Keen with anticipation, the chemists placed the chemical fractions one-by-one into water and watched to see if anything interesting happened. They began with “spot 1,” with molecules that had concentrated in an elongated area close to the original drop on the chromatography plate. Deamer and Pashley watched transfixed as the invisible molecules, once dispersed throughout the meteorite, spontaneously arranged themselves into tiny spheres no more than a hundredth of an inch across—about the size of many modern microbes. What's more, they found, these weren't just little drops of oil or fat floating in water. These structures had an inside and an outside. The molecules had organized themselves into bilayers, just like a cell membrane—an elegant example of emergence.
It was a breakthrough moment for origin researchers. Deamer and Pashley had shown that ancient lipid molecules, synthesized at some distant place in space and delivered intact to Earth, form tiny enclosed structures that are in many ways like the membranes encasing living cells. One of life's most basic requirements—the isolation of inside from outside—suddenly seemed to have been hard-wired into the fabric of the universe.
SELF-ORGANIZATION, REPRISE
Dave Deamer's Murchison experiments were conceptually simple and beautifully executed. So when new lipid-rich samples came along, he repeated the process.
Lou Allamandola and his NASA Ames team realized that their growing inventory of organic molecules, synthesized under simulated deep-space conditions of ultracold vacuum with ultraviolet radiation, contained a significant fraction of yellowish oily stuff just as the Murchison meteorite did. In particular, when they irradiated an ice made principally of water and alcohol with a bit of ammonia and carbon monoxide thrown in, they produced an intriguing residue of fluorescent material. Much of that material was known to be the familiar multiringed hydrocarbons known as PAHs, but other molecules appeared to have an amphiphilic character. Naturally, they turned to Dave Deamer to check it out.
Samples in hand, it took Deamer less than a day to confirm what the Ames researchers had hoped. Once the correct fraction of fluorescent molecules was concentrated, stunning vesicles appeared spontaneously in water. The press trumpeted the result, and a colorful photograph of the delicate tiny spheres graced the front page of the Washington Post above the headline “IN SPACE; CLUES TO THE SEEDS OF LIFE.” The implications were profound: Even before the formation of planets and moons, in the tenuous vacuum of frigid space, the raw materials for life abound, ready to organize spontaneously into cell-like structures.
PYRUVATE REDUX
I got the chance to work with Dave Deamer following a conversation at one of NASA's first astrobiology meetings, in April 2000. Dave had been asked to present a keynote lecture on self-organization to the audience of geologists, chemists, biologists, and astronomers, not to mention a smattering of philosophers and ethicists.
Some scientific lecturers try to snow their audiences. Deamer is different; he meets the audience more than halfway, with comfortable metaphors, familiar examples, and elegant demonstrations. At this lecture, he held up two large beakers, both with colorless solutions. When mixed, the resulting liquid immediately became cloudy white; we were looking at the spontaneous self-organization of lipids, he explained.
At Carnegie, my group's 1996 pyruvate work had been sitting on hold for years. We knew we'd made a lot of interesting organic molecules by heating and squeezing pyruvate, but other than the fact that the reactions occurred rapidly under hydrothermal conditions, the relevance to life's origin wasn't clear. Perhaps, we thought, the yellow, oily goo that oozed out of our gold capsules held self-organizing molecules. That might be worth investigating, because we had started with a core metabolic molecule. It would be newsworthy if there were a facile path from primitive metabolism to membranes.
On hearing my story, Dave immediately invited me to his specially equipped lab at UC Santa Cruz (where he had moved in 1994) to try the experiment. The following winter, I prepared some new pyruvate-plus-water capsules and subjected each of them to two hours at 2,000 atmospheres and 250°C. I brought them, unopened, to the beautiful Santa Cruz campus.
In spite of his insanely busy schedule as faculty member in two departments, supervisor of two laboratories, and mentor to several graduate students and postdocs, Dave was a gracious and attentive host. He welcomed me to his biochemistry lab and we set to work immediately.
