As Brasier warmed to his task, an agitated Schopf stood up and began to pace distractingly a dozen feet behind the podium. Back and forth he walked, hunched over, hands clasped firmly behind his back—a tense backdrop to Brasier's staid delivery.
Ignoring these diversionary tactics, Brasier fired salvo after salvo. Schopf had the geology all wrong, he claimed. A new detailed geological map of the Apex area suggested that the black chert filled a cross-cutting vein—evidence that the chert had formed much later than the surrounding rocks, through the agency of hot circulating water. He outlined chemical experiments that produced cell-like chains of precipitates in a purely inorganic setting—nonliving structures similar to the supposed Apex fossils form with ease under the right chemical circumstances. He demonstrated how carbon-rich deposits might have formed nonbiologically through a familiar industrial process called the Fischer–Tropsch synthesis. He even showed his own Raman spectroscopic data of inorganic carbon that had the same broad features as the purported biological carbon of Schopf's fossils.
As Brasier calmly outlined his arguments, the scene on stage shifted from awkwardly tense to utterly bizarre. We watched amazed as Schopf paced forward to a position just a few feet to the right of the speaker's podium. He leaned sharply toward Brasier and seemed to glare, his eyes boring holes in the unperturbed speaker. After a few seconds, Schopf retreated to the back of the stage, only to return and stare again. Perhaps Schopf was just trying to hear the soft-spoken Brasier in the echoing hall, but the audience was transfixed by the scene.
The two presentations ended in due course and, after an extended period for audience questions and comments, the session concluded. Many of us breathed a sigh of relief that no blows had been exchanged, and then we tried to figure out who won. We all knew, of course, that science isn't about winning. The black smudges in the Apex Chert were either the remains of ancient microbes or they weren't. Eventually, we all assumed, the truth would be found out. A debate like the Schopf–Brasier bout did little but outline the problem and establish our collective state of ignorance. Still, we wondered: Who won?
To be sure, Schopf's intense delivery and unconventional antics hadn't won him any points among my acquaintances. Many scientists were also struck by the sudden softening of his previous claims that his fossils were cyanobacteria. Such waffling undermined a decade of confident, highly public interpretations. But Schopf is also a fine scientist with a long track record; and his systematic point-by-point analysis of the fossils, however quirky in its delivery, appeared both logical and persuasive.
Brasier's cool detachment, by contrast, seemed calculated to provide a veneer of objectivity, yet that very lack of passion and intensity may have cost him some points. So much of the Apex story relied on interpretation of fuzzy objects in a fuzzier context. As doubtful as Schopf's claims might be, it was equally difficult to disprove any biological activity by pointing to irregular black shapes. We have no way of knowing what 3.5 billion years of decay might have done to ancient microbes, and in many ways Brasier's arguments were just as subjective as Schopf's. Rather than providing the audience with the smoking gun that would thoroughly discredit Schopf, Brasier seemed merely to have raised a number of serious doubts—knotty technical issues that deserved further study.
Meanwhile, paleontologists around the world, Schopf and Brasier included, keep searching thin sections of ancient rocks in hopes of finding Earth's earliest fossils.
If there is a moral to the Allan Hills meteorite and Apex Chert controversies, it is that unambiguous identification of ancient life from microscopic structures is fraught with difficulty. Tiny rods and spheres are not always useful indicators of biology. The older the rock, the more difficult the interpretation of such vague features becomes. If fossils are to provide any clues about life's ancient emergence, then we have to look beyond microscopic structures to the tiniest fossils of all.
4
Earth's Smallest Fossils
Millions of brutal years of burial and resurfacing, akin to repeated pressure cooking, permitted very few fossilized cells to survive…. Often geologists must instead rely on other signs of life, or biosignatures—including rather subtle ones, such as smudges of carbon with skewed chemical compositions unique to biology.
