and create an acidic environment in the
surrounding ocean, the Lost City vents
are formed by a reaction between mantle
rocks and seawater, leading to an alkaline
The vents spew methane and hydrogen into the water, which react to form
limestone chimneys, acetate (a potential energy source for early life) and
hydrocarbons (important building
blocks for life). What’s more, pores in
the limestone chimneys suggest a way
for chemicals to undergo reactions
without floating away.
“These microcompartments serve the
function of providing a way for chemicals to be concentrated in a physical way
without cellular membranes,” explains
Martin, who has collaborated with geologists to propose theories on the origins
of life at the Lost City vents. Early cells
could have lived off the chemical cocktails
within the compartments. “Lost City is
the most exciting thing that’s happened
in the past decade in this field,” he says.
While today’s Lost City vents wouldn’t
have been around 4 billion years ago,
similar ones could have spewed life-sustaining chemicals into the early
oceans. It’s up to geologists to determine
whether such vents existed and up to
biologists to figure out whether life could
have thrived there.
Armen Mulkidjanian of the Univer-sität Osnabrück in Germany has a different idea about where life began. He
studies what features modern single-celled organisms share with each other,
a favorite topic among many evolutionary biologists. Two major domains of
such organisms exist —bacteria and
archaea — and both are thought to have
evolved from a common ancestor that
existed at least 3. 5 billion years ago.
© NATHAN YAN
While Earth’s first cells were probably more primitive than this ancestor, studying its characteristics may
help scientists piece together a picture
of even older cells. So far, researchers
have found that bacteria and archaea
have about 60 genes in common. Thus,
scientists deduce, the ancestor, dubbed
LUCA (short for Last Universal Common Ancestor), had these same genes.
“What we decided to do was to analyze
what organic ions are required by each of
the proteins coded by these ubiquitous
genes,” Mulkidjanian says. His team’s
analysis, reported in Proceedings of the
National Academy of Sciences earlier this
year, revealed that these shared proteins
require potassium more than any other
element. And sodium, the researchers
showed, blocked the function of some
of the cellular elements, most of which
are involved in the translation of genetic
material to proteins.
“We know that original membranes
were very leaky. Cells could keep proteins or nucleotides inside, but not
potassium,” Mulkidjanian says. So this
means that LUCA must have been living
somewhere with more potassium than
sodium; otherwise potassium would
have flooded right out of the membrane.
Here’s when the biologists consulted
with geologists. Researchers familiar
with ancient geology agreed that all
the evidence from Hadean rocks suggests the oceans back then were rich in
sodium. But one place on the ancient
Earth, the geologists said, would be
replete with potassium: ponds created
by vapor from volcanic systems, which
the Hadean planet had plenty of.
When magma from volcanoes heats
rocks, some water evaporates, pulling
certain elements from the rocks and
leaving behind others. The resulting
vapor can condense back into water
and form freshwater ponds, rich not
only in potassium but also in zinc and
phosphate — also substances that could
have driven early cellular processes. The
Vaporous ponds, like those
found today at Hot Creek
in California, could have
provided a potassium-rich
setting for early life.
cellular requirements of LUCA proposed by Mulkidjanian’s team matched
the geological descriptions of these
geothermal fields. “That geochemical
knowledge is really what fed our biology
story,” says Mulkidjanian.
Ideas about where life began, whether
it was in an ocean or a pond or somewhere else entirely, are still just proposals, hypotheses with bits of evidence.
The same is true for existing views
about when life emerged and what it
looked like. But as geologists and biologists continue to learn from each other,
they’re turning up new evidence that can
strengthen existing scenarios and lead
to new ones.
For geologists, the challenge going
forward is to find and analyze more
ancient rocks to flesh out the picture of
the Hadean planet. For biologists, the
next task is to combine their theories on
early cells with geologists’ descriptions
of the early Earth.
“We’re starting to narrow the gap
between microbiology and geochemistry,” Martin says.
As new zircons are uncovered and
chemically favorable environments are
explored, the tale of how life began may
gain an agreed upon time, a setting and,
eventually, a plot. s
s N.H. Sleep. “The Hadean-Archaean
environment.” Cold Spring Harbor
Perspectives in Biology. June 2010.
Sarah C.P. Williams is a freelance
science writer based in Hawaii.