by Charles Baum
We saw the Miller camp as self-serving, defending old ideas while dis-
counting new possibilities. Such turf-protecting tactics are not un-
known in science, but we were more than a little dismayed by such
focused efforts to thwart what seemed an interesting and testable idea.
Our sense of a conspiracy arrayed against us was heightened when
anonymous reviewers recommended rejecting one of our grant pro-
posals on the basis of familiar, party-line criticisms.
The most maddening aspect of Stanley Miller’s criticisms wasn’t
his persistence or even the public way he voiced them. The most mad-
dening aspect was that he was basically right. Our first experiments
were admittedly flawed. Our starting mixtures of pyruvate were much
more concentrated than any plausible prebiotic solution; indeed, un-
stable pyruvate is unlikely as an important prebiotic reactant in any
environment. What’s more, we couldn’t control the acidity inside our
gold capsule (acidity can drastically alter reaction pathways), we
couldn’t be sure that all our products formed at high temperature (as
opposed to during cooling), and we hadn’t proved that the gold was an
inert container (might the gold have triggered the observed reactions?).
In short, it was a typical start to a complex scientific research program.
The art of science isn’t necessarily to avoid mistakes; rather,
progress is often made by making mistakes as fast as possible, while
avoiding making the same mistake twice. After several hundred experi-
ments under a wide range of pressure and temperature conditions, we
began to realize that water, carbon dioxide, and nitrogen are not suffi-
cient by themselves to synthesize very many useful biomolecules. Un-
der the right circumstances we could trigger some interesting organic
reactions, but many key biomolecules were missing from the inventory
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of our products; and the ones we did manage to make were indiscrimi-
nate, with numerous molecules of no possible use to biology. We
needed some other chemical ingredient, or ingredients, and so our re-
search led us back to my specialty—mineralogy. Minerals, it turns out,
have the potential to promote key chemical reactions, while protecting
the products from the ravages of high temperature.
Before biology, the only raw materials our planet had to play with
were the ocean, the atmosphere, and rocks. It now appears that rocks
and minerals, in roles as varied as containers, templates, catalysts, and
chemical reactants, may provide a key to understanding life’s ancient
origins.
ENTER WÄCHTERSHÄUSER
The view of minerals’ roles in the origin of life came to the fore with
the publication of a seminal paper in 1988: “Before enzymes and tem-
plates: Theory of surface metabolism,” by the brilliant, combative
Günter Wächtershäuser. Wächtershäuser, a chemist by training but a
Munich patent attorney by day, erected a sweeping theory of organic
evolution in which minerals—mostly iron and nickel sulfides, which
abound at deep-sea volcanic vents—provided catalytic, energy-rich
surfaces for the synthesis and assembly of life.
Among the many striking aspects of Wächtershäuser’s grand
scheme is the unusual extent to which he roots his theory in the phi-
losophy of science. Most researchers tend to ignore science philoso-
phers, whose work often seems removed from the practical day-to-day
details of experimentation. But Wächtershäuser is an avowed disciple
(and a personal friend) of the influential philosopher of science Karl
Popper, who encouraged the new theory’s development. “During
breakfast I mentioned my ideas on an alternative and he urged me to
work it out,” Wächtershäuser told a New York Times reporter not long
ago.
According to Popper’s dictum, theories, to be theories at all in the
scientific sense, must make testable predictions that might be proved
false by empirical tests. A theory that makes no testable predictions is
pseudoscience according to Popper. By the same token, the more pre-
cise (and, therefore, the more falsifiable) its predictions, the more be-
lievable a theory becomes when it survives scrutiny.
Most origins researchers before Wächtershäuser had adopted an
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GENESIS
ad hoc, trial-and-error approach: They cooked up likely geochemical
recipes to see what worked, then patched together a theory around
those observations. Stanley Miller’s experiment made amino acids, so
his followers became convinced that that’s how it happened in nature.
“Ventists,” who found primitive life in hydrothermal zones, often fell
into the same trap. Theories flourished around particular sets of ob-
servations rather than grand falsifiable frameworks. By contrast,
Wächtershäuser set out to formulate his theory from scratch, starting
with a few plausible assumptions about the nature of the first living
entity.
Assumption 1. Reject the “primordial soup” concept. In
Wächtershäuser’s model, random prebiotic synthesis plays no essen-
tial role in life’s origin. He argues that the prebiotic emergence of
biomolecules à la Miller–Urey was irrelevant because the primordial
soup was too dilute and contained too many molecular species that
could not contribute to life. It takes a lot of nerve to throw out decades
of research, but that’s Wächtershäuser’s style.
