by Charles Baum
part of a cross-cutting vein. Given this relationship, with the vein of
chert cutting across older rocks, the exact age of the Apex Chert was
called into question. More damning still, the hydrothermal setting sug-
gested that the chert formed at temperatures far above the permissible
limits for life.
Brasier et al. challenged Schopf ’s claims in an article titled “Ques-
tioning the evidence for Earth’s oldest fossils,” published in 2002 in the
widely read journal Nature. Their bold conclusion: “We reinterpret the
purported microfossil-like structure as secondary artifacts.” The ar-
ticle was a very public attack on Schopf ’s credibility.
In an unusual move, the editors of Nature had delayed the Brasier
et al. article for more than a year, to allow Schopf time to prepare a
rebuttal, “Laser-Raman imagery of Earth’s earliest fossils.” The two con-
flicting articles appeared back-to-back in the March 7, 2002, issue. An
42
GENESIS
accompanying “News and Views” analysis by Nature staffer Henry Gee
emphasized the irony of Schopf ’s predicament.
Seldom has a scientific debate held such high drama. Schopf had
made his reputation in part by staking claim to Earth’s oldest life, while
cutting no slack for the questionable claims of others. More than any
other scientist, he had thrown cold water on the NASA pronounce-
ment of life on Mars. He reveled in reminding the public of past pale-
ontological follies. No wonder then that science journalists were quick
to highlight the controversy: “CRADLE OF LIFE OR CAULDRON OF
CRUD?” one news headline asked.
This debate came to a head on April 9, 2002, at the second biennial
NASA Astrobiology Science Conference, with Schopf and Brasier
squaring off like graying, bespectacled wrestlers. The entertaining spec-
tacle took place deep inside the gargantuan antique dirigible hanger of
Moffett Field, 30 miles south of San Francisco, which is home to the
NASA Ames Research Center. A sturdy lectern embossed with the
NASA logo stood on the stage, to the left of a large projection screen
about 12-feet square. Both speakers were seated on the stage, before a
rapt audience of several hundred scientists.
Schopf spoke first. A flamboyant presenter even under the calmest
of circumstances, Bill Schopf was fighting to preserve his scientific
reputation. Barely controlling his anger, his voice booming, he lectured
Brasier as if the Englishman were a recalcitrant schoolchild. Step by
step, in a talk rich in withering rhetorical questions and exaggerated
dramatic pauses, he reviewed the dozen or so necessary and sufficient
criteria to establish the authenticity of ancient fossil cells. Step by step,
he provided the data to back up his Apex claim, though he did soften
his assertion that the microbes were oxygen-producing cyanobacteria.
After 15 minutes or so, the moderator gestured that Schopf ’s allot-
ted time was almost up. Like a magician pulling a rabbit out of a hat,
Schopf concluded by displaying new analytical data that he claimed
would prove his case once and for all. The smudgey black Apex Chert
“fossils” are composed principally of carbon, the essential element of
life. Carbon concentrations may arise by both biological and nonbio-
logical processes, so carbon in and of itself is not diagnostic of life.
However, Schopf claimed, there is a difference: The carbon remains of
fossil cells are less perfectly ordered than crystalline carbon deposited
as a lifeless mineral. The degree of crystallinity, furthermore, can be
revealed by the established technique of Raman spectroscopy. Schopf
LOOKING FOR LIFE
43
grandly presented a suite of Raman spectra: Indeed, sharp spiky peaks
characteristic of inorganic carbon stood in sharp contrast to the “obvi-
ously biological” broad humps in the Raman spectra from the Apex
Chert. Schopf concluded by summing up all the evidence he had mus-
tered: “If it fits with all other evidence of life, well follks, most likely it’s
life.” [Plate 3]
Brasier gently ascended the stage and began his rebuttal with a
dismissive putdown of his rival’s presentation: “Well, thank you, Bill,
for a truly hydrothermal performance. More heat than light, perhaps.”
In soft-spoken Oxford English, the tone in sharp contrast to what had
come before, he began to cast doubt on Schopf ’s case. The most damn-
ing evidence were the fossils themselves. With the right lighting, field
of view, and level of focus, the Apex features do look like strings of
cells. The size is right, the shape more than a little convincing, and
there are even regularly spaced dark divisions that look like cell walls.
But raise or lower the focus slightly, or shift to another field of view,
and doubts arise. What are all those shapeless black blobs next to the
“fossil?” How can that supposed straight chain of cells suddenly branch
like a “Y”?
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 geologi-
cal 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 pre-
cipitates in a purely inorganic setting—nonliving structures similar to
the supposed Apex fossils form with ease under the right chemical cir-
cumstances. 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 spectro-
scopic 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
44
GENESIS
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 collec-
tive 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 con-
fident, 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 pro-
vide 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 bio-
logical 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 subjec-
tive 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 find-
ing Earth’s earliest fossils.
LOOKING FOR LIFE
45
If there is a moral to the Allan Hills meteorite and Apex Chert contro-
versies, it is that unambiguous identification of ancient life from mi-
croscopic 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 publi-
cized researchers labored to craft a convincing case for fossils even
more ancient than Apex. This new breed of paleontologist doesn’t de-
pend on questionable black blobs. They probe rocks for fossils far
smaller than microscopic cell-like spheres or segmented filaments. Re-
markably, 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 en-
vironment. Earth’s vast but nevertheless finite reservoirs of life-sus-
taining atoms play their parts over and over and over again. Most of
the atoms in your body were once part of mastodons, dinosaurs, trilo-
bites, 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
47
48
GENESIS
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 fos-
sil 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 mas-
sive 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 trilo-
bite—hard parts that survive the rigors of decay and burial. By con-
trast, 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 mi-
crobes, 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 nu-
merous 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 spi-
EARTH’S SMALLEST FOSSILS
49
rals form a panorama of primitive life. Andy’s work surveys the saga of
life’s evolution, culminating in the first enigmatic multicellular organ-
isms 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 pre-
served in black chert or shale—impermeable rocks that have the po-
tential 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 char-
acterize 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. Agree-
ing 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 Af-
rica, 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 poten-
tial 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 hard-
ware 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
50
GENESIS
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
