Robert Hazen is one of five expert speakers on Scientific American’s 175th Anniversary Cruise to the Americas in March 2020. For more information on our 175th Anniversary Cruise, including a detailed itinerary, seminar descriptions and speaker bios, click here.
Look around you. Carbon is everywhere: In the paper of every book, the ink on its pages, and the glue that binds it; in the soles and leather of your shoes, the synthetic fibers and colorful dyes of your clothes, and the Teflon zippers and Velcro strips that fasten them; in every bite of food you eat, in beer and booze, in fizzy water and sparkling wine; in the carpets on your floors, the paint on your walls, and the tiles on your ceilings; in fuels from natural gas to gasoline to candle wax; in sturdy wood and polished marble; in every adhesive and every lubricant; in the lead of pencils and the diamond of rings; in aspirin and nicotine, codeine and caffeine, and every other drug you’ve ever taken; in every plastic from grocery bags to bicycle helmets, cheap furniture to designer sunglasses. From your first baby clothes to your silk-lined coffin, carbon atoms surround you.
Carbon is the giver of life: your skin and hair, blood and bone, muscle and sinews all depend on carbon. Bark, leaf, root and flower; fruit and nut; pollen and nectar; bee and butterfly; Doberman and dinosaur—all incorporate essential carbon. Every cell in your body—indeed, every part of every cell—relies on a sturdy backbone of carbon. The carbon of a mother’s milk becomes the carbon of her child’s beating heart. Carbon is the chemical essence of your lover’s eyes, hands, lips and brain. When you breathe, you exhale carbon; when you kiss, carbon atoms embrace.
It would be easier for you to list everything you touch that lacks carbon—aluminum cans in your fridge, silicon microchips in your iPhone, gold fillings in your teeth, other oddities—than to enumerate even 10 percent of the carbon-bearing objects in your life. We live on a carbon planet and we are carbon life.
Every chemical element is special, but some elements are more special than others. Of all the periodic table’s richly varied denizens, carbon, the sixth element, is unique in its impact on our lives. Carbon is not simply the static element of “stuff.” Carbon provides the most critical chemical link across the vastness of space and time—the key to understanding cosmic evolution. Over the course of almost 14 billion years, the universe has evolved, complexified and become ever more richly patterned with seemingly endless fascinating and quirky behaviors.
Carbon lies at the heart of this evolution—choreographing the emergence of planets, life and us. And, more than any other ingredient, carbon has facilitated the rapid emergence of new technologies, from steam engines of the industrial revolution to our modern “plastic age,” even as it accelerates unprecedented changes in environment and climate on a planetary scale.
So here are a few musings about carbon—the most important element in the cosmos.
Of all the varied high-pressure forms of carbon-bearing minerals, including crystalline forms both known and yet to be discovered, diamond will always hold pride of place. Diamond occupies the ideal niche between scarce and rare: it is sufficiently abundant that almost everyone can own one, but rare enough to command millions of dollars for newsworthy large stones. Hundreds of millions of gems large enough for a ring or necklace have been mined, but hundreds of millions of consumers want to possess one or more. The lure of diamonds extends to their scientific value; the more we study these almost pure fragments of carbon from Earth’s depths, the more we learn about the history and dynamics of our planet.
Diamonds have long been treasured for their rarity, beauty and perfection, but a growing scientific community is finding new reasons to value diamonds above all other gemstones. This new generation of diamond seekers does not crave the flawless stones of high-end engagement rings and tennis bracelets. On the contrary, above all else they prize imperfections in the form of tiny mineral inclusions—unsightly black, red, green and brown mineral specks and microscopic pockets of deep fluid and gas. These blemishes, typically cut away and cast aside in the faceting of precious gemstones, often represent pristine fragments of Earth’s deep interior—bits and pieces that originated long ago, far below our planet’s sunlit surface, where they were trapped and hermetically sealed as the growing diamonds engulfed them.
The stories they tell! Diamonds and their inclusions have the potential to divulge how deep, how long ago and in what surroundings the diamonds grew. Consider secrets that are now being revealed by the world’s largest stones. In the rich lore of diamonds, giant gems stand out: the 603-carat Lesotho Promise, unearthed in 2006 and touted as the greatest find of the new century; the legendary 793-carat Koh-i-Noor diamond found centuries ago in India, now in the crown of the British queen mother; the 813-carat Constellation, sold at auction in 2016 for a record $63 million; and the most outsized treasure of them all, the 3,106-carat Cullinan diamond, which was discovered in 1905 at South Africa’s Premier No. 2 mine as the surviving fragment of what must have been a much larger stone. It turns out that all of these giants share a common, unexpected origin.
