Eventually, over many years of research, Boston and other scientists discovered that the microbes in Lechuguille do a lot more than spit out a little dirt. Lechuguilla is embedded in thick layers of limestone, the fossilized remains of a 250-million-year-old reef. The pipe chambers in such caves are usually formed by rainwater that seeps into the ground and gradually dissolves the limestone. But in Lechuguille, microbes are also the sculptors: Bacteria eating buried oil reserves release hydrogen sulfide gas, which reacts with the oxygen in the groundwater to produce sulfuric acid, which breaks down the limestone. In parallel, various microbes consume hydrogen sulfide and produce sulfuric acid as a byproduct. Similar processes occur in 5 to 10 percent of limestone caves worldwide.
Since Boston’s initial descent into Lechuguilla, scientists around the world have discovered that microorganisms transform the planetary 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 that lies beneath the crust—much closer to the surface. She and her colleagues drill wells up to a quarter of a mile into the uplifted mantle, mining long cylinders of 80-million-year-old rock, some of which are beautifully marbled in striking shades of maroon and green. In laboratory studies, Templeton showed 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 the rock, exhale hydrogen sulfide, and create new deposits of pyrite-like sulfide minerals, also known as fool’s gold.
Through related processes, microbes helped create some of the Earth’s reserves of gold, silver, iron, copper, lead, and zinc, among other metals. As subsurface microbes break down the rock, they often release the metals trapped in it. Certain chemicals released by microbes, such as hydrogen sulfide, combine with free-floating metals to form new solid compounds. Other molecules produced by the microbes grab the soluble metals and bind them together. Some microbes accumulate metals in their cells or grow on a crust of microscopic metal flakes that constantly attract even more metal, potentially creating a substantial layer over a long period of time.
Life, especially microbial life, has created a large number of Earth’s minerals, which are naturally occurring inorganic solid compounds with highly organized atomic structures, or, more simply, very elegant rocks. Today, the Earth has more than 6,000 different mineral species, most of which are crystals such as diamond, quartz, and graphite. However, Earth did not have much mineral diversity in its early days. Over time, the constant crushing, melting, and resolidification of the planet’s early crust shifted and concentrated the unusual elements. Life began to break down the rock and recycle the elements, creating entirely new chemical processes of mineralization. More than half of all minerals on the planet can only occur in high-oxygen environments that did not exist before microbes, algae, and plants oxygenated the ocean and atmosphere.
Through a combination of tectonic activity and the constant hustle and bustle of life, Earth has developed a mineral repertoire unmatched by any other known planetary body. In comparison, the Moon, Mercury, and Mars are mineral-depleted, with a maximum of a few hundred mineral species between them. 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 statistician Grethe Hystad calculated that the chance of two planets having an identical set of mineral species is one in 10³²². Since there are only an estimated 10²⁵ Earth-like planets in the universe, there is almost certainly no other planet with Earth’s exact mineral abundance. “The realization that the evolution of minerals on Earth is so directly dependent on biological evolution is somewhat shocking,” Hazen writes in his book “Symphony in C”. “It represents a major shift from the perspective of several decades ago, when my Ph.D. the counselor told me, ‘Don’t take the biology class. You’ll never use it!”