In particular, Schopf had proposed that his fossils were photosynthetic bacteria that captured sunlight in a shallow lagoon. But Brasier and Steele and co-workers concluded that the rocks had formed in hot water loaded with metals, perhaps around a superheated vent at the bottom of the ocean—hardly the sort of place where a sun-loving microbe could thrive. And microscopic analysis of the rock, Steele says, was ambiguous, as he demonstrated one day in his lab by popping a slide from the Warrawoona chert under a microscope rigged to his computer. “What are we looking at there?” he asks, picking a squiggle at random on his screen. “Some ancient dirt that’s been caught in a rock? Are we looking at life? Maybe, maybe. You can see how easily you can fool yourself. There’s nothing to say that bacteria can’t live in this, but there’s nothing to say that you are looking at bacteria.”
Schopf has responded to Steele’s criticism with new research of his own. Analyzing his samples further, he found that they were made of a form of carbon known as kerogen, which would be expected in the remains of bacteria. Of his critics, Schopf says, “they would like to keep the debate alive, but the evidence is overwhelming.”
The disagreement is typical of the fast-moving field. Geologist Christopher Fedo of George Washington University and geochronologist Martin Whitehouse of the Swedish Museum of Natural History have challenged the 3.83 billionyear- old molecular trace of light carbon from Greenland, saying the rock had formed from volcanic lava, which is much too hot for microbes to withstand. Other recent claims also are under assault. Ayear ago, a team of scientists made headlines with their report of tiny tunnels in 3.5 billion-year-old African rocks. The scientists argued that the tunnels were made by ancient bacteria around the time the rock formed. But Steele points out that bacteria might have dug those tunnels billions of years later. “If you dated the London Underground that way,” says Steele, “you’d say it was 50 million years old, because that’s how old the rocks are around it.”
Such debates may seem indecorous, but most scientists are happy to see them unfold. “What this will do is get a lot of people to roll up their sleeves and look for more stuff,” says MIT geologist John Grotzinger. To be sure, the debates are about subtleties in the fossil record, not about the existence of microbes long, long ago. Even a skeptic like Steele remains fairly confident that microbial biofilms lived 3.2 billion years ago. “You can’t miss them,” Steele says of their distinctive weblike filaments visible under a microscope. And not even critics have challenged the latest from Minik Rosing, of the University of Copenhagen’s Geological Museum, who has found the carbon isotope life signature in a sample of 3.7 billion-year-old rock from Greenland—the oldest undisputed evidence of life on Earth.
At stake in these debates is not just the timing of life’s early evolution, but the path it took. This past September, for example, Michael Tice and Donald Lowe of StanfordUniversity reported on 3.416 billion-year-old mats of microbes preserved in rocks from South Africa. The microbes, they say, carried out photosynthesis but didn’t produce oxygen in the process. A small number of bacterial species today do the same—anoxygenic photosynthesis it’s called—and Tice and Lowe suggest that such microbes, rather than the conventionally photosynthetic ones studied by Schopf and others, flourished during the early evolution of life. Figuring out life’s early chapters will tell scientists not only a great deal about the history of our planet. It will also guide their search for signs of life elsewhere in the universe—starting with Mars.
In January 2004, the NASA rovers Spirit and Opportunity began rolling across the Martian landscape. Within a few weeks, Opportunity had found the best evidence yet that water once flowed on the planet’s surface. The chemistry of rock it sampled from a plain called Meridiani Planum indicated that it had formed billions of years ago in a shallow, long-vanished sea. One of the most important results of the rover mission, says Grotzinger, a member of the rover science team, was the robot’s observation that rocks on Meridiani Planum don’t seem to have been crushed or cooked to the degree that Earth rocks of the same age have been— their crystal structure and layering remain intact. A paleontologist couldn’t ask for a better place to preserve a fossil for billions of years.
The past year has brought a flurry of tantalizing reports. An orbiting probe and ground-based telescopes detected methane in the atmosphere of Mars. On Earth, microbes produce copious amounts of methane, although it can also be produced by volcanic activity or chemical reactions in the planet’s crust. In February, reports raced through the media about a NASA study allegedly concluding that the Martian methane might have been produced by underground microbes. NASA headquarters quickly swooped in—perhaps worried about a repeat of the media frenzy surrounding the Martian meteorite—and declared that it had no direct data supporting claims for life on Mars.
But just a few days later, European scientists announced that they had detected formaldehyde in the Martian atmosphere, another compound that, on Earth, is produced by living things. Shortly thereafter, researchers at the European Space Agency released images of the Elysium Plains, a region along Mars’ equator. The texture of the landscape, they argued, shows that the area was a frozen ocean just a few million years ago—not long, in geological time. Afrozen sea may still be there today, buried under a layer of volcanic dust. While water has yet to be found on Mars’ surface, some researchers studying Martian gullies say that the features may have been produced by underground aquifers, suggesting that water, and the life-forms that require water, might be hidden below the surface.
Andrew Steele is one of the scientists designing the next generation of equipment to probe for life on Mars. One tool he plans to export to Mars is called a microarray, a glass slide onto which different antibodies are attached. Each antibody recognizes and latches onto a specific molecule, and each dot of a particular antibody has been rigged to glow when it finds its molecular partner. Steele has preliminary evidence that the microarray can recognize fossil hopanes, molecules found in the cell walls of bacteria, in the remains of a 25 million- year-old biofilm.
This past September, Steele and his colleagues traveled to the rugged Arctic island of Svalbard, where they tested the tool in the area’s extreme environment as a prelude to deploying it on Mars. As armed Norwegian guards kept a lookout for polar bears, the scientists spent hours sitting on chilly rocks, analyzing fragments of stone. The trip was a success: the microarray antibodies detected proteins made by hardy bacteria in the rock samples, and the scientists avoided becoming food for the bears.