Ultimately, after years of research, Boston and other scientists revealed that the microbes in Lechuguilla do much more than just spit out a bit of dirt. Lechuguilla is hidden in thick layers of limestone, the fossilized remains of a 250 million year old reef. The many chambers in such caves are usually formed by rainwater seeping into the ground and gradually dissolving the limestone. In Lechuguilla, however, microbes are also the sculptors: Bacteria that eat buried oil reserves release hydrogen sulfide gas, which reacts with oxygen in the groundwater to produce sulfuric acid that cuts away limestone. At the same time, various microbes consume hydrogen sulfide and generate sulfuric acid as a byproduct. Similar processes occur in 5 to 10 percent of limestone caves worldwide.
Since the first descent from Boston to Lechuguilla, scientists around the world have discovered that microorganisms are transforming the Earth’s crust wherever they are. Alexis Templeton, a geomicrobiologist at the University of Colorado, Boulder, regularly visits a barren mountain valley in Oman, where tectonic activity has pushed parts of the Earth’s mantle — the layer beneath the crust — much closer to the surface. She and her colleagues drill holes up to a quarter mile into the uplifted mantle and extract long cylinders of 80-million-year-old rock, some of which are beautifully marbled in striking maroon and green hues. In laboratory studies, Templeton has shown that these samples are full of bacteria, some of which change the composition of the Earth’s crust: they eat hydrogen and inhale sulfates in the rock, exhale hydrogen sulfide and create new deposits of sulfide minerals similar to pyrite. known as fool’s gold.
Through related processes, microbes have helped form some of Earth’s reserves of gold, silver, iron, copper, lead and zinc, among others. As underground microbes break down rock, they often free the metals trapped within. Some of the chemicals microbes release, such as hydrogen sulfide, combine with free-floating metals to form new solid compounds. Other molecules produced by microbes grab soluble metals and bind them together. Some microbes store metal in their cells or grow a crust of microscopic metal flakes that continually attract more metal, potentially forming significant deposits over long periods of time.
Life, especially microbial life, has forged a vast amount of minerals on Earth. These are naturally occurring inorganic solid compounds with well-organized atomic structures, or, to put it more plainly, very elegant rocks. Today, the Earth has more than 6,000 different types of minerals, most of which are crystals, such as diamond, quartz and graphite. In its infancy, however, Earth did not have much mineral diversity. Over time, the continuous crumbling, melting, and re-solidification of the Earth’s early crust shifted and concentrated unusual elements. Life began breaking down rocks and recycling elements, creating entirely new chemical mineralization processes. More than half of all minerals on Earth can only occur in a high-oxygen environment, which did not exist before microbes, algae and plants oxygenated the ocean and atmosphere.
Through the combination of tectonic activity and the ceaseless bustle of life, Earth developed a mineral repertoire unmatched by any other known planetary body. By comparison, the Moon, Mercury and Mars are mineral depleted, with perhaps a few hundred types of minerals at most. The variety of minerals on Earth depends not only on the existence of life, but also on its peculiarities. Robert Hazen, a mineralogist and astrobiologist at Carnegie Science, and the statistician Grethe Hystad have calculated that the chance of two planets having an identical set of mineral species is one in 10³²². Given that there are only an estimated 10²⁵ Earth-like planets in the cosmos, there is almost certainly no other planet with Earth’s exact mineral count. “The realization that Earth’s mineral evolution depends so directly on biological evolution is somewhat shocking,” Hazen writes in his book ‘Symphony in C.’ “It represents a fundamental shift from the vantage point of a few decades ago, when my mineralogy Ph.D. advisor told me, “Don’t take a biology course. You’ll never use it!’ ”