Once I had opened the capsules (which responded with the now familiar bang! and intense oily foaming), he led me step-by-step through the chloroform extraction, concentration, and preparation of a 10 × 10-inch glass plate for chromatography. I had pored over his 1989 paper several times, so it was a delight to duplicate that work with my own samples under his supervision.
Within a couple of hours, I had decorated a glass plate with a small yellow-brown dot of unknown chloroform-soluble compounds. We gently lowered the plate into a deep glass tank into which I had poured a half-inch layer of pungent ether. (The strong smell triggered a brief, vivid flashback to an early childhood moment—a menacing masked anesthesiologist bending over me, smothering my face prior to a ton-sillectomy. I had to shake away the disturbing image.) As with Deamer's earlier work, the solvent pulled the glass plate's single yellow-brown spot into a long streak. Then we rotated the plate and the chloroform smeared the streak into what we hoped would be a distinctive pattern of organic compounds.
I felt more than a little tingle of anticipation as the lights went out and the UV fluorescent lamp flicked on. The results were gorgeous! A brilliant yellow, blue, and purple pattern appeared, blazing across the plate in a diffuse 7-inch-long arc of color. We were delighted to see several distinctly fluorescing areas, strikingly similar in detail to the Murchison sample [Plate 6]. Noting the correspondence, Dave suggested that we first concentrate on a blue fluorescing area most closely matching the position of his original “spot 1.”
Again, we followed the 1988 procedures: Carefully mark the glass plate, scrape off the white powder from the area of interest, wash that powder with chloroform to redissolve the fluorescing molecules, and dry the extract (by this time the lab area smelled strongly of the chloroform–ether mix). Then the big test. Would my concentrated extract perform the self-organization trick?
The test was quick and easy. We applied a bit of the extract to a droplet of water on a glass slide and watched in the microscope, which used a UV light to highlight fluorescent molecules. Sure enough, tiny green fluorescing spheres appeared, like a fantastic display of Christmas lights. [Plate 7] Beautiful, but were they vesicles that trapped the surrounding liquid, or simply solid spheres? That was key to determine if we had really made cell-like bilayer membranes.
Deamer's technique was to repeat the microscope observations, but this time starting with a strongly fluorescing red dye in the water. For a second time he applied a bit of the extract to the water and, once again, green fluorescing spheres formed. If we had made hollow vesicles, then they would capture the distinctive red dye. To find out, Dave carefully flushed the slide with new, nonfluorescing water. Lo and behold, the centers of the tiny green vesicles glowed red. We knew we had made bilayer membranes from nothing more than pyruvate and water.
We celebrated that night with a bottle of Napa Valley cabernet and talk of next steps and publications. We both knew that the pyruvate results were at best a footnote to the Murchison and NASA Ames discoveries, but the experiments seemed to underscore the inevitable emergence of self-organizing molecular systems along the path to life.
To be sure, many problems remain to be solved. Recent work by Deamer's group suggests that lipid self-organization may be sharply limited by the presence of dissolved calcium and magnesium, seawater ingredients that would have been present in significant concentrations in Earth's early ocean. Perhaps life can begin only in fresh water, or maybe some as yet unidentified varieties of lipid molecules were involved. And, as many biologists have been quick to point out, the vesicles produced in Deamer's work are a far cry from actual cell m
embranes, which feature a mind-boggling array of protein receptors that regulate the flow of molecules and chemical energy into and out of the cell.
These details will occupy researchers for decades to come, but the emergence of cell-like vesicles from simple molecules is now one of the best-understood features of life's origin.
AEROSOL LIFE
New ideas about the emergence of self-organized molecular systems keep origin-of-life workers on their toes. An especially intriguing recent proposal comes from Oxford chemist Christopher Dobson and his collaborators at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. In 2000, they published a speculative yet persuasive hypothesis on lipid self-organization in the Proceedings of the National Academy of Sciences. Elaborating on earlier unpublished work by the geophysicist Louis Lerman at Stanford, Dobson's group focused on the possible roles of atmospheric aerosols in prebiotic synthesis and molecular organization.
