Reflections From a Warm Little Pond

Back
in 1953, Jim Kasting said, scientists thought they had the origin
of life figured out. Chemists Stanley Miller and Harold Urey at
the University of Chicago had simulated that crucial instant around
3.9 billion years ago when a batch of simple inorganic molecules,
zapped by a bolt of lightning (or maybe just the sun’s warmth during
a break in the clouds), fell together to form the prototypes for
the complex organic compounds that life is made from.

Now that was a moment. Remember it on Star
Trek? The muddy puddle of ooze on the edge of Nowheresville?
The awful humidity? The onset of bubbling? Before, everything was
dead as Play-doh. After came a chain of eye-popping events that
just keeps unfolding, across the eons, into alligators and astronauts,
puppies and banana figs, mosquitos and lichens and particles of
ebola virus . . .

In their lab, Miller and Urey shot flashes of lightning,
in the form of cascades of sparks, through a flask containing an
“ocean” of liquid water and an “atmosphere” of strongly reduced (that is, hydrogen-rich)
gases – methane, ammonia, hydrogen sulfide, and water vapor. After
a couple of days, they tested what was left. “They had formed all
sorts of compounds,” Kasting said, “including large quantities of
amino acids,” the molecules that join to form proteins. This simple
experiment seemed to corroborate a vision Darwin (and not Gene Roddenberry)
had described a hundred years earlier, of life emerging “in some
warm little pond, with all sorts of ammonia and phosphoric salts,
light, heat, electricity, etc., present.”

But
the Miller-Urey experiment, important as it was, had a flaw. Urey
had based his primitive-Earth atmosphere on astronomical data just
then coming in, the first spectra from the giant planets in our
Solar System: Jupiter, Saturn, Uranus, and Neptune. These characteristic
bands of color showed that the giants were swathed in atmospheres
rich in methane and ammonia, thought to be left over from the planets’
formation.

At the time, people thought all of the planets
had once shared a “primordial” atmosphere, the result of their common
birth. Because of their stronger gravity, the giants were believed
to have retained this early atmosphere, while the atmospheres of
Earth and the other, smaller planets had lost some of their lighter
gases, hydrogen among them, to space. Thus, Urey reasoned, an early
Earth atmosphere, before its hydrogen had escaped and the life-driven
process of photosynthesis had boosted its oxygen, would have been
a lot like a present-day giant’s.

Shortly after the Miller-Urey experiment was published,
however, geologists came up with new findings on Earth’s volcanic
emissions – and threw the old reasoning for a loop. “What comes
out of volcanoes is not methane and ammonia,” Kasting said, “but
about 80 percent water vapor, 15 to 20 percent carbon dioxide, and
traces of carbon monoxide and molecular hydrogen.” James C. G. Walker,
one of Kasting’s graduate advisers at the University of Michigan
during the 1970s, took these emissions data and balanced them against
the rate at which hydrogen would be expected to escape from a planet with Earth’s
gravity. (“He did all this stuff on the back of an envelope,” Kasting said.) What Walker
came up with was a much different picture of Earth’s early atmosphere:
an oxygen-rich mix of carbon dioxide, nitrogen, and water vapor.

The catch is that oxygen, although an absolute necessity
for multicellular, advanced life, is poison to pre-biotic synthesis.
Do a Miller-Urey experiment in an oxygen-rich atmosphere, Kasting
said, and “you don’t form things like amino acids. There are too
many oxygen atoms in there.” So, over the years, “enthusiasm for
the warm little pond theory has waned.”

Two competing theories have
emerged instead. The discovery of microbes and other small organisms
living in and around hydrothermal vents – underwater hot springs
boiling from the ocean floor – has led to the idea that life may
have started at the bottom of the sea. Sharp differences in temperature
and oxygen concentration at the boundaries around these vents make
good catalysts for chemical reactions, Kasting said. “The problem
with this theory is that the complex organic compounds likely to
form life cannot remain stable for long at such high temperatures.”
Amino acids, instead of joining up, would tend to break down.