Assumption 2. The first life-form made its own molecules. Most
other workers assume that the first living entity scavenged amino
acids, hydrocarbons, and other useful molecules from its surround-
ings—a strategy called heterotrophy (from the Greek for “other nour-
ishment”). Wächtershäuser denies that possibility, since the soup was
so dilute and unreliable. He counters that the first life must have been
autotrophic (“self-nourishing”), manufacturing its own molecular
building blocks from scratch.
Assumption 3. The first life-form relied on the chemical energy of
minerals, not the Sun. Sunlight and lightning are both too violent, and
they are uncharacteristic of the energy sources most cells use today.
Moreover, photosynthesis is an immensely complex sequence of
chemical reactions, requiring numerous proteins and other specialized
molecules. Surface reactions on minerals, by contrast, are simple and
similar to the synthesis strategies central to many cellular processes
today.
Assumption 4. Metabolism came first. Many researchers claim that
life requires encapsulation and membranes, but simple membranes are
ill suited to let food in or waste out. Others, seduced by discoveries of
modern molecular genetics, are convinced that life began with a self-
replicating genetic molecule like RNA (see Chapter 16), but even the
simplest genetic molecule is vastly more complex than anything in the
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Miller–Urey soup. Wächtershäuser, in sharp contrast to these more
conventional ideas, assumes that lif
e began with metabolism, which he
defines as a simple cycle of chemical reactions that duplicates itself.
These key ideas are amplified by one additional assumption, com-
mon to all origin theories—that of biological continuity. Today’s bio-
chemistry, no matter how intricate and dependent on specialized
catalysts, has evolved in an unbroken path from primordial geochem-
istry. Thus, for example, Wächtershäuser’s scheme builds on the obser-
vation that many of the molecules that enable today’s living cells to
process energy have, at their core, a mineral-like cluster of iron or nickel
and sulfur atoms.
But Wächtershäuser’s theory is much more than a list of assump-
tions. Over 100 pages of detailed chemical reactions—testable steps
that lead from simple geochemical raw materials to biology—amplify
his epic proposition. “You don’t mind if I brag a little,” he told a re-
porter for Earth in 1998, “but something like this has never been done
in the entire field.”
We’ll look at the details of Günter Wächtershäuser’s theory in
Chapter 15, but the central chemical idea is that iron and nickel sul-
fides, notably the common iron–sulfur minerals pyrrhotite and pyrite,
served as template, catalyst, and energy source for biosynthesis. In
Wächtershäuser’s view, simple molecules like carbon monoxide and
hydrogen react on sulfide surfaces to produce larger molecules. These
molecules tend to have negative charges and so they stick to the posi-
tively charged sulfide surfaces, where additional reactions build larger
and larger molecules. Various surface-bound molecules begin to feed
off each other, eventually forming a chemical cycle that copies itself.
Voilà! It’s alive! In Wächtershäuser’s view, this emergent process is both
inevitable and fast. “It takes maybe two weeks,” he estimated in answer
to a question at a Carnegie seminar.
Wächtershäuser’s theory has proven more than a little controver-
sial, and it has evoked much discussion and comment, both pro and
con. “There’s probably nothing there because, otherwise, people would
have found it already,” Jeffrey Bada says. In a recent paper in Science, he and his colleague Antonio Lazcano dismiss Wächtershäuser’s originality: “It’s not a new idea,” they write, pointing to a 1955 paper by the
Lithuanian-American microbiologst Martynas Ycas—a two-page
“Note on the origin of life” in the Proceedings of the National Academy
of Sciences that had comparatively little impact on the origins commu-
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GENESIS
nity. Whatever one thinks of Wächtershäuser’s full-fledged and widely
cited theory, it cannot be dismissed as a mere restatement of earlier
ideas.
More than most scientists, Wächtershäuser seems to keep track of
who cites his work, who offers praise, who follows up with relevant
experiments. Though we had never met, he considered our Carnegie
group a friend. His provocative theoretical ideas had influenced our
experimental program. We had focused on the possibility that in the
presence of minerals—especially iron and nickel sulfides—metabolism
can proceed without protein catalysts. Many of our experiments served
to test his ideas. So in January 1998, as a member of the Lab’s seminar
committee, I invited Wächtershäuser to give a lecture. He responded
by phone less than a week later, and we agreed on a visit near the end of
March.
Intense and earnest, combative and to the point, Wächtershäuser
delivered his lecture like a legal summary to a packed, attentive room.
An hour proved not nearly enough to touch on the philosophy and
outline of his epic hypothesis, but many of us had read his papers and
followed as one does a favorite, oft-told tale. When he did touch on
other theories or dissenting views, he dismissed them quickly, effi-
ciently, as if he were brushing away a pesky fly.