Sarah Simpson, 2004
Even as the Schopf–Brasier battle raged, a small cadre of less publicized researchers labored to craft a convincing case for fossils even more ancient than Apex. This new breed of paleontologist doesn't depend on questionable black blobs. They probe rocks for fossils far smaller than microscopic cell-like spheres or segmented filaments. Remarkably, the fossils they seek consist of the very atoms and molecules of once-living organisms.
When a cell dies, its vital chemical structures quickly fragment and decay. Almost always the essential atoms of biochemistry—carbon, hydrogen, oxygen, nitrogen, and more—disperse and return to the environment. Earth's vast but nevertheless finite reservoirs of life-sustaining atoms play their parts over and over and over again. Most of the atoms in your body were once part of mastodons, dinosaurs, trilobites, even the earliest living cells. Take a moment to look at the palm of your hand and imagine the fantastic yet unknowable histories of its countless trillions of atoms. Earth's biosphere is the ultimate recycling machine.
Atoms almost always recycle, but once in a great while, under an unusual concatenation of geological circumstances, a dying organism will find itself encased in an impermeable rock tomb. If a worm is swept away and buried in a sudden mudslide, if a colony of deep-sea microbes solidifies in chert, if a winged insect dies ensnared in sticky tree sap, then it's just possible that some of the organism's original atoms and molecules will become trapped as well. Such a trapped fossil animal or microbe may persist through eons in its original form, or it may decay to a shapeless dark splotch. Nevertheless, its hermetically sealed atoms and molecules are the remains of past life, so they qualify as fossils just as legitimate as the most elegant coiled ammonite or massive dinosaur.
FOSSIL ATOMS
It's an amazing feeling to hold a 3-billion-year-old rock that once teemed with living organisms—a sample that contains the very atoms and molecules of cells from the dawn of life. Such rare and precious samples demand a new approach to the study of fossils; traditional descriptive paleontology must morph into analytical chemistry.
A casual conversation during the summer of 1997 with longtime friend Andrew Knoll, professor of paleontology at Harvard University, led me into this fascinating field. Andy and I were attending a Gordon Research Conference on the origin of life, held at New England College in Henniker, New Hampshire. He's an engaging, articulate, and friendly speaker, and the author of richly illustrated articles and lectures on the diversity of microbial fossils in Earth's oldest rocks—presentations that opened a new world to me.
When most people hear the word “fossil,” they think of the bones of a dagger-toothed Tyrannosaurus rex or the spiny shell of a trilobite—hard parts that survive the rigors of decay and burial. By contrast, the soft cellular tissues of animals, plants, and microbes almost always rot away without a trace. Only occasionally will an organism die and be buried in rock fast enough to preserve cellular detail. For a micropaleontologist like Andy Knoll, whose specialty is ancient microbes, those rare cellular fossils provide the raw material for a career in science.
The very earliest fossil cells are nondescript objects and difficult to identify, but geologists have documented dozens of localities with numerous clearly identifiable microfossils dating from about 2.9 billion years on. Distinctive bumpy rods and symmetrically spiky spheres, chainlike filaments of repeated rectangles, and curious corkscrew spirals form a panorama of primitive life. Andy's work surveys the saga of life's evolution, culminating in the first enigmatic multicellular organisms about a billion years ago.
Throughout our conversations at the Gordon conference, I was struck by the fact that many ancient microscopic fossil forms are preserved in black chert or shale—impe
rmeable rocks that have the potential to preserve chemical traces of the original bacteria. Over a beer, I asked Andy if paleontologists ever analyzed their microfossils with the kind of machines that we mineralogists routinely employed to characterize the atoms and isotopes of our samples. He shook his head and admitted that, while there had been a few pioneering studies, most paleontologists worried almost exclusively about the sizes and shapes, not the chemistry, of their bugs. Then came his deceptively innocent question: “Do you want to collaborate? I've got a couple of students with really interesting samples….”
