Geochemistry. The descent of minerals.

E volutionary theory states that some differences between individuals in a population are heritable, so that, when the environment changes, individuals bearing traits that provide the best adaptation to the new environment have the greatest chance for reproductive success. In a recent article in the American Mineralogist (1), Hazen et al. apply the term “evolution” to minerals in a similar manner, because new mineral patterns will adapt or evolve with changing environmental conditions throughout geologic time. The authors argue that the vast increase in the number of discrete mineral phases—from a sparse ~60 at the time of Earth’s formation to the more than 4300 known mineral species found on the modern Earth’s surface—requires an evolutionary explanation that goes beyond pure physical and chemical considerations. Hazen et al. define three eras comprising 10 stages, which relate mineral evolution to the chronological divisions of Earth’s history. The eras of Earth’s mineral evolution arise from three basic mechanisms: first, the gradual change in the concentration and distribution of elements from the presolar nebula homogeny to an expanded mineralogical diversity through physical processes during the era of planetary accretion (over 4550 million years ago); second, the introduction of new physicochemical conditions on the Earth’s surface, such as variable pressures and temperatures, as well as the increased activities of H 2 O, CO 2 , and O 2 , led to a marked diversification of minerals in the terrestrial realm during the era of crust and mantle reworking (4550 to 2500 million years ago); and third, the development of widespread nonequilibrium conditions with the rising influence of life on the environment during the era of biologically mediated mineralogy (2500 million years ago to the present). This process of mineral evolution is irreversible, advancing from sparse diversity to increasingly more variable and complex mineral assemblages. With the establishment of these chronological divisions, the authors present the scientific community with a new way to visualize the origin and evolution of Earth in a logical systematic fashion through its evolving mineralogy. Despite their complexity, the history of the planet and the history of life on Earth are profoundly linked. Science and technology have advanced to the point where this relationship can now be explored. Indeed, Hazen et al. relate surface geochemistry and life evolution to mineral diversity. When the Earth’s surface achieved a geochemical state that allowed life to flourish, geomicrobiological processes began to form biominerals, probably via biologically induced precipitation. These processes led to the production of large mineral deposits, such as giant banded iron formations (BIFs), which are a major economic source of iron ore. The deposition of the iron minerals in BIFs is thought to require the presence of microorganisms that can photosynthetically produce O 2 and/or oxidize Fe ions (2). Another example of the early association of minerals and life is exemplified by fossil stromatolites, which are considered to be the oldest evidence for microbial life on Earth. In the rock record, these remarkable structures appear as wavy laminated, lithified sedimentary growth structures, which accrete away from an initiation point or surface (3) (see the figure, left panel). The laminae comprise biominerals, such as carbonates, silica, and phosphates, and represent the former presence of a viable microbial community. This ancient microcosm contained the essential trophic groups needed to maintain life—primary producers, consumers, and decomposers—organized into specific communities that interacted with each other (4). These biological interactions induced mineral precipitation, which may have benefited the microorganisms by providing a mechanism for generating energy. The biochemical process of lamina formation can be observed in modern microbial mats cultured in the laboratory. The microbial community produces discrete carbonate laminae in an organic matrix, or biofilm, as a by-product of distinct metabolic activity, such as photosynthesis or sulfate reduction (5) (see the figure, right panel). Over time, the organic matrix decomposes, allowing the intercalated laminae to coalesce and eventually become amalgamated into lithifed layers, which can potentially be preserved in the rock record. Thus, the study of modern microbial mats suggests that the oldest evidence for life represents the record of a primitive minimal ecosystem, which is, however, a very complex system evolved from a cooperative and sustainable association of organisms. Indeed, the appearance of stromatolites in the early Archean (~3500 million years ago) implies that life and the associated biominerals most likely coevolved on the primitive planet. This early but complex biogeochemical phenomenon directly links biological evolution with mineral evolution. With the introduction of mineral evolution, Hazen et al. provide a new perspective on the study of Earth history. Combining inorganic and organic processes in a chronological association allows scientists to calibrate geologic events in the context of the 10 stages of mineral evolution, from the simplicity of the presolar dust particles to the biomineral explosion in the past 545 million years. Understanding the formation sequence and the interactions of The appearance of minerals during Earth history is closely linked to biological evolution. The Descent of Minerals