by Charles Baum
novel ideas fuel origins research, though most are eventually cast
aside—the victims of faulty chemical reasoning or failed experi-
mental predictions. Whatever the outcome, this story epitomizes the
exhilarating process of scientific exploration.***
In May of 2004, Simon Nicholas (“Nick”) Platts was in trouble. After
almost five years of fruitless graduate work, and approaching his forti-
eth birthday, he was about to be deported to his native Australia with
no degree and no job. [Plate 6]
Nick’s life had defied the traditional arrow-straight science career
path that most of us knew: college, graduate school, postdoc, and ten-
ure track. A gifted chemist and enthusiastic educator, he went from a
master’s degree in chemistry to teaching at Melbourne Grammar
School. Only at the advanced age of 35 did he resolve to return to uni-
versity, get a doctorate, and do some research. Life’s chemical origin
was one topic that really excited him, so he applied to Rensselaer
Polytechnic Institute in Troy, New York, to work with Jim Ferris.
His new RPI colleagues found Nick to be outgoing and supportive,
always ready to help others with their research, and eager to teach un-
THE PRE-RNA WORLD
223
dergraduate chemistry, but progress on his thesis project suffered. Af-
ter three years in Troy, he moved to Washington, D.C., and the Carnegie
Institution as a NASA-sponsored predoctoral fellow, a position that
would allow him to acquire his doctorate from RPI, but do the re-
search in a new setting with fewer distractions. He had two years left to
complete a thesis.
Nick was full of ideas. Early in 2003, he came to me with plans for
an elegant experiment on the possible influence of Earth’s magnetic
field on the origin of biological homochirality. Could I provide some
lab space?
“OK, but for how long?” I asked. My lab is small and space is at a
premium.
“Once it’s set up, the experiment should take only a couple of
weeks,” he assured me, so I offered him the necessary space for a few
months.
“No worries!” his Aussie reply.
Nick started with a flurry of activity, marking out a good fraction
of my lab bench with masking tape, cordoning off a sink that might
otherwise splash onto the lab’s delicate apparatus, assembling an elabo-
rate optical table, and filling cabinets with hardware, but his progress
soon slowed. Crucial supplies were on back order, funds were needed
for a special laser, other difficulties followed. More than a year later, the
sequestered lab space, still piled high with equipment, was gathering
dust. When pressed about his plans, Nick was vague and evasive. Many
of us feared that he would be forced to leave the Geophysical Lab
empty-handed at the end of June 2004. A sense of shared responsibility
weighed heavily on George Cody, Marilyn Fogel, and me.
EUREKA!
Nick Platts’ life changed dramatically for the better on Tuesday, May
25, 2004. That’s when the central idea for his PAH World scenario crys-
tallized. Max Bernstein, a colleague of Lou Allamandola at NASA Ames,
had planted the seeds for the concept at an RPI seminar in November
2001. Bernstein’s talk had emphasized the occurrence of polycyclic aro-
matic hydrocarbons in deep space, and it got Nick thinking about PAH
chemistry. A September 2003 conference in Trieste in celebration of
the 50th anniversary of Stanley Miller’s breakthrough experiment in-
224
GENESIS
spired more progress. “On the return flight,” Nick remembers, “I
scribbled the idea on the back of a United Airlines ticket.”
The germ of an idea gradually developed “on the mental back
burner,” but May 25th was the Eureka moment. “It was 5:51 p.m.,” Nick
recalls. “That’s when I did the drawings. That’s when I realized how big
this was.” Contrary to popular myth, there aren’t many such moments
in science.
Nick’s new idea follows in the tradition of other models of pre-
RNA genetic polymers, but with an original chemical twist. PAHs
would have been abundant among the plethora of prebiotic molecules.
(We met them back in Chapters 3 and 5 as significant components of
carbonaceous meteorites and, by extension, the cosmic debris that
formed our planet.) Each of these flat sturdy molecules consists of
fused 6-member rings of carbon atoms with hydrogen atoms around
the edge. PAHs are relatively insoluble in ocean water, but under the
influence of solar ultraviolet radiation they can be chemically modi-
fied, or “functionalized”; hydrogen atoms can be stripped off, and new
molecular fragments can take their place. If the PAH edges become
decorated with OH, for example, then their solubility in ocean water
increases significantly.
By any calculation, PAHs and their functionalized variants were a
significant repository for organic carbon on the primitive Earth. I sus-
pect that some origins chemists thought it a shame to lock up so many
potentially useful carbon atoms in the relatively inert, biologically use-
less PAHs.
