How Life Began

GBF/SPL/PHOTO RESEARCHERS
ICE LOVERS: These microbes multiply in Antarctic lakes

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While their hardiness was a big surprise, the microbes' ability to eat hydrogen, sulfur, manganese and other chemicals a process known as known as chemoautotrophy was a revelation. Until then, all living systems were thought to depend on photosynthesis, using sunlight as a primary energy source. (Even cave-dwelling or deep-water creatures who never see the sun eat organic matter that ultimately originates from photosynthesis.) But if life could thrive without even indirect contact with sunlight, the amount of potentially habitable real estate on the planet would expand considerably.

And indeed, life began to turn up just about everywhere scientists looked. Geologists had been arguing since the 1920s, in fact, that chemical contaminants found in crude oil suggested that some sort of life was thriving underground. They weren't taken seriously until the 1980s, though, when Department of Energy scientists realized that if subsurface microbes really did exist, they might play a key role in regulating the purity of groundwater. So they began digging boreholes at DOE sites in South Carolina and Washington State.

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."

Deming's specialty is sea ice, which may look solid, but is actually riddled with tiny channels and pores filled with very salty water. Microbes that live in sea ice have to survive temperatures that vary from just below freezing to -31[degrees]F. And although Deming is just beginning to understand how they do it, she has learned that at least one genus is closely related to bacteria that live in the deep ocean suggesting that the adaptations to cold and to high pressure are somehow linked.

This is good news for scientists who hope to find life on Europa, a moon of Jupiter's that is covered with ice but probably has an ocean beneath. According to Deming, indirect analyses of Europa's surface suggest that it contains salts, and that they are the same salts found in Earth's oceans; any microbes that exist on Europa might resemble the ones she has found.

pagebreak They might be even more similar to the bugs Montana State's Priscu is sure lurk in Lake Vostok, not just in its near freezing waters but in the hundreds of feet of sediment he believes lie at the bottom. Russian drillers and their international collaborators aren't going to penetrate the lake itself until they have designed a system that won't taint the pristine water with contaminants from higher up. But when they do probably within the next few years they will be able to see what's going on in a nearly perfect analogue of Europa's oceans. And the techniques they develop will be applicable to eventual searches for life on that distant moon. "It's humbling," Priscu says, "to find microbes where you never imagined they could exist."

It's also clear that there are plenty of surprises left. Says Priscu: "In the '70s, when I first got interested in this field, many colleagues called claims of life in extreme environments 'hand waving.' " Since then, he and the other extremologists have found life inside glaciers, at the bottom of mines, in searing heat, freezing cold, crushing pressure and lethal toxicity. And that's after exploring only a tiny fraction of the planet. What they have discovered so far has transformed biology. What they will find next is anybody's guess.