The other scenario has life first coalescing in
the frigid climes of outer space – specifically, within the cold
dark hearts of interstellar dust clouds. “Long, complex organic
molecules can be made when ionizing radiation leads to ion-molecule
reactions,” Kasting explained. “The intense cold prevents them from
breaking down.” In this so-called “seeding from space” model, these
complex molecules are brought to Earth by incoming meteorites and
comets. The weak link here is that most of a meteor is vaporized
on impact with our atmosphere. “The survival potential for organisms
is low. They get pyrolized: Burned to a crisp.”

Kasting,
for his part, is not ready to give up on the warm little pond. Using
computer models of light-triggered atmospheric processes, he is
working to reconcile Darwin’s vision with the constraints imposed
by a relatively oxygen-rich atmosphere.

“My idea,” Kasting said, “is that this atmosphere
did contain some methane: just enough to allow for the formation
of hydrogen-cyanide molecules, one of the key starting materials
for making both amino and nucleic acids. Ten to 100 parts per million
would be enough.”

Present-day life, he explained, requires three types
of molecules: DNA, to store the genetic information that allows
cells to replicate; RNA, which transfers that genetic information
from the nucleus to the rest of the cell; and the proteins that
catalyze these reactions. “It’s a very complicated system.” Yet
in 1989, molecular biologists Thomas Cech of the University of Colorado
and Sidney Altman of Yale shared a Nobel prize for showing that
under some circumstances RNA can replicate on its own. Not only
that, but it can store genetic information.

RNA, in other words, can do it all. “Early life
is now believed to have passed through a stage in which only RNA
was present,” Kasting said: the so-called “RNA world.” All you need
for life, besides those crucial amino acids, are the ingredients
for RNA: ribose, a sugar; phosphate, a salt; and the four bases
– adenine, cytosine, guanine, and uracil (the last replaces the thymine
in DNA). The question is, can you get these molecules in an atmosphere
where significant oxygen is present? The answer, Kasting said, is
yes – assuming there’s a little bit of methane around.

Ribose, Kasting explained, “is simply five molecules
of formaldehyde strung together,” and formaldehyde is easy to make
where there is carbon dioxide and light. Phosphate occurs routinely
with the weathering of rocks. And all four bases, A, C, G, and U,
can be synthesized from hydrogen cyanide, for which you need that
sprinkling of methane.

“So the key to making Darwin’s little pond,” Kasting
said, “is to figure out if there was a good source for methane in
the early atmosphere.” That source, he suggests, is under the sea,
in the volcanic activity that fires up those super-hot hydrothermal
vents. Currently, the carbon released from the vents run about 99
percent carbon dioxide, he said, and about one percent methane,
a slightly different mix than what comes from volcanoes on land.
“And there are good geochemical reasons to believe that the Earth’s
mantle 3.9 billion years ago was much more strongly reduced than
it is today, which means the methane component of these emissions
would have been that much higher.” Plenty high enough to allow for
the formation of organic molecules.

That’s not to say this is the way life sparked into
being, Kasting quickly added. But it’s a plausible scenario. And
if it did happen that way here, what’s to stop the same process
from repeating itself, around the universe, wherever conditions
happen to be the same?

“Astrobiology: Looking for Life in the Universe,” the 2000 Penn State Lectures on the Frontiers of Science, was organized by the Eberly College of Science and sponsored by the pharmaceutical company Pfizer Inc. The series of six lectures took place on consecutive Saturday mornings from January 22 through February 26, 2000, on the Penn State University Park Campus. This special report was funded by the Penn State Astrobiology Research Center, the Pennsylvania Space Grant Consortium, the Education office of the NASA Astrobiology Institute, Pfizer Inc., and the Eberly College of Science. It was written and produced by Research Publications in the Office of the Vice President for Research, Penn State University, 320 Kern Graduate Building, University Park, PA 16802; 814-865-3477; editor@research.psu.edu.

The Penn State Astrobiology Research Center, directed by Hiroshi Ohmoto, Ph.D., professor of geosciences in the College of Earth and Mineral Sciences, is one of 11 lead members of the NASA Astrobiology Institute. PSARC is affiliated with Penn State’s Life Sciences Consortium, the Environment Institute, and the Pennsylvania Space Grant Consortium. For more information, contact PSARC at 814-863-8761, or lxd1@psu.edu, or see http://psarc.geosc.psu.edu.