For centuries, it was assumed that such magnificent gems are just large versions of more common, smaller stones. Not so. Hints at a different genesis come from optical studies. Most diamonds, though stunningly transparent to visible light, absorb wavelengths of infrared and ultraviolet light as a consequence of impurities at the atomic scale. Nitrogen atoms are the most common offenders. In “Type I” diamonds, nitrogen typically replaces about one in every 1,000 carbon atoms. When those nitrogen atoms congregate into little clusters, they may impart a yellow or brown color to the gems. The remaining diamonds, less than two percent of all mined gems, are “Type II.” Distinguished by their unparalleled transparency to both visible and ultraviolet light, Type II diamonds have no discernible nitrogen impurities and they tend to be larger and more optically perfect—characteristics that have led some scientists to posit a slower, deeper crystallization environment. Nevertheless, the exact origins of Type II diamonds remained a mystery.
In a headline-grabbing 2016 discovery, an international team of scientists headed by the Gemological Institute of America in New York (GIA) showed that Type II diamonds, including many of Earth’s biggest gemstones, host a distinct and curious suite of inclusions: silvery specks of iron-nickel metal quite different from the usual oxide and silicate mineral inclusions of their smaller cousins.
This research is a triumph on sociological as well as scientific grounds. Mine owners, gem cutters and collectors jealously guard their hoards; the bigger the diamond, the more difficult to gain access for scientific study. To win the opportunity for even a cursory examination of inclusions in one or two big diamonds would be an unexpected treat for most scientists. Those who had tried, who caught brief glimpses of the silvery inclusions in big stones, mistakenly assumed them to be the common mineral graphite—a result that was not particularly newsworthy. The GIA, teaming up with other diamond experts from the United States, Europe and Africa, had laid the groundwork for studies at an altogether grander scale. The nonprofit GIA in New York is tasked with certifying diamonds of all kinds: weighing them, grading them, teasing out their countries of origins, and constantly devising new tests to weed out the next generation of crafty synthetic fakes or illicit “conflict diamonds.”
GIA certification is the universal standard of excellence for diamonds. From their numerous contacts at mines and museums, they were able to assemble and probe in detail an astonishing collection of gems and cutting fragments from 53 big Type II diamonds. They even recut and polished five of the fragments to expose the silvery inclusions to the meticulous probing of advanced analytical instruments.
The first surprise came from composition studies. The metal-rich inclusions contain no oxygen, the mantle’s most abundant chemical element, but they are rich in carbon and sulfur—telltale impurities that reveal the metal must have been in a molten state when the diamonds formed. Remarkably, metal inclusions point to deep regions of our planet similar in composition to Earth’s inaccessible core, with its ocean of dense liquid iron and nickel surrounding a 1,520-mile diameter inner sphere of even denser crystalline iron and nickel alloy.
The inference: Big diamonds grow hundreds of miles beneath the surface in isolated mantle pockets of metal-rich liquid. Diamonds grow easily in such environments because iron metal has the unusual ability to soak up lots of carbon atoms. At sufficient pressure and temperature, diamonds nucleate and grow, with mobile carbon atoms passing easily through the metal melt, adding layer upon layer to potentially giant crystals. It’s not a complete surprise to scientists that some diamonds form in this metal-mediated way; metal solvents have been employed to grow large crystals in the synthetic diamond business since the early 1950s. But no one realized that nature had learned the same trick billions of years earlier.
The implications of this finding, that big diamonds have their own special provenance, go far beyond the quest for fancy gems. This distinctive population of Type II diamonds reveals a previously undocumented heterogeneity in the mantle. One might think that the mantle’s high temperatures, coupled with billions of years of mixing by convection, would have blended the mantle into a smoothie-like uniformity. Now, thanks to big diamonds and their telltale inclusions, we have clear evidence that the mantle is more like a fruitcake, with some relatively uniform regions but with swirls of novelty and lots of fruits and nuts (read metal and diamonds) thrown in.
What’s more, these local variations in the mantle’s rocks and minerals point to deep regions with wildly different chemical environments. We have long assumed that the mantle was made almost exclusively of oxygen-rich minerals. That’s what we typically see in the volcanic rocks called kimberlites that transport their trove of diamond gemstones to the surface and host the world’s richest diamond mines. But metal inclusions point to other mantle zones that are devoid of oxygen—regions where different chemical processes can occur.
As in so many facets of Earth’s evolution, the closer we look and the more data we collect, the more complicated and fascinating the story becomes.
CARBON AND GLOBAL CHANGE
We should not be coy about carbon and its role in climate change. Four facts are indisputable.
