A photomicrograph of a thin section of serpentinite rock. Photo: Emily Pope |
About 3.8
billion years ago, Earth was teeming with unicellular life. A little more than
4.5 billion years ago, the Earth was a ball of vaporous rock. And somewhere in
between, the first organisms spontaneously arose. Pinpointing exactly when and
how that shift happened has proven a difficult bit of interdisciplinary
detective work.
A team of Stanford University geologists hasn’t quite
solved the problem, but they’ve come closer. By examining the geology and
environment of the early Earth, the researchers demonstrate the plausibility of
one theory: that life originated above serpentinite rock on the ocean bottom.
Because the necessary conditions only existed for a few million years, the
findings provide a potential timestamp for the appearance of the Earth’s first organism.
The paper, authored by geophysics professor Norm Sleep, geological and
environmental sciences professor Dennis Bird, and former graduate student Emily
Pope, appears in Philosophical Transactions of the Royal Society B.
Serpentinite under the sea
Greenish-colored serpentinite is common enough in California to be the official state rock.
But geologists are more interested in deep-sea serpentinite deposits, where the
mineral forms “white smoker chimneys”—hydrothermal vents—in which
alkaline vent fluids interact with more acidic seawater.
The resulting
reaction can form microscopic “pore spaces” in the chimney stone.
This honeycombed rock acts as a percolator for white smoker fluid,
concentrating dissolved substances inside the tiny spaces. Because the nucleic
acids that make up RNA may have occurred naturally in vent fluids, this process
increased the probability of spontaneously forming complete RNA strands. The
tiny pores could have even allowed the resultant organisms to survive without
cell membranes, using the rock itself for structure and protection.
The pH
difference between the vent fluids and the ocean also could have provided an
important energy supply for early organisms. When serpentinite is oxidized by
seawater, hydrogen is formed. Microbes can react hydrogen with carbon dioxide
to form methane or acetate, both of which serve as sources of chemical energy.
“These
same conditions exist wherever water comes out of serpentine in the Bay
Area,” explains Sleep. “If you look, you can see hydrogen bubbling
out of the ground.”
But this model
of life’s origins is only feasible under very specific conditions.
Serpentinite, a cool Earth, and an acidic ocean all must have coexisted for a
time.
Serpentinite
was likely present when life arose. Unfortunately, the geological record only
reliably goes back approximately 3.8 billion years, making a definitive
statement impossible. Still, under the West Greenland
ice fields, Bird and Pope have recently identified serpentinite among some of
the oldest rocks yet found.
The temperature of the Earth was also habitable at the time in question. A
few hundred million years after its formation, the planet had cooled below 120 C—hot
by human standards, but livable for certain microorganisms.
Acid enough?
The single most time-restricted requirement for early life would have been the
acidity of the ocean. In order for early life to make use of a pH gradient
between hydrothermal vents and seawater, the oceans must have been 100 times as
acidic as they are today—a state of affairs that overlapped with a cool Earth
for only a few million years.
The early
oceans would have remained acidic as long as the early earth’s atmosphere
remained high in carbon dioxide. Much of the gas was eventually trapped in the
earth’s mantle by subducting continental plates.
“This
leaves a relatively brief window for the origin of life, at least by this
mechanism,” says Sleep.
Smoking-gun
evidence in support of the origin-of-life theory remains hard to come by.
Geologists are currently looking deep in the Earth’s crust for ancient white
smoker structures. And the search continues for a modern-day version of
membrane-less rock-living microbes.
“It’s
conceivable that a biologist might get lucky,” Sleep says. “But I’m
not holding my breath.”
