Earth’s first living organisms didn’t leave behind footprints or bite marks or bones. These single cells thrived quietly in a
tiny pocket somewhere on the planet. For
centuries, scientists trying to describe
this earliest life have relied on evidence
provided by biology, studying what features modern life-forms have in common
to deduce the most primitive components
of cells. By working backward, biologists
have developed proposals describing
when and where such simple forms of
life could have arisen. But the ideas so far
are guesses at best, impossible to prove.
Researchers from a different field —
geology — have more recently joined in
the effort. With guidance from biologists, geologists are looking to Earth’s
oldest rocks to uncover traces of life
left behind by the very first cells. Geologists are also pointing biologists toward
unusual environments where early cells
might have gained a foothold. Where the
two fields intersect, more concrete scenarios regarding life’s formative years
are now taking shape.
Life, by definition, alters its surroundings, exchanging energy and chemicals
with the world around it. So early cells
should have left indelible chemical traces
of their existence — clusters of elements
that would never have come together
without help from a metabolizing organism. Today, materials that could contain
chemical signatures of Earth’s earliest life are few and far between, mostly
buried deep within the planet’s interior,
occasionally pushed to the surface when
volcanoes erupt or mountains form. But
geologists are determined to find and
analyze these rocks for signs of life.
“The geological record is like a rug in
an old house,” says Stephen Mojzsis of the
University of Colorado Boulder. “Over
centuries of people walking over it, it gets
completely worn away and all you have is
a few colored threads left. But if you look
closely enough at those few threads, you
This banded iron formation reveals the
presence of primitive bacteria more than
3 billion years ago. Older rocks may
yield even earlier signs of life.
might be able — thread by thread — to fig-
ure out what the rug once looked like.”
Finding the threads left by early life is
only part of the challenge. Geologists are
also contributing to discussions about
where those threads were spun. One
story, based on the discovery of a new
type of underwater vent, begins deep in
the ocean. Another proposal gets its start
in vapor-fed ponds, where geologists say
the requirements biologists have laid out
for life’s existence could have been met.
As the collaboration plays out, geologists and biologists are providing a reality check for each other, determining
what was possible.
“There’s a need for the biologists who
are thinking about this to think in terms
of real Earth processes and conditions
rather than what they can do in a test
tube, because clearly there weren’t test
tubes lying around on early Earth,” says
geophysicist Norm Sleep of Stanford
University. “I consider it my duty to
provide a shopping list of early environ-
ments to these biologists.”
organisms integrate carbon into the solid
matter of the planet. So the presence of
carbon-rich rocks mean photosynthesis
was occurring. And it’s not just carbon.
Ancient rocks with “banded iron forma-
tions” — red layers rich in iron — also indi-
cate photosynthesis; the oxygen released
by photosynthesis rusted iron on the
planet’s surface (SN: 6/20/09, p. 24).
Life’s first step Scientists suspect that
the root of all life on Earth emerged sometime
after the cataclysmic event that is believed to
have formed the moon, some 4.35 billion years
ago. The event would also have reset the Earth
environs, opening the way for life.
Hadean
4. 5 billion years ago Earth forms
4.35 bya Moon forms
When life formed?
Rocks of life
Four and a half billion years ago, the
infant Earth was a hot and volatile place.
Shortly after its formation, within 150
million years or so, it is thought to have
been hit by a smaller young planet. The
collision formed the moon and altered
the Earth dramatically.
“There are bookends to the possible
time for life to emerge on the planet,”
Mojzsis says. “And one bookend is the
formation of the moon — this was such a
catastrophic event that it remelted the
Earth and reset everything. There’s no
way any life could have survived this.”
But as the Earth cooled through this
period, called the Hadean eon, it slowly
became a more habitable place. By the
start of the Archean, 3. 8 billion years ago,
life was thriving.
Geologists know that life existed then
because rocks from the Archean have
high concentrations of carbon. Before
living organisms were around, most of
the carbon on the planet was in the atmosphere. But the chemical reactions used
to generate energy by photosynthetic
2 mya Evolution of Homo genus
May 19, 2012 | SCIENCE NEWS | 23