Fact one: Carbon dioxide and methane are potent greenhouse gases. Their molecules trap the sun’s radiation, reducing the amount of energy radiated into space. Higher concentrations of carbon dioxide and methane in the atmosphere mean more solar energy is trapped.
Fact two: The amounts of carbon dioxide and methane in Earth’s atmosphere are increasing rapidly.
Fact three: Human activities, primarily the burning of billions of tons annually of carbon-rich fuels, are driving almost all of the changes in atmospheric composition.
Fact four: Earth has been warming for more than a century.
Almost every scientist who has examined these compelling and unassailable facts arrives at the same unambiguous conclusion. Human activities are causing Earth to heat up. This conclusion is not a matter of opinion or speculation. It is not driven by politics or economics. It is not a ploy for researchers to obtain more funding or environmentalists to revel in hyperbolic press coverage.
Some things about Earth are true and this is one of those things.
CARBON AND THE SYMPHONY
Carbon chemistry pervades our lives. Almost every object we see, every material good we buy, every bite of food we consume, is based on element six. Every activity is influenced by carbon—work and sports, sleeping and waking, birthing and dying.
And what of other pursuits? What of music? A symphony orchestra—every section, every instrument—sings a song of carbon. The string section—violins and violas, cellos and basses—are composed almost entirely of carbon compounds: Wooden belly, fingerboard, sound post, pegs and tailpiece; gut strings, horsehair bow and plastic chin rest. String instruments also depend on slippery grease for the pegs and sticky rosin for the bow.
The woodwind section? The name gives the game away—wood forms the bodies of oboes, clarinets and bassoons. Bamboo provides their reeds; cork the linings of their elegant jointed bodies. Even metal flutes rely on lubricating oil and airtight leather pads for their stunning array of keys.
The percussion section bangs on a riot of carbon: ash drumsticks and calfskin drum heads, teak xylophones and ebony piano keys, castanets and tambourines, woodblocks and claves, maracas and marimbas, conga drums and bongo drums.
Pianos are much the same, with wooden frame, felt-lined hammers and rubber stops, all hidden in a curvaceous case elegantly finished with carbon-based paints, stains and lacquer. And, once upon a time, the 88 keys of each piano were sheathed in sturdy veneers of ivory—an expensive embellishment that led to the slaughter of thousands of elephants per year. One tusk provided enough pieces for 45 keyboards; thin plates, three rectangles to a key, were meticulously cut and then arrayed in the sun for weeks to achieve the preferred “white” key hue. Today tough plastics—ivory-colored polymers that simulate the banned carbon-based biomaterial—provide a benign synthetic substitute.
Ah, you say, but what of the brass family—surely trumpets and horns, trombones and tubas have no need of carbon. Silver-plated mouthpieces, copper lead-pipes, steel valves, brass tubing, U-shaped tuning slides and flaring bells are all crafted from solid metal. But fail to oil your valves or grease your slides and within a week all you have is a useless chunk of frozen metal.
Without carbon all would be silence.
LIFE: WHY CARBON?
Carbon is the element of crystals, of cycles and of stuff. Carbon, incorporated into myriad solid, liquid and gaseous forms, plays countless chemical roles that touch every facet of our lives. But what of living organisms, which display structures and functions far more complex than any inanimate material of nature or industry? What element will provide the vital spark of life?
For a chemical element to be central to life’s origins, it had better conform to a few basic expectations. Without question, any element essential to life has to be reasonably abundant, widely available in Earth’s crust, oceans or atmosphere. The element has to have the potential to undergo lots of chemical reactions; it can’t be so inert that it just sits there doing nothing. On the other hand, life’s core element can’t be too reactive; it can’t burst into flame or explode at the slightest chemical provocation. And, even if an element finds itself at a happy medium of chemical reactivity, in that ideal realm between explosive and dead, it must do more than just one chemical trick. It must be adept at forming sturdy and stable structural membranes and fibers—the bricks and mortar of life. It must be able to store, copy and interpret information.
And that special element, in combination with other ubiquitous elemental construction materials, must find a way to harness energy from combinations of other chemicals or perhaps the sun’s abundant light. Clever combinations of elements must store that energy in convenient chemical form like a battery and then release controlled pulses of energy whenever and wherever it is needed. The essential element of life has to multitask.
In that restrictive context, consider the many elemental alternatives. The most common elements in the cosmos are hydrogen and helium, the first and second occupants of the periodic table—the entire upper row—but they will never do as the foundation of a biosphere. Hydrogen, which can only bond strongly to one other atom at a time, fails the versatility test. Hydrogen is not unimportant, mind you. It helps to shape many of life’s molecules through “hydrogen bonding”—a kind of molecular glue—while it plays a vital co-starring role with oxygen in water, the medium of all known life forms. But element one cannot provide the versatile chemical foundation for life.
