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ocean hydrothermal zones. Once they loaded and sealed the apparatus,
hot, pressurized water compressed and heated their sample container
uniformly on all sides.
The principle of the gold-bag apparatus is much the same as that
of our smaller gold-tube experiments. Both rely on soft, inert, deform-
able gold to exert uniform pressure and temperature on a fluid sample.
The great advantage of the gold-bag setup is that small samples of the
reacting fluid can be extracted every few hours or days throughout the
duration of a long experiment. But trade-offs are a fact of scientific
life. The disadvantage is that the thin-walled gold bag is frustratingly
fragile and can be a pain to use. A slight miscalculation and the bag will
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rip, ruining an experiment and requiring a tedious and exacting welded
repair. But once properly filled with reactants, the device usually works
wonderfully well.
Lemke and Ross gingerly placed the bag into a water-filled pres-
sure vessel, slowly filled the gold container with a solution of the amino
acid glycine and water, sealed the assembly, and ramped up the pres-
sure and temperature. They ran their samples for weeks, monitoring
the solution, watching for the glycine to decompose. Over time, the
concentration of glycine steadily declined, but even though they didn’t
use minerals in their experiments, they observed a much slower rate
of breakdown than had been reported in previous studies at lower
pressure.
In addition, they found a surprisingly fast rate of peptide-bond
formation—amino acids linking together to form molecular chains.
This result was unexpected, because hot water tends to break apart
rather than form peptide bonds. (That’s one reason boiled foods are so
squishy—the sturdy bonds between amino acids that give food texture
break down.) Once formed, these peptides decomposed rather quickly,
but their formation pointed to more complex behavior than had been
expected.
Lemke and Ross found hints of another potentially important be-
havior in their gold bag. Single amino acid molecules and small clus-
ters with just two or three molecules linked by peptide bonds readily
dissolve in water at room temperature, but longer peptide chains
proved much less soluble. Lemke and Ross imagine a scenario in which
peptides form rapidly in vents and are then exposed to the cooler sea-
water. Given a high enough concentration of long amino acid chains,
these molecules might separate out as a relatively stable concentrated
phase—just the kind of emergent molecular selection and organiza-
tion that life’s origin required. There’s a lot more work to be done, but
it appears that the book is not yet closed on amino acid stability in
hydrothermal systems.
FIXING CARBON
The most fundamental biological reaction—and one of our group’s
primary goals in prebiotic-synthesis experiments—is carbon fixation,
the incorporation of more carbon atoms (starting with carbon diox-
ide) into organic molecules. After all, the first chemical step in the path
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to life must be to make bigger molecules, like amino acids and sugars,
out of smaller ones, like carbon dioxide, ammonia, and water. Such
reactions occur rapidly in our experiments, but they follow two rather
different paths, depending on the mineral employed.
Many common minerals, including most oxides and sulfides of
iron, copper, and zinc, promote carbon addition by a routine, industri-
ally important process known as the Fischer–Tropsch (F–T) synthesis.
In its idealized form, F–T synthesis builds long chainlike organic mol-
ecules from carbon dioxide and hydrogen that are exposed to hot, dry
metal surfaces. Our gold-tube experiments and studies in several other
labs display similar reactions in the presence of wet mineral surfaces at high pressure, though a lot less efficiently than the industrial process.
Field studies complement these experiments. Recent intriguing
analyses of organic molecules emanating from hydrothermal vents re-
veal similar Fischer–Tropsch-like products, and it now appears that
F-T synthesis constantly manufactures larger organic molecules from
smaller building blocks in Earth’s hydrothermal zones. Many of these
molecules are hydrocarbons of the type that form petroleum. (Who
knows, maybe Tommy Gold is correct and at least some petroleum
forms abiotically at depth.)
Alternatively, when we use nickel or cobalt sulfides, we observe
that carbon addition occurs primarily by the insertion of carbon mon-
oxide, a molecule with one carbon atom and one oxygen atom, which
readily attaches itself to nickel or cobalt atoms. By repeating these
simple kinds of reactions—add a carbon atom here, an oxygen or hy-
drogen atom there—over and over again, new and more complex or-
ganic molecules emerge.
One conclusion seems certain: Mineral-rich hydrothermal systems
contributed to the early Earth’s varied inventory of potential bio-build-
ing blocks. With tens of thousands of miles of deep-ocean hydrother-
mal ridges, billions of cubic kilometers of warm wet crust, and
hundreds of millions of years to process the raw materials, organic
molecules must have been produced in prodigious amounts. But the
Geophysical Lab synthesis experiments have done more than simply
add to the catalog of interesting molecules that could have been formed
on the early Earth. These experiments are now uncovering something
quite new about the possible role of minerals in the origin of life.
