCUE
163
A
B
C
GROWTH
CLEAVAGE
Graham Cairns-Smith suggests that clay minerals, which have a sheetlike layered
structure, can carry genetic information in the form of three different layer orientations (A). Individual clay particles can possess a complex pattern of these orientations (B). If sequences of layers can be copied, then this genetic information might be passed on from one generation of clays to the next (C) (after Cairns-Smith, 1988).
ticles crystallize over and over again, and particularly stable sequences
of layers or elements ultimately win out in a mineralogical version of
evolution by natural selection.
Moreover, according to Cairns-Smith’s model, different exposed
layer edges or elements at the clay surface have affinities for specific
organic molecules. Not only do the negatively charged clay surfaces
readily attract organic molecules, but clays also have the ability to cata-
lyze reactions between surface-bound molecules. The self-replicating
inorganic world of clays could have acted as the long-abandoned scaf-
folding for the organic world—a process he calls “genetic takeover.”
The resulting macromolecule might then possess exactly the sort of
164
GENESIS
information-rich structure needed for the emergence of a carbon-
based genetic mechanism.
TESTING
Any acceptable scientific theory must make testable predictions; oth-
erwise, as Karl Popper consistently maintained, a theory is just idle
speculation. Graham Cairns-Smith is well aware of this requirement,
and he does not shy away from predictions.
A potentially testable feature of the clay-life hypothesis follows
from the central requirement that clays evolve. In 1988, Cairns-Smith
wrote a provocative essay entitled “The Chemistry of Materials for Ar-
tificial Darwinian Systems,” in which he elaborated on his idea that
clays carry “genetic information” in the form of variable crystal pat-
terns that can be passed from one generation of crystals to the next.
Clays continuously form and dissolve in many geological environ-
ments. Perhaps, he posited, clays evolve by the selective replication of
favorable patterns, coupled with the selective dissolution of unfavor-
able patterns. In this way, the evolution of clays is analogous to that of
microorganisms. “The first step is to grow up a batch of the organ-
isms,” Cairns-Smith explains. “Then a small sample of this
is used to
seed a new batch, which is in turn grown up—and so on. In such cir-
cumstances those types which reproduce fastest will be most likely to
be carried on between the batches and will eventually become the only
types present.”
By analogy, Cairns-Smith predicts, more successful (that is, more
likely to be replicated) element arrangements will gradually dominate
less successful variants. But can we test this idea in a laboratory experi-
ment? Like a study of microorganism evolution, we would have to char-
acterize the detailed state of numerous particles in the first generation
of clays, and then monitor changes in subsequent generations. Cairns-
Smith recognizes many experimental challenges. “Can the material be
synthesized in the laboratory on a practical time scale? Can we find
conditions that will allow crystals to grow? . . . And then the $64,000
one: Do sequences replicate accurately enough through growth?” Un-
derlying all of these questions is the technical barrier: how to deter-
mine the exact elemental sequence of a clay particle.
For microbial populations, these problems have been solved. Mi-
crobes grow rapidly, they divide predictably, and they copy their DNA
MINERALS TO THE RESCUE
165
with fidelity. And it’s relatively easy to conduct a broad survey of mi-
crobial DNA sequences—a task that has become automated by decades
of genetic research and billions of dollars in capital investment. Mo-
lecular biologists have thus learned to document evolution in a micro-
bial community.
But for clays, no such sequencing technology exists. To be sure,
modern imaging instruments can provide some insights. Cairns-Smith
advocates high-resolution transmission electron microscopy
(HRTEM): “We should go for materials for which we can expect
HRTEM to be applicable,” he writes. Nevertheless, characterization of
the element arrangement in a single clay particle, much less the thou-
sands of separate analyses necessary for an adequate clay-particle
population survey, is beyond any current technique. This situation
is nothing new—scientific progress has often had to wait for new
technology.
Many earth scientists are drawn to the concept that clays, or some other
mineral, played a crucial role in biogenesis. There’s a satisfying com-