O ne could easily be forgiven for thinking that life bears little connection to rocks. From high-school science curricula to Wikipedia, the institutional separation of geology and biology seems as ingrained today as when the 18th-century Swedish botanist Carl Linnaeus first distinguished animals, vegetables, and minerals. After all, what could be more different than a fragrant rose and a cold chunk of granite?
Minerals are usually defined as naturally occurring inorganic substances that combine to form rocks. Until recently, many geologists assumed that most rocks had been around since the origins of Earth , well before life formed on this planet. Even biominerals such as calcite and apatite, which organisms secrete to form shells, teeth and bones, are merely recent examples of very ancient and rather common non-biological materials. No wonder, then, that when I asked my PhD adviser if I should take a biology course as a capstone to my graduate studies, his response was: Why? You're a mineralogist. You'll never use biology!
For more than 20 years, my career flourished in blissful ignorance of microbes and mollusks, teeth and bone. But my perceptions changed a bit in 1996, when I began to research the origins of life.
Four decades earlier, the US chemists Stanley Miller and Harold Urey had performed their famous simulation of Earth's earliest years by mixing water (ie, oceans), gases (the atmosphere) and little sparks (lightning) in order to generate the basic biological building blocks. Minerals weren't part of the Miller -Urey experiment – their amino acids and sugars formed perfectly well without crystals cluttering up the reactions. But as the decades passed, the assembly of those building blocks into more lifelike macromolecules – such as proteins and DNA – proved difficult. And so theorists gradually brought minerals such as quartz-rich beach sand, shiny iron pyrite and fine-grained clay into the story. They said these minerals helped to select, concentrate and assemble small molecules into bigger molecules, while protecting the more delicate bio-bits from the ravages of Earth's hostile early environment. Now, almost everyone in the field agrees that mineral variety is essential to biogenesis, which means life and rocks have been together since the very beginning.
Life is thought to have arisen in the Archaean aeon , some 4 billion years ago, when Earth was blanketed by a thick noxious atmosphere tinged orange with hydrocarbon smog. The seas and the land were both barren then, and the raw materials of life – water, air, rocks – were bathed in lethal ultraviolet radiation. The planet's surface was marred by a steady stream of violent volcanic eruptions, asteroid bombardments and icy comet impacts. How could that alien, volcanic, comet-battered globe produce living molecules? The story is that minerals – quartz, pyrite, clays, and the like – must have provided the safe, shielded environments that spawned life. But for this theory to be true, those critical life-triggering minerals had to have been present when life first formed. Recent discoveries indicate that planetary mineralogy evolves, and that some minerals took a billion years or more to appear. How can we know that the crucial ones were around way back then?
Earth's near-surface mineralogy has changed radically in both diversity and distribution during its 4.5 billion year history. And we now know what caused these radical changes: basic chemistry, physics and, most surprisingly, biology. We also know that these general principles apply to the trillions upon trillions of rocky planets and moons that exist throughout the cosmos. This new evolutionary perspective is detailed enough to let us travel back in time, to the formation of the first minerals in the universe . We know that no minerals could have formed right after the Big Bang , for the universe was much too hot, and there weren't enough mineral-forming elements around. Almost nothing except the gaseous elements hydrogen and helium could have existed then. Nor did any minerals form inside the first stars, which were too hot to support solid crystals.
Modelling the outcome of the entangled and rapidly declining relationship between humans and Earth has become a key task for scientists across a range of fields, which has led to a blurring of scientific and political futures. The closer this relationship gets, the harder it becomes to disentangle the two from each other. For someone like myself, born in the mid-1990s, models of planetary conditions have almost become images of the future itself. The planet is suddenly everywhere . Planning, designing, research, strategising – even the grounds for thinking itself – are now adjusted to the planetary scale. As this scale becomes ubiquitous, it can sometimes be difficult to see that this form of the planet has a history of its own. If the planet just recently resurfaced as a new kind of environmental category, where did it come from?
But when those first stars exploded a few million years after the universe's birth, their expanding remains cooled, allowing carbon atoms within them to condense into diamonds, and a few other kinds of minute crystals, which are called the ur-minerals lead ), corundum (most familiar as ruby and sapphire in its coloured forms), and moissanite (a tough compound of silicon and carbon that's often used as a cheap substitute for diamond gemstones). These ancient species of crystals still fall to Earth today in the form of microscopic interstellar dust, left over from the great nebula that formed the Sun and planets more than 4.5 billion years ago.
The driving question behind mineral evolution is how vast quantities of dust, composed of the original dozen ur-minerals, were processed and reworked to yield the thousands of different minerals on Earth today. All of Earth's chemical richness – what we enjoy today as iPhones, skateboards, automobiles, flat-screen TVs, and countless other toys – was sequestered into those primordial dust grains, but in trace quantities. All but a dozen of the 80 or so chemical elements that make up planets were impossibly dilute, constituting only a few atoms in a million, or a billion, or even less. Barring some remarkably efficient concentration processes, the chances of those rare chemical elements clumping together to form separate, distinct mineral species were vanishingly small. To have the mineral diversity we enjoy today, the rocks that make up Earth needed to experience a number of interventions.