But what if PAHs played a key role in the ancient ocean? What
Nick realized was that a functionalized PAH with OH around the edges
is an amphiphile—a molecule that both loves and hates water, similar
to Dave Deamer’s lipids (see Chapter 11). The flat surfaces of the car-
bon rings repel water (they’re hydrophobic), but the OH-bearing edges
of the PAHs attract water (they’re hydrophilic). So what happens when
lots of functionalized PAHs are placed in seawater? How might the
hydrophobic parts stay away from water, while the hydrophilic parts
remain in contact with the wet surroundings? The simple answer is to
assemble a pile of PAHs like a stack of plates in the cupboard. The flat
hydrophobic parts shield each other from water, while the edges form
the water-bathed outside of the stack. The PAHs self-organize.
Platts predicted that, once stacked, the system would preferentially
bind small flat molecules, like the DNA bases, to the PAH edges and
THE PRE-RNA WORLD
225
thus concentrate them from the surrounding prebiotic soup. These
baselike molecules would attach and break free in a constant game of
molecular musical chairs. Gradually, however, a selection process
would take place. Because the PAHs are loosely stacked, adjacent flat
PAH molecules would slide back and forth and rotate next to each
other, like your hands do when you rub them together on a cold day.
This mechanical action would tend to break off any edge-bound mol-
ecule that isn’t, itself, as flat as a PAH. Consequently, the small, flat,
ring-shaped base molecules (the A, C, G, and U of RNA, for example)
would bind preferentially around the edges of the stack. What’s more,
these RNA bases are also amphiphilic, so they might have a tendency
to line up more or less on top of each other, forming a kind of
ministack. Remarkably, the spacing between the PAH layers (and hence
&
nbsp; the vertical separation between bases) is 0.34 billionths of a meter—
exactly the same spacing as found between the bases of DNA and RNA.
The result of all this self-organization, according to Nick’s PAH
World hypothesis, would be a stack of PAHs decorated along the side
by an array of small flat molecules, including bases—an arrangement
that looks for all the world like the information-rich genetic sequence
of DNA or RNA.
Once bases are effectively stacked, Platts suspects, other small mol-
ecules would start to form a backbone linking the bases together into a
true information-rich molecule, though those key chemical steps re-
main fuzzy. Then a change in the ocean water’s acidity, as might be
experienced by moving from a deep hydrothermal environment to a
more shallow zone, might allow the sequence of bases to break free as a
true pre-RNA genetic molecule that could fold back on itself to match
up base pairs. Eventually, complex assemblies of these polymers might
act as catalysts for self-replication and growth.
Nick’s proposal lacked any experimental support. Nevertheless, the
PAH World hypothesis seemed to provide a geochemically plausible,
self-consistent, conceptually simple, and seamless chemical path from
the dilute soup to an RNA-like genetic sequence.
SHARING
Nick Platts’ model crystallized in a flash, but was it reasonable? Did it
make sense? He decided the best strategy was to bounce the idea off
other people.
226
GENESIS
A
B
C
THE PRE-RNA WORLD
227
D
E
(A) Nick Platts’ PAH World hypothesis rests on the ability of polycyclic molecules to self-organize into stacks. (B) Once stacked, the PAHs would tend to attract small flat molecules (notably the bases of DNA and RNA) to the edges. (C) A molecular backbone forms to link the bases into a long chain. (D) The RNA-like chain of bases
separates from the PAHs and folds into a molecule that carries information. (E) Complex assemblages of these chains have the potential to catalyze reactions. These drawings are adapted from Nick Platts’ unpublished manuscript.
228
GENESIS
I must have been about tenth on his list. Nick appeared at my of-
fice door on the afternoon of Thursday, May 27th. “You got a few min-
utes?” he asked, taking a seat. I nodded, hoping for a progress report on
the stalled thesis experiments. “I’ve found something extraordinary,”
he began. “I think I’ve discovered how life began.”
And so he described his hypothesis—the self-assembly of
functionalized PAHs, the selection of flat molecules along the edges,
the fortuitous spacing of the bases. He sat on the edge of his chair,
leaning toward me and gesticulating as he spoke. Nick was clearly ex-
hausted from almost two days without sleep; his voice took on a manic
intensity. “This is a once-in-a-lifetime moment! I’ve never been part of
anything this big!”
I was a bit taken aback by what sounded like a wildly speculative
idea. It seemed at first like another distraction, just weeks before his
scheduled doctoral defense.
“I told Dave Deamer and he loves the idea.” Nick rattled off the
names of half a dozen other origin-of-life experts he’d already con-
tacted. “No one can think of an objection.” Then, paradoxically, “We’ve
got to keep this secret. Someone else will be sure to steal the idea, so
don’t tell anyone.”
“Can you propose any experimental tests?” I asked him. A safe,
neutral question, while I considered how else to respond.
He deflected the question. “There’s lots we can do, but we have to
get this out fast. I’ve been drafting a manuscript. I’d like you to be a
coauthor. Where do you think we should submit it?”
