Symphony in C

Carbon and the Evolution of (Almost) Everything

Robert Hazen

• Publisher: William Collins
• Year: 2019
• ISBN:
  9780008292409
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

Composed in four movements, Symphony in C explores carbon’s multi-faceted characteristics, as epitomised by the classical elements of the ancients – Earth, Air, Fire and Water.

MOVEMENT IV—WATER

Carbon, the Element of Life

Earth, Air, and Fire—enough for a majestic world,
enough for a benign environment, enough for rich stores of
material goods—yet the key essence, Water, is missing.
Carbon is also the element of the living world.

The periodic table has been made. Stars have exploded.
Planets have formed, and a wealth of chemical compounds—
crystal, liquid, gas—have been forged.
Earth is poised to perform its most creative act;
the emergence of carbon-based life is at hand.

Evolution and radiation bring innovation after innovation—
gather the carbon atoms, harness sunlight,
build hard carbon-rich shells, venture onto land.
Life evolves and, in so doing, forever alters Earth’s cycle of
carbon as the domains of Earth, Air, Fire, and Water coevolve.

INTRODUCTION—The Primeval Earth

IMAGINE EARTH 4.5 BILLION years ago—an alien, inhospitable world, bombarded by rocks from space, scorched by floods of lava and venting steam, bathed in the young Sun’s lethal, unfiltered ultraviolet radiation. No life could have emerged—much less survived—such extreme environmental insults. Yet, in spite of the unforgiving tantrums of infant Earth, the raw ingredients for life were all in place.

Water? Check. The biosphere depends on water. Cells are more than half water by weight, and virtually all origin scenarios demand a watery context. Water is the universal solvent of biology—the milieu in which cells emerge, thrive, and multiply.

Energy? Check. All life-forms require reliable sources of energy, whether the chemical energy of food or the radiant energy of sunlight. And if those tried-and-true sources weren’t enough, early Earth also boasted the inexhaustible energy of internal geothermal heat, the pulsed energy of lightning, and the pervasive nuclear energy of radioactive decay.

Carbon? Check. The carbon-based molecules of life rained down on early Earth in a steady barrage of carbonaceous meteorites. Even-richer supplies of life’s building blocks emerged from Earth itself—its atmosphere, oceans, and rocks, as our planet became an engine of molecular novelty.

The stage was set. Earth, Air, Fire, and Water were about to organize themselves into something new: Life.

EXPOSITION—Origins of Life

THE STORY OF LIFE’S origins and evolution is an epic tale, best recounted in the language of carbon chemistry. The single greatest transition in Earth’s history was the emergence of the biosphere. We know it happened; we are here and driven to understand how it happened. Yet that saga of emergence remains only a vague narrative, largely hidden in the shadows of deep time. A small army of scientists—explorers driven by their curiosity to know what no one yet knows—devote their professional lives to this pursuit. We take on the challenge with no guarantee that a convincing resolution will be found before we die, for this is a journey of discovery that has taken centuries—a quest with far more questions than answers.

Life’s Origins—The Five Ws

A plucky reporter will investigate the “five Ws” of any story: who, what, where, when, and why. Add “how” to the list and you have a comprehensive summary of the challenging questions confronting origins-of-life researchers. To these classic questions, we can respond with varying degrees of confidence, though none of these puzzles has been fully solved.

Who and Why?1

“Who?” and “why?” in the context of life’s origins are questions more suited to philosophers and theologians than to lab-based scientists. Strong, even dogmatic, opinions abound, but science must remain neutral on the why, entwined as it is with the age-old question of life’s meaning and purpose. Science relies on independently reproducible observations, experiments, and mathematical logic—an epistemology in sharp contrast to philosophy and theology. Yet science surely informs philosophy; after all, how can we understand the meaning and purpose of the Universe without knowing the ground rules of the cosmic game? Nevertheless, scientists are powerless to tell us why the Cosmos, with all its richly varied living and nonliving bits, exists.

“Who?” is equally unanswerable by the rigorously constrained scientific method, which demands objective, independently verifiable results. Unless we find that life on Earth was purposely seeded by an alien intelligence—a curious concept dubbed “directed panspermia” with its own amusingly speculative literature—the question of who is also beyond the narrow purview of science.

When?

We can be a lot more confident about when life emerged because we have discovered two rigid, bookend-like constraints. On the one hand, the Moon emerged following a spectacular collision between Earth and the smaller hypothetical planet Theia—a cataclysm that disrupted both worlds about 4.5 billion years ago, as suggested by the isotopic ages teased from the oldest lunar crystals.2 Even if some form of primitive life emerged before that catastrophic Moon-forming event, the impact of Theia created a globe-encircling magma ocean that obliterated any living realm on Earth. Forming the Moon represented a globally sterilizing reset for oceans, atmosphere, and life.

