After years of research, Boston and other scientists finally found that the microbes in Lechuguilla do much more than just spit out a bit of dirt. Lechuguilla is surrounded by thick layers of limestone, the fossilized remains of a 250-million-year-old reef. The numerous chambers in such caves are usually formed by rainwater seeping into the ground and gradually dissolving the limestone. But in Lechuguilla, microbes are also the sculptors: Bacteria eating buried oil reserves release hydrogen sulfide gas, which reacts with oxygen in groundwater to produce sulfuric acid, which erodes 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 first descent into Lechuguilla, scientists around the world have discovered that microorganisms are altering the Earth’s crust wherever they inhabit it. Alexis Templeton, a geomicrobiologist at the University of Colorado at 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 holes up to a quarter-mile deep into the uplifted mantle and extract long cylinders of 80-million-year-old rock, some beautifully marbled with striking shades of brown and green. In lab studies, Templeton has demonstrated that these samples are teeming with bacteria, some of which are altering the composition of the Earth’s crust: They feed on hydrogen and inhale sulfates in the rock, exhale hydrogen sulfide, and create new deposits of sulfide minerals that resemble pyrite, also known as fool’s gold.
Through similar processes, microbes have helped form some of Earth’s stores of gold, silver, iron, copper, lead, zinc, and other metals. When subterranean microbes break down rocks, they often release the metals trapped within them. Some of the chemicals released by microbes, 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 form a crust of microscopic metal flakes that continually attract more metal, potentially building up a significant deposit over long periods of time.
Life, particularly microbial life, gave rise to a large proportion of Earth’s minerals, which are naturally occurring inorganic solid compounds with highly organized atomic structures, or, more simply, very elegant rocks. Today, there are more than 6,000 different types of minerals on Earth, most of which are crystals such as diamond, quartz, and graphite. However, in its early days, Earth did not have a great diversity of minerals. Over time, the continuous crumbling, melting, and re-solidification of the early Earth’s crust shifted and concentrated unusual elements. Life began to break up rock and recycle elements, creating entirely new chemical mineralization processes. More than half of all minerals on the planet can only form in an oxygen-rich environment, which did not exist before microbes, algae, and plants oxygenated the ocean and atmosphere.
Through the combination of tectonic activity and the constant activity of life, the Earth developed a mineral repertoire that no other known celestial body can boast. The Moon, Mercury and Mars are comparatively poor in minerals, containing at most a few hundred mineral species. The diversity of minerals on Earth depends not only on the existence of life, but also on its peculiarities. Robert Hazen, mineralogist and astrobiologist at Carnegie Science, and statistician Grethe Hystad have calculated that the probability of two planets having an identical combination 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 as many minerals as the Earth. “The realization that the mineral development of the Earth depends so directly on biological evolution is quite shocking,” Hazen writes in his book Symphony in C: “It represents a fundamental shift from the view of a few decades ago, when my PhD supervisor in mineralogy told me, ‘Don’t take a biology course. It will never do you any good!'”