I should be clear: None of us was a Wächtershäuser disciple, nor
did any of us believe everything he proposed; indeed, the idea that
prebiotic chemistry and the primordial soup played no role in life’s
origins runs counter to the assumptions of many of our hydrothermal
synthesis experiments. But his origin-of-life model is amazingly rich
and detailed. Virtually every chemical step is testable by experiments.
Nothing is fuzzy or left to speculation. His daring lies in the prediction
of specific chemicals that arise by specific reaction pathways. Right or
wrong, Wächtershäuser has produced a synthesis that will be studied
by scientists for decades to come (and historians of science long after
that).
Following the seminar, several of us were eager to help test the
model in any way we could. That afternoon we showed him our exten-
sive experimental facilities, described our origins research, and raised
the possibility of collaboration. Our offer was for him to propose the
experiments and be the lead author on any resulting publications. A
week later I wrote, “We would like to explore the possibility of col-
laborative experimental work, in particular the effects of modest pres-
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sure on the rates of reactions you propose.” He received our offers
politely, but was clearly reluctant and declined to accept our help. I
was left with the strong impression that Günter Wächtershäuser is
eager to maintain control over both his theory and its experimental
verification.
EXPERIMENTS
The hypothesis that minerals might enhance the stability of some
biomolecules received a boost from a series of experiments in 1999–
2000 conducted by my Carnegie colleague Jay Brandes. Brandes had
arrived at the Geophysical Lab fresh from his 1996 University of Wash-
ington PhD in oceanography—a study of nitrogen chemistry in the
Central Amazonian Basin of Brazil. Jay’s original proposal for work at
the Lab was to elaborate on the nitrogen work, but he soon shifted his
efforts to our origin-of-life program. It proved a good decision.
His first paper at Carnegie, “Abiotic nitrogen reduction on the early
Earth,” appeared in the September 24, 1998, issue of Nature and imme-
diately gained widespread attention. Brandes demonstrated that
hydrothermal vents could have provided a reliable source of ammo-
nia—an essential nitrogen-bearing compound for prebiotic synthesis.
He followed up by demonstrating the synthesis of amino acids under
ammonia-rich vent conditions.
In spite of these successes, studies by Stanley Miller’s group on the
rapid breakdown of amino acids had raised a question that would not
go away. Miller and his colleagues objected to the hydrothermal-ori-
gin hypothesis, in part, because amino acids decompose rapidly at the
elevated temperatures of ocean vents. How could we be making ap-
preciable quantities of amino acids in our runs at 200°C, if these
chemicals aren’t stable? So Jay’s next round of experiments focused on
the breakdown of amino acids under a variety of geochemically rel-
evant co
nditions.
Brandes’s new experiments found evidence that the amino acid
leucine, which breaks down in a few minutes in 200°C pressurized wa-
ter, may persist for days when pyrrhotite, an iron–sulfur mineral com-
monly found at submarine volcanic vents, is added to the mix. While
the exact protection mechanism is still under study, Brandes’s experi-
ments seemed to demonstrate that minerals may greatly enhance the
stability of essential biomolecules.
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GENESIS
Two other intriguing lines of research point to the complexity of
determining amino acid stability. One recent set of experiments fo-
cuses on fossil bones, which often preserve ancient proteins. Fossil
bones are composite materials in which a strong but brittle mineral
matrix interweaves with flexible fibers of the proteins osteocalcin and
collagen. Unprotected, these proteins would break down in a matter of
a few centuries, but some fossil bones are known to preserve proteins
for many millions of years. Recent work in Andrew Steele’s laboratory
has even revealed hints of fossil collagen in dinosaur bones more than
70 million years old. The secret to such exceptional preservation is
strong bonding between minerals and the amino acid constituents of
the proteins. Minerals can thus protect and preserve amino acids.
Additional evidence for amino acid survivability comes from ex-
periments conducted by Stanford graduate student Kono Lemke and
geochemist David Ross of the U.S. Geological Survey laboratories in
Menlo Park. Working with financial support from our NASA Astrobi-
ology Institute grant, they placed amino acids in a superhot pressure
cooker and watched for them to decompose. But, unlike the protocol
of our restrictive gold-tube experiments, Lemke and Ross employed an
exotic flexible “gold bag” apparatus—a great improvement on the
sealed-tube experiments.
The heart of the gold-bag apparatus is a thin-walled flexible bag
about the size of a grapefruit, meticulously crafted from pure gold foil.
The bag opens at one end into a titanium-valve system, with which
Lemke and Ross filled and emptied their experimental solutions. They
immersed the entire gold assembly in a water-filled pressure chamber
that was compressed to several hundred atmospheres and heated to
several hundred degrees—conditions similar to those found at deep-