Many organic molecules—especially lipid molecules like the ones Deamer isolated from the Murchison meteorite—could have accumulated at the ocean's surface like an oil slick. As wind kicked up white-caps and waves crashed onto the earliest shores, a continuous fine mist of aerosol particles—tiny droplets, some smaller than a thousandth of an inch across—would have sprayed into the atmosphere from the oily surface. Each water droplet would have contained a significant concentration of organic molecules that almost immediately would have formed a membranous shell around the wet interior. The largest of these droplets would have fallen quickly back into the foam, but smaller aerosol particles are quite robust and could have remained suspended in the atmosphere for months or even years, riding wind currents like microscopic gliders high into the stratosphere.
Dobson and colleagues speculate that lipids in each aerosol particle formed a spherical, single-layer structure with the hydrophobic ends facing the atmospheric exterior and the hydrophilic ends facing the aqueous interior. Many of these aerosol particles would have incorporated reactive, water-soluble organic molecules, which might have undergone further chemical reactions in sunlight. Each particle would have had weeks or months to experience such energetic transformations; each would have been, in effect, a tiny chemical experiment.
For hundreds of millions of years, aerosol particles in numbers beyond imagining drifted into the skies. Upon their return to the ocean, each hydrophobic aerosol particle would have been spontaneously coated by more lipid molecules at the ocean's surface to form a bilayer structure—the emergence of the familiar membrane structure of cellular life. In the words of Dobson, “Organic aerosols offer more than freedom from the tyranny of the tidal pool or Darwin's ‘warm little pond'; they offer a possible mechanism for the precursory production and the subsequent evolution of populations of cells.”
In either scenario, whether in the form of wind-blown aerosols or water-bound vesicles, lipid self-organization seems to have been an essential step in isolating the insides from the outsides of cell-like structures. But a membrane, by itself, is not life. Other essential biomolecules, including proteins, carbohydrates, and nucleic acids, had to be assembled from the soup. The trouble is that the building blocks of these macromolecules—amino acids, sugars, bases—are all water soluble. By themselves, they can't self-assemble in water.
According to the theory of Christopher Dobson and colleagues, the surface of the ancient ocean was coated with amphiphilic molecules. The action of ancient waves and winds would have formed aerosol particles surrounded by lipid molecules (A). These particles might have remained in Earth's atmosphere for months (B), but would eventually return to the ocean (C), and form cell-like bilayer structures (D) (after Dobson et al., 2000).
What to do? Call in the rocks.
12
Minerals to the Rescue
But I happen to know exactly how life arose; it's brand-new news, at least to the average layman like yourself. Clay. Clay is the answer. Crystal formation in fine clays provided the template, the scaffolding, for the organic compounds and the primitive forms of life. All life did, you see, was take over the phenotype that crystalline clays had evolved on their own.
John Updike, Roger's Version, 1986
The first living entity emerged from interactions of air, water, and rock—the same raw materials that sustain life today. Of these three chemical ingredients, rocks—and the minerals of which they are made—have generally received little more than a footnote in theories of life's emergence. The atmosphere and oceans have long enjoyed the starring roles in origin scenarios, while rocks and minerals sneak in and out as bit players—or simply as props—and then only when all other chemical tricks fail.
Some recent and fascinating experiments promise to change that misperception. Origin-of-life researchers have begun to realize that minerals must have played a sequence of crucial roles, beginning with the synthesis of biomolecules and during their subsequent assembly into growing and evolving structures.
The Miller–Urey chemical process works by ionizing gas molecules—blasting them with lightning or ultraviolet radiation and thereby stripping off electrons, so that small groups of atoms readily recombine into larger organic molecules. Interesting molecules inevitably emerge, but those energetic processes effectively prevent the formation of essential macromolecular structures, including the polymers and membranes required by all known life-forms. The Miller–Urey scenario can't have been the entire story. That's why many researchers, especially those trained in geology, turn to rocks and minerals.