Helium, the second element in the periodic table, is of no use whatsoever—impossibly inert, a snooty “noble gas” that refuses to bond to anything, not even to itself.
Scanning across the periodic table, elements three through five (lithium, beryllium and boron) are much too scarce to build a biosphere. At concentrations of a few atoms per million in the crust, and even less in the oceans and atmosphere, you can safely cross them off the list of prospective life-giving ingredients.
Carbon, element 6, is the chemical hero of biology; we’ll come back to it.
Element seven, nitrogen, is an interesting case. Abundant in the near-surface environment, nitrogen forms about 80 percent of the atmosphere. It bonds with itself in pairs as N2, an unreactive molecule that comprises most of the gas we breathe. Nitrogen also bonds with many other elements—hydrogen, oxygen and carbon among them—to form a variety of interesting chemicals of relevance to biochemistry. Proteins are fabricated from long chains of amino acids, each holding at least one nitrogen atom. The vital genetic molecules DNA and RNA also incorporate nitrogen in their structural units, the so-called “bases” that define the genetic alphabet—A, T, G and C. But nitrogen, which is three electrons shy of the magic number 10, winds up being a little too greedy for electrons—its chemical reactions are a bit too energetic and the resulting bonds a bit too inflexible to play the multi-faceted role of leading actor. As a consequence, we can eliminate nitrogen from the competition.
Why not oxygen? After all, atom for atom oxygen is the most abundant element in Earth’s crust and mantle, representing more than half of the atoms in most rocks and minerals. In the feldspar mineral group, which accounts for as much as 60 volume percent of Earth’s varied continents and ocean crust, oxygen outnumbers other atoms by an eight-to-five margin. The ubiquitous pyroxene group features a three-to-two mix of oxygen with common metal elements like magnesium, iron and calcium. And quartz, the commonest mineral of most sandy beaches, is SiO2. It’s remarkable to think that when you lie on the beach, soaking up the sun, two thirds of what’s holding you up are atoms of oxygen.
As a consequence, oxygen is atom for atom about a thousand times more concentrated in the crust than carbon. But oxygen, in spite of its overwhelming abundance, is chemically boring. An isolated oxygen atom starts with only eight electrons, two electrons short of its desires, so it engages in indiscriminate hookups with just about any atom that will make up the deficit. True, oxygen is absolutely essential to all manner of biologically critical chemicals—sugars, bases, amino acids and of course water. Yet oxygen can’t form the requisite chains and rings and branching geometries that are so central to life’s intricate architecture. And so we can cross off abundant oxygen from the short list of life’s most critical atomic building block.
Fluorine, occupying the periodic table’s ninth position, is much worse, being only a single electron shy of the desired complement of 10. Fluorine sucks up electrons voraciously from almost any other element. Reactive fluorine corrodes metal, etches glass and explodes on contact with water. Breathe a lung full of fluorine gas and you will die horribly, in agony as your lungs blister with chemical burns.
And so it goes. Elements 10 and 18, neon and argon, are inert gases, so give them no further consideration. Sodium, magnesium and aluminum (elements 11 through 13) are too eager to give away electrons, while phosphorus, sulfur and chlorine (elements 15 through 17) are too eager to accept them. And as we delve deeper into the periodic table the elements become less common and the possibilities for life’s core chemistry dwindle.
An exception might be found in the abundant element silicon, which falls in the middle of the periodic table’s third row. Silicon is element 14, occupying the significant position right below carbon. Elements sharing a column of the periodic table often have similar properties, so perhaps silicon is a viable biological backup to carbon? Science fiction writers have seized on this option more than once.
I vividly recall an episode from the first season of the classic Star Trek TV show—the original one with William Shatner as Captain James T. Kirk and Leonard Nimoy as Mr. Spock—in which the crew of the Enterprise discovers a race of intelligent and potentially dangerous silicon-based life forms shaped like rocks. The concept of the show was fun, especially with the satisfying peaceful resolution as rocks and humans learned to get along. But the mineralogical premise was flawed; silicon is a biological dead end. Silicon at Earth’s surface has only one bonding imperative—find four oxygen atoms and make a crystal. Once formed, those silicon-oxygen bonds are too strong and too inflexible to do interesting chemistry. You simply cannot base a biosphere on a single-minded element like silicon.
