How Life Began

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Sure enough, they found bugs living more than 1,500 ft. down--cut off, like their ocean-vent cousins, from any conceivable contact with the surface. No one knows how deep the biosphere extends, but Tullis Onstott, a geologist at Princeton, has followed the trail two miles straight down: he began exploring South African gold mines in 1998, and so far he and his international colleagues have pulled out scores of heat-tolerant, hydrogen-eating bugs from subsurface water.

Many of them are bacteria, but some, just as biologists discovered at ocean vents and hot springs, turn out to be something else entirely. By looking carefully at their genes, Carl Woese, of the University of Illinois, realized that while they shared bacteria's key feature--the absence of a distinct nucleus--their genes more closely resembled those of more advanced cells. He proposed, and his colleagues accepted, that these odd organisms should be given their own kingdom. Before, bacteria were classified as Prokaryota, while everything else, including fungi, plants and people, were considered Eukaryota. To these, Woese added Archaea. "Archaea," says University of Colorado biologist Norman Pace of Woese's new classification, "has shaken up how we think of the origin of life."

Basically, Pace and others believe that Archaea may be the closest thing we have to the original life forms that populated the earth. The case is circumstantial, but compelling. For one thing, their genetic makeup suggests they've evolved less than the other two kingdoms from the common ancestor of all earthly life. For another, most of (though not all) the Archaea are heat-loving extremophiles--and, as Baross points out, "the early earth was riddled with ridge systems and submarine volcanic eruptions." The young earth was also awash in the same harsh chemicals and metals that extremophiles thrive on today.

Most extremophile communities are dominated by bacteria, not Archaea. But after seven years of looking, Francis Chapelle, of the U.S. Geological Survey in Columbia, S.C., found an exception. Lidy Hot Springs, in Idaho's Beaverhead Mountains, is some 600 ft. underground. Its temperature hovers at 137[degrees]F, and more than 90% of its microbes are Archaea that consume hydrogen gas and belch out methane. That ratio, wrote Chapelle in Nature, makes this community "unlike any previously described on earth." If Pace and others are right about Archaea's pedigree, says Chapelle, then "systems like Lidy Hot Springs become very important to understanding what life may have looked like on the early earth."

If the heat lovers represent the original inhabitants of earth, cold-loving extremophiles could show us what kinds of creatures might live beyond the earth, in parts of the solar system previously thought uninhabitable. Says microbial ecologist Jody Deming, of the University of Washington: "The surfaces and planets we have to consider are frozen, so we needed to know more about frozen environments."