On the other hand, fossil evidence (scrappy fossils from some of Earth’s most ancient rock formations in Greenland) reveals that microbial life was well established by about 3.7 billion years ago. Those distinctive stromatolites—mound-like structures with layer upon microscopic layer of minerals deposited by microbes—speak to cellular life that was already highly evolved. We must conclude that life emerged long before those most ancient of fossils.

Exactly when life arose between 4.5 and 3.7 billion years ago remains uncertain. Some experts believe that Earth was habitable, with oceans and an atmosphere, as early as 4.4 billion years ago; a quick emergence of life favors such an early date. Other experts prefer an origin event closer to 3.9 billion years ago, a time after a suspected interval of intense disruption by a swarm of big asteroids and comets. Direct evidence of those globe-rattling collisions has been thoroughly erased from Earth’s rapidly resurfaced crust, but the timing and abundance of the so-called Great Bombardment remain vividly etched on the Moon’s scarred surface. In any event, we can state with confidence that Earth has been a living world for more than 80 percent of its dynamic history.

Where?

“Where?” in the context of life’s origins is intriguing, as it raises questions about unknowable locations that have long been erased from the globe. If, as most of us expect, life emerged on Earth as opposed to some distant world, and if it emerged quickly within a few million years of the catastrophic impact that blasted Earth’s outer layers and led to the Moon’s formation, then we should consider Earth’s cooler polar regions as the most likely place for life’s emergence.

Rocks at the poles would have solidified first and were least affected by the immense tidal forces generated by the nearby, newly assembled Moon. Orbiting frenetically, the young Moon cycled through the familiar sequence of lunar phases once every few days. In the millennia after its assembly, the Moon appeared as an immense ball in the sky at less than a tenth of its present distance. Thousand-foot tides must have swept across the perpetually disrupted globe. Only Earth’s cooler, stable poles would have been safe from the precocious Moon’s pervasive, destructive influence 4.4 billion years ago.

If we assume a more leisurely time frame for life’s origins—say, a few hundred million years after the planet formed, when Earth had cooled and the Moon had receded to a safer distance—then the question of where life began on the globe is lost to time. Poles, mid-latitudes, the equator—it hardly makes a difference, and we will never be able to pinpoint the exact GPS coordinates.

But intriguing twists complicate the “Where?” question. Perhaps life originated on some other world and Earth was seeded from afar. This speculative, untested idea comes in at least two distinct flavors. The more “scientific” version looks to a nearby planet, almost certainly Mars, where warm and wet conditions conducive to life may have occurred tens to hundreds of millions of years before Earth became habitable.3 If life is a cosmic imperative that pops up quickly on any habitable world, then microbes on Mars likely came first. A few of those hardy microscopic bugs, safely nestled in a protective layer of rock, may have hitched rides on Martian meteorites ejected in the violent aftermath of powerful asteroid impacts. Such violent events must have peppered the surface of Mars with regularity.

It may seem counterintuitive, but mathematical models of large impacts reveal that huge chunks of the surface could have been launched into space with relatively little disruption to the rocks or their encased microbial communities. After their relatively short ride to Earth, those microbial hitchhikers could have become the first colonizers, the ancestors to all life today. It might sound far-fetched, but one of NASA’s rationales for continued Mars exploration is the search for Earthlike microbes in protected ecosystems beneath today’s red, desiccated surface. If discovered, and if those microbes share the biochemical quirks of Earth life, then many of us will conclude that Mars did it first and that we are all descended from Martians.

For the record, a few scholars postulate a more distant origin of life. Astrophysicist Fred Hoyle, who gained fame by discovering the triple-alpha process that makes carbon in stars, was an outspoken proponent of a version of panspermia by which virus-bearing comets brought the first life to Earth.4 What’s more, he argued, they continue to infect the planet with new viral diseases that rain down from space. Most scientists view such a scenario as nonsense.

Others speculate on the possibility of life having come from another star system, perhaps even intelligently designed and purposefully seeded in an act of directed panspermia. Such a hypothesis is, at least for now, untestable by science. The pseudoscientific idea is also intellectually flaccid, for it merely transposes the question of life’s origins to another place and time. After all, who designed the designers?

What Is Life?5

And then there’s the maddening “What?” question. If we are to solve the riddle of life’s ancient origins, then we should probably know what life is. But we don’t.

