Signs of Earliest Life in Ocean Depths

By Kathy Sawyer, Washington Post, Thursday 8 June 2000; A03

Geologists have found apparent fossil evidence of microbial life in scalding hot deep-ocean vent systems on the very young Earth, 2.7 billion years earlier than previously known.

The signs of ancient single-celled organisms—threadlike filaments etched in volcanic rock about 3.2 billion years old—bolster a theory favored by many biologists, based on genetic studies, that the first life on Earth arose in just such a sunless, toxic and hot environment.

The cradle of life may have been a sulfurous, subterranean inferno, not unlike a medieval vision of hell, said Birger Rasmussen, of the University of Western Australia, who describes his discovery in today's issue of the journal Nature.

In recent decades, scientists have learned that life can thrive in extreme environments previously thought to be lethal. On Earth today, deep-ocean hydrothermal vent systems—rifts in the ocean floor where water and chemicals from the Earth's hotter interior emerge—are known to sustain a variety of creatures, including diverse bacterial communities. But microfossils of any kind in such ancient deposits are extremely rare, and until now, none like these had been found in rock more than about half a billion years old.

Paleobiologist Andrew Knoll, of Harvard, said Rasmussen's find represents a rare and welcome insight into a 2 billion-year stretch of Earth's biological history that remains cloaked in mystery. It's a clear view of life at a moment in time in a particular place on the early Earth. . . . It's really, I think, several steps forward from any micropaleontology that we've seen before in rocks of this age.

The first murky chemical sign of life anywhere on Earth dates to 3.8 billion years ago, he noted, and by 3.5 billion years ago, the record indicates that primitive biology of some sort was a going concern.

Rasmussen's fossils appear to be the imprints left by strings of single-celled heat-loving organisms (in more technical terms, thermophilic chemotropic prokaryotes) a thousandth of a millimeter in diameter and a tenth of a millimeter in length. He found the stringy formations unexpectedly while studying core samples of rock drilled from a 3.2 billion-year-old metal sulfide deposit in the Pilbara region of northwestern Australia. It is a type of deposit usually formed on a sea floor at the darkest depths and at high temperatures. He was examining the rocks with optical and scanning electron microscopes—not looking for fossils, but trying to determine oxygen levels in the atmosphere of the primordial Earth.

After looking at hundreds of slides, I noticed some unusual structures that contained a dense assemblage of interwined filaments, he said. After careful examination, I came to the conclusion that the filaments had to be biological.

It is their sinuous shape, uniform thickness along their length and pattern of intertwining that led Rasmussen to this inference, he said. At least as compelling, both to him and to independent experts interviewed, are the patterns by which the filaments orient themselves, suggesting behavioral variations distinct to living organisms as they react to varying stimuli. As for their abundance, he found more than 300 such filaments intertwined within a tiny spot in one very thin polished section of rock.

These fossils strongly resemble, in size and shape, microbes found around deep-sea vent systems today, Rasmussen said.

Geologist Euan Nisbet of the University of London, in an accompanying commentary, said: The work of Rasmussen and of others . . . adds a new realm—predicted but previously only surmised—to the map of ecology for this epoch. Although it does not prove that deep-water hydrothermal life came before life based on the sun's energy, he added, it lends circumstantial support to the argument that steps in the early history of life—and perhaps even the very first step—took place around hydrothermal systems.

But Nisbet also cautioned that the study of rocks this old is like a forensic investigation based on heavily smudged fingerprints, carrying a high risk of error.

Rasmussen said he believes he has ruled out known alternative explanations. For example, the filaments . . . bear little resemblance to structures generated by [non-biological] means. The nature and location of the fossil evidence suggests to Rasmussen that the microorganisms probably lived in the pores and crevices of rocks at shallow depths below the sea floor, a habitat that would have been bathed in a hot, rich brew of metals and nutrients.

The planet's surface, under heavy meteor bombardment, was even less hospitable than the volcanic submarine depths. At 1,000 meters (3,300 feet) down, with a sediment cover, the organisms would have been shielded from ultraviolet radiation and could not have depended on sunlight for energy like most familiar forms of life. Instead, Rasmussen speculates, they would have relied somehow on chemical energy, possibly from the sulfur common in such hot springs.

Biogeochemist John Hayes of the Woods Hole Oceanographic Institute predicted that the paper will stir up debate on this issue of how the bugs made a living. Rasmussen, he said, leaves unanswered the vexing major question of exactly what chemical reaction on early Earth they might have been exploiting to drive their growth. It's a stimulating paper, without a doubt, he said, even though it lands us in the middle of this remarkable puzzle.