Experts Dissect a Primordial Banquet


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 planet.
"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 sulfur-eating bacteria.