Without life, two-thirds of all the known minerals on Earth would never have formed, a new study suggests. The discovery could aid the search for extraterrestrial life and improve climate change models.
The data to support the claim that the mineral and biological kingdoms co-evolved have been in the mineralogical literature for years. But this new study, by Robert Hazen of the Carnegie Institution in Washington DC and his colleagues, is the first to collect and summarize the mineralogical evolution of the planet from roughly 4.5 billion years ago to the present day.
In the beginning...
Scientists believe there were roughly a dozen minerals in the interstellar medium. According to the study, around a further 60 different minerals formed 4.5 billion years ago, as clumps of matter collided and coalesced to begin forming the Solar System. The smaller fragments congealed into larger, planet-sized bodies, where volcanism and the effects of water took the mineral count into the hundreds.
The planets Mars and Venus have got this far, Hazen notes. But it was plate tectonics and the origin of life that pushed Earth's mineral count into the thousands. That's because the movement of the planet's crust created new kinds of physical and chemical environments in which minerals could continue to diversify. As algae appeared in Earth's oceans, they released oxygen into the atmosphere through photosynthesis, which caused existing minerals to 'rust' and form metals such as iron and copper oxides.
In the ocean, algae evolved into more complex microorganisms with calcium carbonate shells. When these organisms died, they started to leave thick layers of calcite on the ocean floor. On land, the evolution of soil-based microbes and plants led to the hasty production of diverse clay minerals, like the talcs used in makeup and powdered laundry detergents and smectites in lip stick and nail polish. The study is published in American Mineralogist1.
Metal oxides, calcites and clay minerals would all be rare on a lifeless planet, but they are abundant on a living one, Hazen says. Astronomers searching for extraterrestrial life could look for signatures of these specific minerals in the atmospheres of other planets and determine whether they have life too, he explains.
Gary Ernst, a mineralogist at Stanford University in Palo Alto, California, says the "provocative evolutionary history" will encourage a wide range of new studies for years to come. The authors argue, for example, that mineral evolution occurred in a more punctuated style rather than by a 'Darwinian' mode of gradual succession. As Earth lurched from one state to the next, mineral diversity increased in spurts.
Peter Heaney of Pennsylvania State University in University Park says the work will force mineralogists to "confront some questions from a more holistic perspective that stretches our collective comfort zone".
Understanding mineral evolution provides the basis for cutting-edge geoscience research, research that includes the fate of carbon dioxide produced by burning fossil fuels, he says.
Scientists will also have to look at whether it is possible to quantify past fluctuations in mineral diversity with standardized metrics, he says. They will have to ask whether global oxidation destroyed minerals just as it poisoned organisms. "These are the kinds of ideas that this paper has made it safer to ask among the mineralogical cognoscenti," says Heaney.