Previous origin-of-life studies, such as those of Günter
Wächtershäuser, treat minerals essentially as solid and relatively stable
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119
platforms for synthesis and assembly of organic molecules. But our
experiments reveal another, more complex behavior that may have
important consequences for origin-of-life chemistry. We find that in
the presence of high-temperature and high-pressure water, minerals
often start to dissolve. In the process, the dissolved atoms and mol-
ecules from the minerals themselves become crucial reactants in the
prebiotic milieu. Sulfur, dissolved from sulfide minerals, combines with
carbon dioxide and water to form thiols and thioesters—reactive mol-
ecules that can jump-start new synthetic pathways.
Even more dramatic is the behavior of iron, which can dissolve in
water to form brilliantly colorful organic solutions. After one experi-
mental run, George Cody bounced from office to office on the second
floor, showing off a particularly striking orange-red solution he had
just extracted from a pressure capsule. The deep color was exciting
because it pointed to the formation of iron complexes—iron atoms
surrounded by a starburst of organic molecules. Chemists have long
known that similar iron complexes promote chemical reactions, so
Cody speculated that our cheerful solutions might contain a kind of
primitive catalyst that promoted the as
sembly of more complex mo-
lecular structures.
Such behavior is not entirely unexpected, for hydrothermal fluids
are well known to dissolve and concentrate mineral matter. Many of
the world’s richest ore deposits arise from hydrothermal processes.
Similarly, spectacular sulfide pillars tens of meters tall grow rapidly at
volcanic vents called black smokers, where rising plumes of hot, min-
eral-rich solutions contact the frigid water of the deep ocean.
Yet there’s so much we don’t know about hydrothermal systems
and the chemical processes that might occur in their vicinity. And in
spite of their prevalence, the role of these dissolved ingredients has not
yet figured significantly in origin scenarios. No one yet knows how this
rich mix of organic compounds and dissolved minerals might influ-
ence the synthesis and assembly of biomolecules. But we’re poised to
find out.
9
Productive Environments
The limits of life on this planet have expanded to such a
degree that our thoughts of both past and future life have been
altered.
Kenneth Nealson, 1997
Even as the debate between Miller’s advocates and the ventists was
heating up, an explosion of new research dramatically changed the
research community’s view of the emergence of biomolecules. When
Miller first reported organic synthesis on a benchtop in 1953, the re-
sults seemed almost magical. Fifty years ago, no one could have pre-
dicted how easy it would be to make amino acids, sugars, and other key
biomolecules from water and gas. But the more scientists study carbon
chemistry in a wide range of plausible, energetic prebiotic environ-
ments, the more diverse and facile organic synthesis seems to be. It
now appears that anywhere energy and simple carbon-rich molecules
are found together, a suite of interesting organic molecules is sure to
emerge. It’s all a matter of environment, and it now appears that the
universe boasts an extraordinary range of productive environments.
MOLECULES FROM DEEP SPACE
The last place you might think to look for life-forming molecules is the
black void of interstellar space, but new research reveals that organic
molecules from space must have predated Earth by billions of years.
Deep space, we now realize, is home to immense tenuous clouds where
carbon, hydrogen, oxygen, and nitrogen combine in complex sequences
of reactions.
A research team at NASA Ames Research Center at Moffett Field,
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GENESIS
California, led by veteran astrochemist Louis Allamandola, has simu-
lated the ultracold deep-space environments of these so-called dense
molecular clouds (though these vast volumes of dust and gas are far
less dense than the highest vacuum attainable on Earth). A typical in-
terstellar cloud harbors only a measly million atoms per cubic inch, at
temperatures colder than –100°C. Such high vacuums and frigid tem-
peratures would seem to preclude any sort of chemical reaction, but in
these remote regions, minute ice-covered dust particles are subjected
to ultraviolet radiation from distant stars. Gradually, as molecules ab-
sorb this radiation, they become sufficiently reactive to form larger
collections of atoms. Radio astronomers have long recognized the dis-
tinctive signatures of numerous organic species in these clouds. Each
type of molecule absorbs or emits characteristic wavelengths of light—
features that appear as sharp lines on a radio spectrum. The most abun-
dant molecules are the diatomic and triatomic species, such as CO, H ,
2
CO , and H O, but more than 140 different compounds are known,
2
2
including many larger molecules with a dozen atoms or more.