My own research on life's emergence had to that point focused on bottom-up chemical experiments, trying to synthesize life's molecular building blocks, but the top-down approach also has great appeal. I've always loved fossils and was more than happy to associate myself with a real paleontological pro, albeit in a modest support capacity. Agreeing to the offer, I immediately envisioned an arsenal of microanalytical tools that might be brought to bear on the problem. Our conversation soon turned to technical details: the number of samples, their size, the degree of chemical alteration, and more.
The first samples arrived at Carnegie's Geophysical Laboratory from Harvard within a few months, and many more followed. A suite of 400-million-year-old plants from Canada, slices of ancient black soils from Australia, 3-billion-year-old microbial mats from South Africa, bizarre spiky spores from a billion-year-old Chinese formation—wonderful fossils holding some of the secrets of life's past. Our Carnegie team quickly confirmed that ancient fossils have the potential to provide three important types of microanalytical data: chemical elements, isotopes, and molecules. Of the three, the composition of chemical elements is arguably the easiest to measure.
The mineralogist's tool of choice for analyzing chemical elements is the electron microprobe, a costly but indispensable piece of hardware in geology departments around the world. The machine works by firing a narrowly focused beam of electrons at a highly polished piece of rock, typically an inch or so across. The energetic electron beam excites the rock's atoms, which in turn emit a spray of X-rays. It turns out that every element of the periodic table produces its own slightly different suite of X-rays of different wavelengths. The task, then, is to capture these X-rays and measure their diagnostic wavelengths.
Microprobe analysis of most elements has become routine and automated. The machine is a workhorse, operating 24 hours a day for analyses of silicon, magnesium, iron, and other rock-forming elements. But the lightest elements, including the one of greatest interest to us—the key biochemical element carbon—pose a severe analytical challenge. Lighter elements tend to be rather inefficient at producing X-rays, while the relatively few X-rays that are produced have rather low energies. Both of these factors complicate carbon analysis. What's more, we routinely use carbon to coat rock samples prior to probing, in order to make them electrically conductive. The coat is essential to prevent the sample from building up an electric charge while being bombarded by electrons, but it can mask any carbon in our samples. In short, the Lab's usual microprobe procedures wouldn't work. We'd have to devise new protocols.
The Geophysical Lab's deviser of protocols is Christos Hadidiacos, microprobe jockey extraordinaire. For more than 30 years, Chris has maintained and upgraded the Lab's electron microprobe. He knows every trick in the book and constantly invents new ones to push the limits of analysis. Complex circuit diagrams, cryptic numerical tables, and other papers decorate his office, while technical manuals and electronics catalogs fill his bookcases and rows of volleyball trophies line the shelves above.
I explained the carbon analytical problem to him—one he had never faced before. The Geophysical Laboratory probe had been used almost constantly for decades but almost always to study rocks and experimental run products—samples with mostly heavier elements. But it took Chris less than a minute to figure out a possible fix. “We can try raising the current,” he said. “That might work.” He began asking detailed questions about the expected amount of carbon and what other elements might be present. Then a frown. “We're not going to be able to use a carbon coat, are we?”
I shook my head, knowing that this was a potential deal killer. We needed an electrically conductive coating, and that meant a metal. But a carbon coat would mask our fossils, and most metals absorb X-rays so efficiently that we'd never see the low-energy carbon X-ray signal. “Any ideas?” I asked.
Again, it took him less than a minute. “Maybe we could use aluminum. Just vaporize a bit of aluminum foil.” He was smiling again, pleased at the simplicity of his solution. Aluminum, element 15, should be light enough itself to allow most of the carbon signal through. It was definitely worth a try.
I handed over the first of my fossil specimens, a pair of 2 × 3-inch rectangular thin sections of 400-million-year-old plant fossils from Rhynie, Scotland, a classic chert locality. The samples had been collected many years ago as isolated flinty boulders in old stone walls; no rock outcrop has ever been found. The precious thin sections arrived courtesy of Kevin Boyce, a bright-eyed, soft-spoken grad student in Andy Knoll's group. Kevin had trolled through Harvard's somewhat neglected paleobotany collection as part of his thesis work on the evolution of leaves. He hoped that the Rhynie samples, which preserve cellular structures of some of the oldest known land plants, might reveal clues about the chemical evolution of plants.
