H00102--00A, Front mat Genesis

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-

  162

  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-