Theorists easily explain such molecular diversity. They calculate
the efficiency with which small cold molecules condense onto tiny dust
particles, forming submicroscopic ice coatings. They predict details of
how icy particles occasionally absorb ultraviolet radiation, which can
shuffle electrons and trigger chemical reactions. They plot reaction cas-
cades by which small groups of atoms clump together and slowly cause
new larger molecules to accumulate in the cloud. Eventually, under the
pervasive inward pull of gravity, local regions of a molecular cloud can
collapse into a new planetary system with a central massive star and an
array of planets and moons. As each body forms, a steady rain of
organic-rich comets and asteroids contributes to the life-forming in-
ventory. So, the theorists tell us, organic molecules inevitably consti-
tute part of any planet-forming mix.
Regardless of how convincing a theory may sound, experiments
carry a lot of weight in science. Allamandola and co-workers’ experi-
ments at NASA Ames have exploited an elegant chilled vacuum cham-
ber, about 8 inches in diameter, crafted of shiny stainless steel, and
equipped with thick glass observation ports, to produce suites of or-
ganic molecules. First, they introduce a fine spray of simple gas mol-
ecules, such as water, carbon monoxide, methane, and ammonia, into
the chamber, where the gases freeze onto an aluminum disk. Then they
bathe the thin ice layer with a beam of ultraviolet radiation, which
PRODUCTIVE ENVIRONMENTS
123
triggers the formation of larger molecules—compounds that match
the distinctive molecular emissions from those distant clouds.
[Plate 5]
The NASA team has used their benchtop apparatus to produce a
rich variety of interesting molecules: reactive nitriles, ethers, and
alcohols abound, as do ringlike hydrocarbons. One set of experiments
yielded nitrogen-bearing precursor molecules to amino acids. Another
set generated long chainlike molecules reminiscent of the building
blocks of cell membranes.
Evidence from space amply buttresses these nifty experiments. The
Murchison meteorite and many other carbon-rich meteorites are
loaded with organic molecules thought to be of extraterrestrial origin.
Comets, too, are known to be rich in the molecular precursors of life,
as are the microscopic interplanetary dust particles that incessantly
drift down to Earth’s surface. Armed with their vacuum chamber, the
Ames team can reproduce the supposed deep-space synthesis processes
in the lab. Theory, observations, and experiments agree: The prebiotic
Earth was seeded abundantly with extraterrestrial organics.
Nevertheless, the Miller crowd is unpersuaded by these studies,
too. Says Miller, “Organics from outer space, that’s garbage, it really is.”
Jeff Bada echoes, “Even if cosmic debris struck the prebiotic Earth at
10,000 times the present levels, the resultant prebiotic soup would still
have been much too weak to engender life.”
MOLECULES FROM GIANT IMPACTS
Meteorites and comets carry a rich inventory of organic molecules, but
can these molecules survive the catastrophic insults of collisions with
Earth? Deep-space synthesis, no matter how
fecund, would be irrel-
evant to life’s origin if the intense temperatures and pressures of im-
pact disintegrated molecules.
It’s hard to imagine an environment more destructive to life and
its molecules than the shattering surface impact of an asteroid or
comet. Nevertheless, carbon-rich meteorites like the Murchison con-
tain a significant store of amino acids and other potential biomol-
ecules; evidently impacts don’t destroy all organic molecules. In fact,
recent experiments suggest quite the opposite. Jennifer Blank and her
colleagues at the Lawrence Berkeley National Laboratory, in Berkeley,
California, use a giant experimental gas gun that hurls hyperfast chunks
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GENESIS
of metal at innocent rocks. Their goal is to trace the fates of organic
molecules during these violent collisions.
Blank’s experiments begin with a flattened cylindrical stainless-
steel capsule about 1 inch in diameter that is filled with a solution of
five different amino acids in water. She carefully positions the sealed
sample in a metal well—the target at the end of a 40-foot-long gun
barrel.
“Clear the room!” she demands, as they close the gun chamber.
A technician powers up her weapon. “Three, two, one, fire!” and
blam! , a tremendous shock wave shakes the building as a massive metal
projectile hurls down the barrel at more than 4,000 miles per hour and
squashes her neatly prepared sample like a bug. For a few microsec-
onds, the amino acid solution experiences pressures in excess of
200,000 atmospheres at temperatures approaching 900°C.
Then the fun begins. Blank pries out her deformed steel capsule
and mills down the metal to extract a few drops of liquid. The original
clear solution has turned a dark brown color—something interesting
has happened to the amino acids. The organic chemists’ standard ana-
lytical techniques, chromatography and mass spectrometry, tell the
story. To be sure, most of the original amino acids are lost in every run.
But, remarkably, some of the delicate molecules react with each other
to form pairs of amino acids. The formation of these peptide bonds
between amino acids is a crucial step in the assembly of proteins.
Jennifer Blank’s highly publicized conclusion: Impacts on the early