Planet Earth

Works in Progress

Geologists and biologists know where to look for the earliest life—but the rocks are hard to read.

By Karen WrightMar 1, 2003 6:00 AM


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For today's civilized world, with its dotcoms, sitcoms, ATMs, and ATVs, the first 3.5 billion years of life on Earth are a bit of an embarrassment. It was only a few hundred million years ago that trilobites prowled the seas. More primitive life subscribed to two or three basic lifestyles: algal mat, spineless worm, or bacterial blob. Before that, in the Archean Eon more than 2.5 billion years ago—well, that kind of life is what Lysol is for.

Scientists, of course, see it differently. "Almost everything of any biological importance happened back in the Archean," says Andrew Knoll of Harvard University, author of the upcoming book Life on a Young Planet. Soon after the infant Earth cooled down, he says, primeval microbes began processing essential elements—carbon, sulfur, and nitrogen, among others—that allowed for the eventual emergence of higher life-forms, including us. To this day, says Knoll, bacteria still do the biosphere's heavy lifting. "We just sit back and live off the fruits of their labors."

Folks like Knoll would like to know whom to thank for those first trophic cycles. But in the quest to identify Earth's earliest life, geology can look a lot like biology. It's not always easy to tell the dead organisms from the dead ends. One of the few things experts all agree on is where to conduct the search: in the three far-flung provinces that host the world's most ancient sedimentary rocks. Deposits in Australia, Greenland, and South Africa offer a cryptic view of the earth's surface as it was between 3.2 billion and 3.8 billion years ago. The deposits are made up of layers of accumulated particles that were later buried, heated, and compressed. Rounded pebbles and smoothed sand grains in the sediments indicate that they were seabeds, so any life they record would be marine.

The oldest fossil of that life comes from a remote desert site in Western Australia called North Pole. The rocks there bear the marks of stromatolites—sizable mounds of mud and minerals trapped or precipitated by microbial colonies living in shallow ocean water. Modern stromatolites grow knee-high in Australia and the Bahamas, and the organisms that build them leave distinctive patterns in the mud pedestal that can't be duplicated by mere geologic manipulation. At North Pole, those patterns appear in rocks that are almost 3.5 billion years old.

The sediment layers in the North Pole fossils are much finer than those in modern stromatolites, suggesting that much smaller life-forms inhabited them. Even so, there's evidence of a food chain of sorts. The principal architects of stromatolites are photosynthetic. They get their energy directly from sunlight instead of feeding off other creatures. But geochemists found the chemical signature of a microbe that was feasting on dead organic matter, a scavenger of sorts. "We had quite sophisticated ecological communities back then, even if they were just tiny little microbes," says astrobiologist Roger Buick of the University of Washington, who discovered the North Pole stromatolites.

Unfortunately, the vestiges of microbial communities are far more conspicuous than the remains of their individual members. Lacking bones, shells, teeth, and other hard parts, the first Earthlings didn't fossilize well. In the oldest rocks, chemical leftovers may be the only evidence of animation. So it happens that the earliest evidence of life is not a lithic imprint but a skewed ratio of carbon isotopes in a chunk of rock from southwest Greenland. Microscopic globules of graphite in the rock, documented in 1999 by geologist Minik Rosing at the University of Copenhagen, are unusually low in a heavy carbon isotope that gets excluded when inorganic carbon is converted into living material. Rosing thinks the C-13-poor graphite globules might have come from free-living planktonlike organisms that fell to the seafloor when they died. Their remains, he says, are at least 3.7 billion years old.

In 1996 geochemist Stephen Mojzsis, now at the University of Colorado at Boulder, trumped Rosing's find in a report of heavy-isotope depletion in graphite grains from the Isua formation in Greenland and another site on the Greenland island of Akilia. Mojzsis says the grains are 3.85 billion years old—the oldest yet. But his interpretations of both the biological markers and the rock itself have been put through the wringer. One of Mojzsis's former coauthors, geochemist Gustaf Arrhenius of the Scripps Institution of Oceanography, showed how the Isua carbon-isotopic ratio could arise by geologic activity alone, if certain iron minerals in the rock were melted and pressed together over time. He and other investigators also think that the putative sedimentary rocks are actually igneous formations that have been severely transformed by heat.

Thus, rocks of advanced vintage seem to confound even the most basic geologic distinction: igneous, metamorphic, or sedimentary? "These rocks have been buried and cooked at least three times," says Buick. "They've been severely squashed and strained and tied in knots at least three times too. Then they sat around for at least a billion years and got polished by glaciers. These are not ordinary rocks."

The ambiguity of chemical evidence leaves geologists hungry for a well-defined, and ideally photogenic, fossil or two. In the early 1990s they thought their hopes had been answered when paleobiologist William Schopf of the University of California at Los Angeles described microscopic structures embedded in a Western Australia formation almost 3.5 billion years old. In his report, dark, slender silhouettes appear in translucent sections of thinly sliced quartz. Schopf says the silhouettes are a complex carbon polymer made by chains of bacteria that may have been anchored to the seafloor. After examining hundreds of present-day microbes, he named 11 possible species in his collection and gave the back story in a 1999 book called Cradle of Life. His menagerie made the Guinness Book of World Records, as the Earth's oldest fossils.

"I found a whole bunch of different things," says Schopf. "The question was, what were they?"

Schopf decided that at least half could be cyanobacteria, or blue-green algae, the first organisms in the evolutionary record to produce oxygen. That challenges orthodox thinking about conditions on the young Earth, which would not have had a significant oxygen atmosphere for at least another billion years. When geologist Martin Brasier of the University of Oxford had a look at the structures, he decided Schopf was wrong, wrong, and wrong again. The tubes are too branched to come from bacteria, he says. The rock is an extrusion from a hydrothermal vent, not seafloor sediment. And the silhouettes are inorganic carbon injected by the vent and molded into suggestive shapes by the growth of mineral crystals. "Ancient filamentous structures should not be accepted as being of biological origin until all possibilities of their nonbiological origin have been exhausted," Brasier and his coauthors wrote in a report last year.

The hubbub over Schopf's fossils has humbled disciples of early life. "People have become more critical about what they'll accept as evidence of biology," says Knoll. And, as demonstrated by the recent retraction of evidence for life in a Mars meteorite, the stakes are astronomical. Once biologists know where and how life emerged, astrobiologists will be better prepared to look for it elsewhere in the solar system. If life on Earth was a freak accident born of unique and peculiar conditions, it's probably rare elsewhere. But, says Buick, "if life can arise quickly and easily, given the right environment, there might be quite a bit of it out there."

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