Eventually, over many years of research, Boston and other scientists discovered that the microbes in Lechuguilla do much more than excrete a little dirt. Lechuguilla is encased in thick layers of limestone, the solidified remains of a 250-million-year-old bedrock. The multiple chambers in such caves are usually formed by rainwater seeping into the ground and gradually dissolving the limestone. At Lechuguilla, however, the microbes are also the sculptors: bacteria eating the buried oil reserves release hydrogen-sulfide gas, which reacts with the oxygen in the groundwater, producing sulfuric acid that carves the limestone. In parallel, various microbes consume hydrogen sulfide and generate sulfuric acid as a byproduct. Similar processes occur in 5 to 10 percent of global limestone caves.
Since Boston’s initial landing on Lechuguilla, scientists around the world have discovered that microorganisms transform the planet’s crust wherever they inhabit it. 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 of a mile into the uplifted mantle and extract long cylinders of 80-million-year-old rock, some of which are beautifully marbled in gorgeous shades of maroon and green. 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 breathe sulfates in rock, excrete hydrogen sulfide, and create new deposits of sulfide minerals similar to pyrite, too. known as fool’s gold.
Through interconnected processes, microbes have helped form some of Earth’s reserves of gold, silver, iron, copper, lead, and zinc, among other metals. As underground microbes break down the rock, they often release metals trapped within it. Some of the chemicals released by microbes, such as hydrogen sulfide, combine with free floating metals, forming new solid compounds. Other molecules produced by microbes capture soluble metals and bind them together. Some microbes accumulate metal within their cells or grow a crust of microscopic metal flakes that continuously attract even more metal, potentially forming a significant deposit over long periods of time.
Life, particularly microbial life, has forged a large amount of Earth’s minerals, which are naturally occurring solid inorganic compounds with highly organized atomic structures, or, to put it more clearly, highly elegant rocks. Today the Earth has more than 6,000 distinct types of minerals, most of which are crystals such as diamond, quartz, and graphite. However, in its early days, Earth did not have much mineral diversity. Over time, the constant crumbling, melting, and resolidification of the planet’s early crust displaced and concentrated unusual elements. Life began to break down rocks and recycle elements, generating entirely new chemical processes of mineralization. More than half of all minerals on the planet 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 hustle and 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 several hundred mineral species between them. The variety of minerals on Earth depends not only on the existence of life, but also on its characteristics. Robert Hazen, a mineralogist and astrobiologist at Carnegie Science, and statistician Grethe Hystad have calculated that the chance of two planets having an identical set of mineral species is one in 10³²². Since there are only 1,025 Earth-like planets in the cosmos, there is almost certainly no other planet with Earth’s exact complement of minerals. “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 change from the perspective of a few decades ago, when my mineralogy Ph.D. the counselor told me: ‘Don’t take a biology course. You’ll never use it!’ “