We may think we are the first organisms to remake the planet, but life has been transforming the Earth for eons.
Adapted for publication and use by Big History Project from the essay, “Mineral Fodder,” by Robert Hazen, published on Aeon.co.
If you look at science textbooks and sources like Wikipedia, the separation of geology and biology seems as strong today as hundreds of years ago. Even now, it would be easy to think that life and rocks are not connected. After all, what could be more different than a rose and a cold chunk of granite?
Minerals are nonliving substances that occur in nature. When minerals combine, they form rocks. Until recently, many geologists assumed that most rocks had been around since the Earth first formed, long before life appeared. Even biominerals—the minerals living organisms produce to form shells, teeth, and bones—are just more-recent examples of ancient and common nonbiological materials.
My perception of the relationship between geology and biology changed in 1996, when I began to research the origins of life. Forty years earlier, chemists Stanley Miller and Harold Urey had performed a simulation of Earth’s earliest years by mixing water, gases, and little sparks to generate the basic building blocks of life. Minerals weren’t part of their experiment. As the decades passed, the assembly of those basic building blocks into larger molecules, such as proteins and DNA, proved difficult. And so theorists brought minerals into the story. They suggested that minerals helped small molecules assemble into bigger molecules. Now, most scientists agree that mineral variety is essential to the development of life.
Around 4 billion years ago, Earth was covered by a thick, poisonous atmosphere. There was no life on the planet, and the raw materials of life – water, air, rocks – were bathed in deadly ultraviolet radiation. The planet’s surface was disrupted by volcanic eruptions and the impact of asteroids and icy comets. How could such a planet produce living molecules? The theory is that minerals provided the protected environments that gave rise to life. But for this theory to be true, those critical minerals had to have been present when life first formed. Recent discoveries have taught us 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?
The diversity and distribution of those minerals present near Earth’s surface have changed a lot during our planet’s history. And we now know what caused these changes: basic chemistry, physics, and biology. We also know that these general principles apply to the 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 right after the Big Bang, it was impossible for minerals to form. The Universe was much too hot and there weren’t enough of the necessary elements around. No minerals formed inside the first stars, either—they were too hot as well.
But when those first stars exploded a few million years after the Big Bang, their remains cooled, allowing carbon atoms within them to condense into diamonds and a few other kinds of tiny crystals. These crystals are called the ur-minerals. About a dozen mineral species emerged. These ancient species of crystals still fall to Earth today in the form of microscopic interstellar dust.
How did vast quantities of dust, composed of the original dozen ur-minerals, give rise to the many minerals on Earth today? All of Earth’s chemical richness was bound up in those first dust grains. Most of the 80 or so chemical elements that make up planets were very rare. The chances of those rare chemical elements clumping together to form distinct mineral species were very, very small. To have the mineral diversity we enjoy today, a number of things had to happen to the rocks that make up Earth.
For Earth’s first half-billion years, there were no living cells on its harsh surface. The earliest signs of new minerals occurred when the Sun began to flare, sending plumes of fire into the dust and gas spinning around it. The original 12 ur-minerals melted and remixed to generate 60 species, followed by another 100, which emerged when gravity clumped dust and gas into larger and larger objects. Heat, pressure, and water transformed crystals into new forms, and collisions between rocks produced shocks that led to even more mineral diversity.
As Earth grew, dense metal sank through a layer of rocks to form the planet’s core. Around this core was an outer crust made primarily of black volcanic basalt. The near-surface of Earth contained dozens of rare elements. Mineral-forming elements, metals, radioactive elements, and elements of life were all present in high concentrations in Earth’s evolving crust.
Early Earth became an engine of mineral production, thanks to the water on its surface, the heat in its deeper layers, and the rock-recycling system of plate tectonics. These, and other physical and chemical processes, combined to form 1,500 different minerals. That’s a large number, but today we know of 5,000 different minerals species. What created the remaining 3,500 minerals? We’ve come to the conclusion that most minerals are the result of biological processes.
The story of how rocks and life evolved together began around 4 billion years ago, when there were only rocks, air, and oceans on Earth. Scientists realized that air and oceans aren’t enough to create life. Only when certain minerals are added will simple, nonliving molecules concentrate and combine in complex ways that result in life. The earliest microbes made their own biomolecules by getting energy from minerals. These microbes breakdown unstable minerals such as iron, sulfur, and carbon, and get a small boost of energy in the process.
The rock-eating microbes transformed the Earth’s surface, but not nearly as much as the Great Oxidation Event (GOE), which began about 2.4 billion years ago. The GOE was the most dramatic event in Earth’s mineral evolution. During the GOE, special microorganisms began to flourish along coastlines that were rich in eroded mineral nutrients. These microorganisms, called photosynthetic microbes, used the Sun for energy and produced oxygen as a by-product. Oxygen is a gas that attacks and changes most rocks and minerals, transforming them into new forms, such as rust. More oxygen in the ancient atmosphere meant more erosion, which led to more nutrients, and even more photosynthetic microorganisms. And that’s just one cycle among many that demonstrates how nearly every aspect of Earth’s near-surface environment is an example of the interplay between life and rocks. Life arose from minerals. And then minerals arose from life.
As we look back on Earth’s history, it’s natural to wonder if we are living in a new era. The feedback between life and rocks has wildly accelerated, mostly because of one organism: Homo sapiens. During the past 10,000 years, humans have begun to alter many of Earth’s near-surface cycles through mining, agriculture, the leveling of forests, the damming of rivers, the production of new chemicals, and the burning of fossil fuels.
These shifts within the Earth system are radical, even on a geological time scale that spans millions of years. Although we might not be able to understand the consequences of these shifts for some time, we had better try. After all, the Earth will continue to evolve as a dynamic living world whether or not our species survives. Saving ourselves will require a deeper understanding of the complicated relationship between rocks and life, a relationship that sustains the only home we have ever known.
About the author: This piece was adapted for publication and use by Big History Project from the essay,“Mineral Fodder,” by Robert Hazen. Robert Hazen is a research scientist at the Carnegie Institution of Washington’s Geophysical Laboratory and professor of earth science at George Mason University. His latest book is The Story of Earth (2012).
Cover image: Banded iron formation, Karijini National Park, Western Australia, by Graeme Churchard, CC BY 2.0.