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Paleontologists now know that the Precambrian actually did swarm
with living creatures, and it was swarming more than 3.85 billion years
ago. The earliest evidence of life comes from the southwestern coast of
Greenland. There are no fossils to be found there, at least not in the
conventional sense. An organism can leave behind a visible part of its
body -- a skull, a shell, the impression of a flower petal -- but it
also leaves behind a special chemistry, and scientists now have the
means of detecting it.
The ratio of carbon 13 to carbon 12 is lower in organic carbon, such
as wood or hair, compared to inorganic carbon that escapes out of a
volcano as carbon dioxide. This makes it possible to tell if the carbon
in a rock had ever been inside a living thing. Consider, for instance,
a leaf growing on an elm tree. It builds up a low ratio of C-13 to C-12.
A caterpillar that nibbles that leaf will incorporate the carbon in its
prey into its own tissue, and it will take on a low C-13 ratio as well,
as will the bird that eats the caterpillar. Birds, caterpillars, and
leaves all die sooner or later, and when they do, they all become part
of soil, which eventually washes out to the ocean and becomes sedimentary
rock. And even those rocks, made partly from carbon that has cycled
through life's metabolism, will bear life's low C-13 ratio. Any
sedimentary rocks that formed before life appeared on Earth would have
the high C-13 ratio of a volcanic origin.
In 1996 a team of American and Australian scientists traveled to the
twisted fjords and bare islands of southwestern Greenland, where the
oldest sedimentary rocks on Earth can be found. A layer of volcanic rock
cuts through them, and the scientists used the uranium-lead clock inside
its zircons to date it to 3.85 billion years. They then sifted through
the surrounding rock. Over its lifetime, it has been cooked, compressed,
and otherwise ravaged almost beyond recognition. But the researchers
found microscopic bits of carbon in a mineral known as apatite in the
sedimentary rocks. They brought these samples back to their labs and
blasted off bits of the apatite with a beam of ions and counted up the
carbon isotopes it contained. They found that carbon in the apatite had
the same low C-13 ratio as biological carbon today -- a ratio that could
only have come from life.
Scientists cannot say just how long life existed on Earth before it
left this mark in the rocks of Greenland, because no rocks older than 4
billion years have survived. But it's safe to say that life must have had
a hellish birth. Giant asteroids and miniature planets pummeled Earth for
its first 600 million years. Some of them were big enough to boil off the
top few meters of the oceans and kill any life it held. Perhaps life
survived these cataclysms hidden around the thermal springs at the ocean
floor, where bacteria can be found today. When rains filled the seas again,
the microbes were able to emerge from their refuges.
However life got started, it had to have been in full swing by the time
it left its mark in the Greenland rocks. At the time, the oceans were
teeming with bacteria generating their own food as they do today, either
from sunlight or from the energy contained in the chemistry of hot springs.
These self-sustaining microbes were probably food for predatory bacteria,
as well as hosts for viruses.
The oldest actual fossils of bacteria date back 3.5 billion years,
about 350 million years after the earliest chemical signs of life.
These fossils, discovered in the 1970s in western Australia, consist of
delicate chains of microbes that look exactly like living blue-green
algae (otherwise known as cyanobacteria). For billions of years, these
bacteria formed vast slimy carpets in shallow coastal waters; by 2.6
billion years ago they had also formed a thin crust on land.
Of course, life is not limited to bacteria. We humans belong to an
enormous group of organisms called eukaryotes, which include animals,
plants, fungi, and protozoans. The evidence for the oldest eukaryotes
doesn't come from traditional fossils, which date back only about 1.2
billion years. It comes, once again, from molecular fossils. Among the
many things that distinguish eukaryotes from bacteria and other life-forms
is how their cell membranes are constructed. Eukaryotes stiffen them with
a family of fatty acids known as sterols. (Cholesterol belongs to the
sterol family; while it may be dangerous when too much of it gets into
the bloodstream, you couldn't live without it. Your cells would simply
disintegrate.)
In the mid-1990s, a group of geologists led by Jochen Brocks of
Australian National University drilled 700 meters down into the ancient
shales of northwest Australia to formations that have been dated with
uranium and lead to 2.7 billion years ago. Inside the shale, the
geologists found microscopic traces of oil that contained sterols.
Because eukaryotes are the only organisms on Earth that can make
these molecules, Brocks's team concluded that eukaryotes -- probably
simple, amoeba-like creatures -- must have evolved by 2.7 billion years
ago.
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