Keep going, but you will search in vain for another promising elemental option. True, your eye might fall on iron, element 26, the fourth most abundant element in the crust after oxygen, silicon and magnesium. Why not iron? Iron loves to bond, and it’s flexible in its choices. Bond with oxygen? Sure, form red rust with ionic bonds. Bond with sulfur? Of course, make golden, lustrous metallic pyrite (aptly called “fool’s gold”) with covalent bonds. Iron bonds to arsenic and antimony, to chlorine and fluorine, to nitrogen and phosphorus, even to carbon in a variety of iron carbide minerals. And if no other elements are handy, iron happily bonds with itself in iron metal. Such a diverse bonding portfolio might seem ideal for the core element of life. But iron has a flaw. It readily forms minerals with big crystals, but it shies away from making small molecules. Life demands a huge variety of molecules, with chains and rings and branches and cages—tricks that iron rarely attempts.
And so we are left with carbon, the most versatile, most adaptable, most useful element of all. Carbon is the element of life.
What is our role in the evolutionary scheme of things, in the great carbon symphony? Humans are at once ordinary and unique. On the one hand, we are just another evolutionary step in a four-billion-year story that will likely continue long after our lineage has gone extinct or morphed into some new species. Some argue that we alone have the ability to radically alter Earth’s climate and environment, but oxygen-producing photosynthetic microbes and the diverse green plants that followed them have changed Earth’s near-surface environment in ways far more profound than any human actions.
Others point to humanity’s global influence on the continents through building cities, roadways, mines and farms, but trees and grasses far outstrip our impact on the landscape. Some say our species is unique in its potential to “destroy the planet,” but repeated catastrophic impacts of asteroids and explosive eruptions of megavolcanos have had far greater destructive consequences than any weapon devised by humans.
At the same time, our human species does possess unprecedented abilities. We are unique in the history of life in our technological prowess to adapt and alter our environments at scales from local to global. We are unique in our inventive exploitation of other species—animal, vegetable and microbial. We are unique in our exuberant desire and ability to explore beyond our world, perhaps eventually to colonize other planets and moons. And we are unique in our impact on Earth’s carbon cycle—a cycle that profoundly affects every aspect of our planet—earth, air, fire and water.
Humans are unique among life-forms because of the frenetic pace of the changes we impose. We are altering the planet at rates much faster than any prior species—at rates exceeded only by the sudden cataclysms of volcanoes exploding and rocks falling from the skies. Microbes took hundreds of millions of years to oxygenate the atmosphere, and perhaps a billion years more to oxygenate the oceans. Multicellular life required tens of millions of years to colonize the land after the earliest tentative encroachments.
These changes were profound, but they occurred over geological timescales that enabled life and rocks to co-evolve gradually. Earth’s ecosystems are remarkably resilient but they need generations to shift, to evolve, and to reset themselves in response to new environmental conditions. If humans pose a unique threat to Earth, as some scholars fear, then it is the unprecedented rate of environmental change that carries the greatest risk for damage to the biosphere.
That being said, the rocks and the varied microbes that live among them will do just fine whatever injuries we might do to our home and, inadvertently, to our own species. Earth will go on, life will go on, and the powerful process of evolution by natural selection will ensure that new creatures continue to inhabit every niche on the planet.
Carbon’s grand, eternal symphony unifies all of the elemental essences—earth, air, fire, water. Nothing exists in isolation; all are essential parts of the whole. Earth grows the solid crystals of carbon—sturdy foundation stones of land and oceans alike. Air holds the molecules of carbon that embrace us all—forever cycling, protecting and sustaining life. Fire, born of carbon, energizes the world, while providing unrivaled molecular variety to the material and living worlds. Water, which gave birth to carbon life, nurtures that life as it evolves and radiates to every corner of the globe. In a crescendo of exquisite harmony and complex counterpoint, each essence of carbon celebrates, and is celebrated by, the others.
Humans have learned to impose their own urgent themes and ever-accelerating tempi on this ancient score. We strip earth of its minerals. We flood air with our waste. We harness fire to satisfy our wants and needs. We exploit the teeming, living sphere of water, often careless of which species live or die.
We must, each of us, step back from the urgency of our desires to see our precious planetary home as a unique, but vulnerable, dwelling place. If we are wise, if we can temper our wants with a renewed sense of awe and wonder, if we can learn to cherish our rhapsodically beautiful carbon-rich world as it so urgently deserves, then we may hope to leave an unrivaled, priceless legacy for our children, their children and all the generations to come.
This essay is excerpted and adapted from: Symphony in C: Carbon and the Evolution of (Almost) Everything (W.W. Norton & Co., New York, 2019), by Robert M. Hazen.