Experts Dissect a Primordial Banquet
by KENNETH CHANG
March 20, 2001, New York Times
Extending their knowledge of some of the earliest life on earth,
scientists believe they have figured out what one group of bacteria
was feasting on 3.5 billion years ago.
Analyzing rocks from western Australia, the researchers did not find
any fossils of the bacteria, but they said the chemical composition of
sulfur compounds in the rock could only be the byproduct of living
organisms - the leftovers of an ancient meal.
"In some ways, we've got a record what the first dinner party was
like," said Dr. Roger Buick, a lecturer in paleontology and
stratigraphy at the University of Sydney. "We can work out who the
diners were. We can see what they were eating. We can even see what
they were excreting. It's reconstructing a primordial banquet."
The findings also suggest indirectly that the early atmosphere and
oceans contained little oxygen.
Dr. Buick, along with Dr. Yanan Shen and Dr. Donald E. Canfield of
Odense University in Denmark, reported the findings in the March 1
issue of Nature.
"What we've demonstrated is the earliest recognizable type of
metabolism in the geologic record," said Dr. Canfield, a professor of ecology.
Chemical traces of life date to about 3.8 billion years ago, but
almost nothing was known about the earth's initial inhabitants, not
even what they ate or the metabolic processes that powered them. Until
now, that sort of information was not available for any organisms
before 2.75 billion years ago, more than a billion years into the
history of life.
The new research "really gives clarity where clarity was wanting
before," said Dr. Andrew H. Knoll, a professor of biology at
Harvard. "You don't have to rewrite what you think about the history
of life from it. You feel better about what we've been all speculating
on for the last 20 years."
The work focuses on two types of sulfur compounds known as sulfates
and sulfides. Certain bacteria, some of which still exist, consume
sulfate and produce sulfide as a waste product. That makes
sulfate-eating bacteria among the oldest lineages of life on the
"We've nailed down the oldest date of the oldest ancestor on that
tree," Dr. Canfield said. "It's like doing your family history and
finding out when your great-great-great-grandfather was born."
The most common form of sulfur contains 16 protons and 16 neutrons in
its nucleus. Another heavier version, or isotope, of sulfur contains
two additional neutrons. When sulfate is bountiful, the bacteria
prefer to eat the sulfate with the lighter sulfur, because that takes
less energy to digest.
"It's the fast food mentality," Dr. Buick said. "No slow dining for
bacteria. They want it quick and fast."
In the Australian rocks, which formed in shallow salty ponds near the
ocean, the sulfides - the supposed bacterial waste - contain about 12
percent less of the heavier sulfur atoms than the sulfates
contain. Some natural chemical and geological processes can separate
the heavier sulfur from light sulfur, but not to that degree.
"It's a very strong indication this was a biologically produced
sulfide," Dr. Canfield said. "I actually can't think of another
explanation when you take all of the indications into account."
Information about the bacterial metabolism tells about the environment
they lived in. Sulfates form in the presence of oxygen, and the
presence of sulfate-eating bacteria implies the pond they lived in was
rich in oxygen. That implies oxygen was not plentiful elsewhere on the
planet, because other rocks from the same era lack signs of