A manuscript? Coauthorship? Nick had just raised the stakes. I
was uncomfortable with being an author unless I could contribute
something original to the paper, but I was happy to discuss publication
strategy. I thought the ideal forum for a short, high-impact outline
would be a 700-word “Brief Communication” in Nature or a similarly
concise “Brevia” in Science. Proceedings of the National Academy of Sciences was another option, but PNAS articles are generally longer, and Nick had no data yet with which to flesh out his hypothesis. He agreed
to adopt the short Nature format, while I promised to read his paper
and comment quickly.
The next day, Nick e-mailed me a 700-word draft for “Edge-
derivatised and stacked polycyclic aromatic hydrocarbons (PAHs) as
essential scaffolding at the origin of life.” I could tell from the title that
THE PRE-RNA WORLD
229
the paper was going to need some work. Even so, as I read the text I
warmed to the elegant theory.
The hallmark of any useful scientific hypothesis is that it makes
unambiguous predictions. Nick’s hypothesis made testable predictions
by the bundle. First, functionalized PAHs must self-assemble into
stacks. The stacked PAHs, furthermore, must be of similar size and
shape. PAH edges must attract a variety of molecules, but there must
be a preferential selection for flat, baselike molecules. And the bases
must also be aligned vertically. If only we could confirm at least one of
these predictions.
George Cody provided the chemical evidence that made me a be-
liever. Coal, George’s specialty, is loaded with PAHs. It turns out that
there’s already an extensive scientific literature on the ability of
functionalized PAHs to self-organize into stacks—a process known as
discotic organization. A quarter-century of publications had already
elaborated on Nick Platts’ prediction. Neither Nick nor I had ever heard
the word “discotic” until Cody mentioned it, drawing on seminars he’d
heard on the subject while working at Exxon. I returned to Nick’s
manuscript with a red pen and renewed intensity. My principal contri-
bution was to come up with a catchier title, “The PAH World.”
By Tuesday, June 1st, Nick had received comments from more than
a dozen scientists from around the world and the draft manuscript was
taking final form. He called a meeting of Carnegie coauthors—nine of
us in all—at the Lab’s library. We sat around a massive mission oak
table and worked through the paper one last time. Should we talk about
the common mineral graphite, which also has flat carbon rings? Had
we included the most appropriate references? Should we propose spe-
cific experiments? Our biggest concern was Nick’s figure, which needed
to make the essentials of the model as clear as possible, and he agreed
to redo it. He submitted the PAH World paper to Nature on Thursday,
June 3rd, just nine days after his epiphany.
OPINIONS
Nick had no time to relax. Whether or not Nature accepted the paper,
he wanted us to stake a claim. We began e-mailing the manuscript,
designated “in review,” to a dozen astrobiologists and origins experts
to request their comments. We focused on leading RNA World propo-
230<
br />
GENESIS
nents—Jerry Joyce, Leslie Orgel, Jack Szostak—figuring they would
have the most to gain from the novel idea.
Responses came quickly and with varying degrees of enthusiasm.
A few scientists who were good friends of the Lab were warmly con-
gratulatory, but most respondents remained cautious, and almost ev-
eryone cited the need for experimental evidence. Jack Szostak
responded in less than an hour with some skepticism, adding “I think
it’s worth pursuing experimentally—it would certainly be cool if an
effect could be demonstrated.” The next day Leslie Orgel chimed in,
“An experimental demonstration of your scheme might be interesting,
but I wouldn’t advise publishing without showing that it works well.”
“I thought it was interesting and certainly appealing,” echoed Jerry
Joyce. “However, it would help tremendously if there were some ex-
perimental support.”
Andy Knoll joined the chorus: “For now it is fascinating specula-
tion, but the ideas seem amenable to what you guys do best—careful
experimentation.”
Evidently the editors at Nature agreed. We received the form re-
jection letter a week later. “Thank you for submitting your contribu-
tion, . . . which we are unable to consider for publication.” Boilerplate
didn’t make their decision any easier to swallow. “Because of severe
space constraints . . . we are unable to return individual explanations
to authors. . . .”
EXPERIMENTS
Experimental evidence seemed the obvious key to securing support
for the PAH World hypothesis. Unfortunately, experiments take time,
and that was one commodity of which Nick Platts had precious little at
this stage in his graduate work.
In such circumstances, it’s best to think boldly. Nick envisioned a
single sequence of experiments that might validate every facet of his
theory. His plan: First obtain a sample of a modest-sized PAH, one
with a dozen or so interlocking rings. He settled on the elegant, sym-
metrical hexabenzocoronene (HBC), with its starlike pattern of 13
rings. Put the HBC into a flask with some water, functionalize the mol-
ecules by irradiation with an ultraviolet lamp, then measure the system
for discotic organization. That much we were pretty confident would
work, based on a survey of the discotics literature. Then add base mol-