We almost always know life when we see it, but surprisingly, biologists have so far failed to craft a universally accepted definition. This lexical shortcoming stems not from any difficulty in recognizing hopping frogs or swaying birches, but rather from our relative ignorance of cosmic possibilities: we have only one biosphere to study, only one sampling of “life.” If verdant Earth is the only living world in the Cosmos, then we could easily compile a satisfying laundry list of chemical idiosyncrasies and physical characteristics unique to our own biosphere. If we are truly alone in the vastness of space, then our Earth-based taxonomy would provide a comprehensive definition of life. We would point to essential chemical ingredients such as carbon and water, ubiquitous molecular modules like proteins and DNA, distinctive structures including ribosomes and mitochondria, all enclosed in microscopic cells, the most fundamental common unit of Earth’s diverse biosphere.

On the other hand, if the Universe holds innumerable other living worlds, as many of us who study cosmic history suspect that it does, then it would be presumptuous for us to define life in such a narrow, Earth-centric way. That’s why scientists, attempting to distinguish the living from the nonliving realm, resort to lists of more general characteristics and behaviors. All imaginable life-forms—collectively, if not individually—will have the ability to reproduce, to grow, to respond to environmental changes, and to evolve new and novel attributes. NASA, whose long-term mission includes the search for life on other worlds, adds the proviso that life must be a chemical system, composed of interacting atoms and molecules. Accordingly, a computer-based electronic “life-form”—a growing, evolving entity of zeros and ones confined by silicon semiconductors, for example—would be something quite different, requiring a new taxonomy and a different set of organizational rules.

The “What?” question thus encompasses the ambiguity of rigorously defining the essence of life. Scientists approach that taxonomic question with caution and respect, for we have at present only one example of a living world. That state of ignorance might change on any date with a transformative discovery of alien life by one of our planetary probes or direct contact by a more distant alien species. But as of today, we have no scientific basis on which to catalog the range of natural phenomena that might be said to be “alive” (the infinitely creative musings of science fiction writers notwithstanding).

Whatever the still-unproven cosmic diversity of life, efforts to understand the origin (or origins) of life focus on the accessible biology we know best: carbon-based life on Earth. Exploring the ancient transition from a lifeless geochemical world to a planet rich in biochemistry represents one of the most daunting scientific challenges. That ancient transformational leap was too profound to explain with any one theory or to explore in any one set of experiments. Better to divide the story into many comprehensible chapters, each adding a degree of structure and complexity to the evolving world of carbon chemistry.

And that leaves us with the remaining big question: “How did life arise?”

Life’s Origins: The Chemical Ground Rules

When tackling one of nature’s great mysteries, it’s best to start by examining the ground rules. Three core assumptions frame the study of life’s origins. First, most researchers would agree that planets provide all the raw materials—oceans, atmosphere, and a host of rocks and minerals. Many of us also conclude that the origin of life required a sequence of chemical steps, each one adding a degree of complexity and function. And the third and most basic assumption of virtually every origins-of-life investigation is the central role of carbon. Carbon is the cornerstone element of life on Earth today, so most of us in the origins game conclude that it must have been that way from the beginning. But can we be sure?

Making 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? Which element will provide the vital spark of life?

For a chemical element to be central to life’s origins, it must 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, and 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 would never do as the foundation of a biosphere.

Hydrogen, which can bond strongly to only 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 1 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 3 through 5 (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, they can safely be crossed off the list of prospective life-giving ingredients.

Carbon, Element 6, is the chemical hero of biology; we’ll come back to it.

Element 7, 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 “bases” that define the genetic alphabet—A, T, G, and C (adenine, thymine, guanine, and cytosine). But nitrogen, which is 3 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 multifaceted 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 percent of the volume of Earth’s varied continents and ocean crust, oxygen outnumbers other atoms by an 8-to-5 margin. The ubiquitous pyroxene group features a 3-to-2 mix of oxygen with common metal elements like magnesium, iron, and calcium. And quartz, the most common mineral on the majority of 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 is.

But oxygen, in spite of its overwhelming abundance, is chemically boring. ...

Table of contents

• TITLE PAGE
• COPYRIGHT
• DEDICATION
• CONTENTS
• PROLOGUE
• SILENCE
• MOVEMENT I—EARTH: Carbon, the Element of Crystals
• MOVEMENT II—AIR: Carbon, the Element of Cycles
• MOVEMENT III—FIRE: Carbon, the Element of Stuff
• MOVEMENT IV—WATER: Carbon, the Element of Life
• PICTURE SECTION
• NOTES
• INDEX
• ACKNOWLEDGMENTS
• ABOUT THE AUTHOR
• ABOUT THE BOOK
• ALSO BY ROBER T M. HAZEN
• ABOUT THE PUBLISHER