The first step was to apply the thin aluminum coating. Our antiquated but serviceable vacuum coating system consists of a well-worn metal housing about the size of a washing machine with vacuum pumps and hoses arranged inside. A 4-inch-square platform with wire electrodes sits on top, while vacuum gauges and control valves project from the side.
Chris snipped a 1-centimeter-square piece of aluminum foil, crumpled it up and placed it in a small wire basket attached to the electrodes. He arranged the two Rhynie chert thin sections on the metal platform, and then lowered a 2-foot-tall dome-topped bell jar so that its rubber-lined base made an airtight seal. The old pumping system labored for a quarter of an hour to achieve the desired vacuum, but then it took only a fraction of a second to apply an electric current and vaporize the aluminum foil. Aluminum atoms flew off in all directions, coating everything inside the bell jar, including our samples. Chris released the vacuum, raised the bell jar, and the fossils were ready for the probe.
The Geophysical Lab electron microprobe is an awkward-looking tabletop machine that sits in its own small room. [Plate 3] Two chairs flank the workbench, which is dominated by a 4-foot-high cylindrical tower that looks like a model of some futuristic fortification. The tower houses the electron gun, the heart of the probe. At the tower's top, a coiled tungsten filament generates electrons; a series of ring-shaped electromagnets focus the electrons into a narrow beam as they accelerate downward onto the sample. The base of the tower is cluttered with five boxlike attachments, called spectrometers, that measure X-rays, plus various vacuum lines, power cables, and viewing ports.
A curious combination of instrument panels controls the electron gun hardware and X-ray detectors. To the right, a computer monitor displays all the machine's vital statistics—beam current, spectrometer settings, sample position, and more. In sharp contrast, a 1980s-vintage slant-front console, sporting two antiquated 6-inch black-and-white video screens and more than a dozen plastic knobs reminiscent of a classic Star Trek set, dominates the central table. Like an old house that has undergone decades of renovation, the Geophysical Lab probe has been through a lot of upgrades.
I carefully secured one of the thin fossil sections into a shiny metal sample holder, closed the sample port, and waited a couple of minutes for the machine to achieve the high vacuum necessary to stabilize the electron beam. Meanwhile, Chris fiddled with the computer controls, raising the electron current to about ten times its normal settings. It took him a few moments to center and focus the intense beam onto a carbon-rich portion of our sample. We were about to discover whether or not we could detect fossil carbon atoms.
It worked! A carbon signal of 600 X-ray counts per second stood out sharply from the 30-count-per-second background. We were in business. It was a simple matter to select half a dozen areas, each about a fiftieth of an inch square, to map. Slowly but surely, the microprobe beam scanned across the sample, measuring the carbon concentration point by point. It took about 3 hours to produce one map. We put the probe on automatic, happy with our rapid progress, and headed outside to the Lab's sand court for our afternoon game of volleyball.
The analytical procedures took a bit of tweaking. The initial aluminum coatings were too thick, the beam settings not quite optimal. But within a few weeks, we were producing a steady stream of colorful maps; regions rich in carbon atoms from ancient life-forms stood out boldly against the carbon-poor fossil matrix. Cellular features less than a ten-thousandth of an inch across were clearly visible. Armed with these maps, Kevin Boyce, with his sophisticated botanical eye, was able to describe and interpret cellular detail never previously seen. [Plate 3]
Making these carbon maps, watching the fine details emerge, is great fun. Each map is formed from a two-dimensional array of point analyses, just like the pixels on your computer screen. We typically employ a quick 400 × 400-point array for reconnaissance, while slower 500 × 500-point arrays yield beautifully detailed maps with colors representing the concentration of carbon—red for the highest carbon content, followed by orange, yellow, and the other spectral colors. We play with map colors like a high-tech video game to heighten the contrast and highlight features of special interest.
Genesis: The Scientific Quest for Life's Origin Page 7