MINERALS AS PROTECTION
Rocky outcrops and overhangs—especially in tidal zones, where seawater evaporates and thus concentrates organic molecules—might have promoted macromolecular formation. Imagine a shaded cove where increasingly concentrated mixtures of organic molecules accumulated and reacted, protected by a rocky ledge from the Sun's harmful radiation. Rocks might have served as Earth's earliest sunblock. They may well have provided protection at a smaller scale as well. Many volcanic rocks of the early Earth were laced with countless air pockets left by expanding volcanic gases. Evaporating seawater might have deposited a rich mix of organic molecules in such tiny hollows, each like a small test tube where further reactions could proceed.
Mineralogist Joseph V. Smith, professor emeritus at the University of Chicago, envisions even smaller protected environments. He cites electron microscopy studies of weathered mineral surfaces, which often display myriad microscopic cracks and pores. Feldspar, the commonest of all rock-forming minerals, sometimes features millions of tiny weathered pockets, each the approximate size and shape of living cells, each providing a place for molecules to congregate, each pore and crack a separate experiment in molecular self-organization. [Plate 7]
POLYMERIZATION ON THE ROCKS
The production of macromolecules requires two concerted steps: The correct molecules must first be concentrated and then organized into the desired structure. In the case of lipid membranes, these two tasks occur virtually simultaneously and spontaneously; lipids in water separate and self-organize into a bilayer. But other key biological macromolecules, including proteins and carbohydrates, form from water-soluble units—amino acids and sugars. Consequently, they tend to break down, not form, in water.
One promising way to assemble such molecules from a dilute solution is to concentrate them on a surface. For decades, the prevailing paradigm has been that the molecules of life assembled at or near the ocean–atmosphere interface. The surface of a calm tidal pool, or perhaps a primitive slick of water-insoluble molecules might have done the job. But then, as noted, these environments are open to lightning storms and ultraviolet radiation.
Origin scientists with a penchant for geology have long recognized that rocks might provide attractive alternative surfaces for concentration and assembly—a kind of scaffolding for the assembly of protolife. More than a half-century ago, the British biophysicist John Desmond Bernal advocated the special role of clays, which are ubiquitous minerals with regularly layered atomic
structures.
Clays come in a wide range of compositional and structural variants, but all of them feature layers of strongly bonded silicon and aluminum atoms. The proclivity of clays to exhibit a surface electrostatic charge enhances their ability to adsorb organic molecules—a kind of molecular-scale static cling. What's more, clays tend to occur as exceptionally fine-grained flat particles. Consequently, a palm-sized pile of ordinary clay can boast a reactive surface area greater than 1,000 square feet.
Subsequent experiments have supported Bernal's speculations. In a 1978 study, Israeli biochemist Noam Lahav and colleagues discovered that amino acids concentrate and polymerize on clays to form small, protein-like molecules. Such reactions occur when a solution containing amino acids evaporates in the presence of clays—a situation not unlike the evaporation that dries up a shallow pond or tidal pool. Of special note is the fact that this process relies on cycles of heating and evaporation—and cycles, recall, are one of the key factors in the emergence of complexity. Patterns of daily and seasonal changes doubtless fostered the emergence of new molecular structures.
More recently, research by NASA-sponsored teams in California and New York has demonstrated that a variety of layered minerals can adsorb and assemble a variety of other organic molecules. In a tour de force series of experiments during the past two decades, chemist James Ferris and colleagues at Rensselaer Polytechnic Institute induced clays to act as scaffolds in the formation of RNA, the polymer that carries the genetic message enabling protein synthesis.
Ferris relied on the simplest of procedures. First, he prepared a solution of “activated” RNA nucleotides, each consisting of a ribose sugar bonded to a phosphate and a base, plus a reactive molecule called imidazole that promotes, or “activates,” bonding between nucleotides. Such a solution can sit on the lab bench for weeks with little change. But sprinkle in a bit of a suitable clay mineral and the RNA pieces start to link up. In a matter of hours, lengths of 10 nucleotides form. By the end of 2-week experiments, the RPI team produced RNA strands of more than 50 nucleotides. The fine-grained clay particles had induced polymerization by a process not yet fully understood.
Genesis: The Scientific Quest for Life's Origin Page 18