Shadows of Forgotten Ancestors

by Carl Sagan

  Closer in, where the disk gas has by now been cleaned away, some of the worlds are becoming Earth-like planets, another class of survivors in this game of world-annihilating gravitational roulette. The final accumulation of the terrestrial planets takes no more than 100 million years, about as long compared to the lifespan of the Solar System as the first nine months is relative to the lifetime of an average human being. A doughnut-shaped zone of millions of rocky, metallic, and organic worldlets, the asteroid belt, survives. Trillions of icy worldlets, the comets, slowly orbit the Sun in the darkness beyond the outermost planet.

  The principal bodies of the Solar System have now formed. Sunlight pours through a transparent, nearly dust-free interplanetary space, warming and illuminating the worlds. They continue to course and careen about the Sun. But look more closely still and you can make out that further change is being worked.

  None of these worlds, you remind yourself, has volition; none intends to be in a particular orbit. But those that are on well-behaved, circular orbits tend to grow and prosper, while those on giddy, wild, eccentric, or recklessly tilted orbits tend to be removed. As time goes on, the confusion and chaos of the early Solar System slowly settle down into a steadily more orderly, simple, regularly spaced, and, to your eyes, increasingly beautiful set of trajectories. Some bodies are selected to survive, others to be annihilated or exiled. This selection of worlds occurs through the operation of a few extremely simple laws of motion and gravity. Despite the good neighbor policy of the well-mannered worlds, you can occasionally make out a flagrant rogue worldlet on collision trajectory. Even a body with the most circumspect circular orbit has no warrantee against utter annihilation. To continue to survive, an Earth-like world must also continue to be lucky.

  The role of something close to random chance in all this is striking. Which worldlet will be shattered or ejected, and which will safely grow to planethood, is not obvious. There are so many objects in so complicated a set of mutual interactions that it is very hard to tell—just by looking at the initial configuration of gas and dust, or even after the planets have mainly formed—what the final distribution of worlds will be. Perhaps some other, sufficiently advanced observer could figure it out and predict its future—or even set it all in motion so that, billions of years later, through some intricate and subtle sequence of processes, a desired outcome will slowly emerge. But that is not yet for humans.

  You started with a chaotic, irregular cloud of gas and dust, tumbling and contracting in the interstellar night. You ended with an elegant, jewel-like solar system, brightly illuminated, the individual planets neatly spaced out one from another, everything running like clockwork. The planets are nicely separated, you realize, because those that aren’t are gone.

  ——

  It’s easy to see why some of those early physicists who first penetrated the reality of the nonintersecting, coplanar orbits of the planets thought that the hand of a Creator was discernible. They were unable to conceive of any alternative hypothesis that could account for such magnificent precision and order. But in the light of modern understanding, there is no sign of divine guidance here, or at least nothing beyond physics and chemistry. Instead we see evidence of a time of remorseless and sustained violence, when vastly more worlds were destroyed than preserved. Today we understand something of how the exquisite precision that the Solar System now exhibits was extracted from the disorder of an evolving interstellar cloud by laws of Nature that we are able to grasp—motion, and gravitation, and fluid dynamics, and physical chemistry. The continued operation of a mindless selective process can convert chaos into order.

  Our Earth was born in such circumstances about 4.5 or 4.6 billion years ago, a little world of rock and metal, third from the Sun. But we musn’t think of it as placidly emerging into sunlight from its catastrophic origins. There was no moment in which collisions of small worlds with the Earth ceased entirely. Even today objects from space run into the Earth or the Earth overtakes them. Our planet displays unmistakable impact scars from recent collisions with asteroids and comets. But the Earth has machinery that fills in or covers over these blemishes—running water, lava flows, mountain building, plate tectonics. The very ancient craters have vanished. The Moon, though, wears no makeup. When we look there, or to the Southern Highlands of Mars, or to the moons of the outer planets, we find a myriad of impact craters, piled one on top of the other, the record of catastrophes of ages past. Since we humans have returned pieces of the Moon to the Earth and determined their antiquity, it is now possible to reconstruct the chronology of cratering and glimpse the collisional drama that once sculpted the Solar System. Not just occasional small impacts, but massive, stupefying, apocalyptic collisions is the inescapable conclusion from the record preserved on the surfaces of nearby worlds.

  By now, in the Sun’s middle age, this part of the Solar System has been swept free of almost all the rogue worldlets. There is a handful of small asteroids that come near the Earth, but the chance that any of the bigger ones will hit our planet soon is small. A few comets visit our part of the Solar System from their distant homeland. Out there, they are occasionally jostled by a passing star or a nearby, massive interstellar cloud—and a shower of icy worldlets comes careening into the inner Solar System. These days, though, big comets hit the Earth very rarely.

  Shortly, we will sharpen our focus to one world only, the Earth. We will examine the evolution of its atmosphere, surface, and interior, and the steps that led to life and animals and us. Our focus will then progressively narrow, and it will be easy to think of us as isolated from the Cosmos, a self-sufficient world minding its own business. In fact, the history and fate of our planet and the beings upon it have been profoundly, crucially influenced, through the whole history of the Earth and not just in the time of its origins, by what’s out there. Our oceans, our climate, the building blocks of life, biological mutation, massive extinctions of species, the pace and timing of the evolution of life, all cannot be understood if we imagine the Earth hermetically sealed from the rest of the Universe, with only a little sunlight trickling in from the outside.

  The matter that makes up our world came together in the skies. Enormous quantities of organic matter fell to Earth, or were generated by sunlight, setting the stage for the origin of life. Once begun, life mutated and adapted to a changing environment, partially driven by radiation and collisions from outside. Today, nearly all life on Earth runs off energy harvested from the nearest star. Out there and down here are not separate compartments. Indeed, every atom that is down here was once out there.5

  Not all of our ancestors made the same sharp distinction we do between the Earth and the sky. Some recognized the connection. The grandparents of the Olympian gods and therefore the ancestors of humans were, in the myths of the ancient Greeks, Uranus,6 god of the sky, and his wife Gaia, goddess of the Earth. Ancient Mesopotamian religions had the same idea. In dynastic Egypt the gender roles were reversed: Nut was goddess of the sky, and Geb god of Earth. The chief gods of the Konyak Nagas on the Himalayan frontier of India today are called Gawang, “Earth-Sky,” and Zangban, “Sky-Earth.” The Quiché Maya (of what is now Mexico and Guatemala) called the Universe cahuleu, literally “Sky-Earth.”

  That’s where we live. That’s where we come from. The sky and the Earth are one.

  Chapter 2

  SNOWFLAKES FALLEN ON THE HEARTH

  There is not yet one person, one animal, bird, fish, crab, tree, rock, hollow, canyon, meadow, forest. Only the sky alone is there …

  Popol Vuh: The Mayan Book of the Dawn of Life1

  Before the High and Far-Off Times, O my Best Beloved, came the Time of the Very Beginnings; and that was in the days when the Eldest Magician was getting Things ready. First he got the Earth ready; then he got the Sea ready; and then he told all the Animals that they could come out and play.

  RUDYARD KIPLING

  “The Crab That Played with the Sea”2

  If you could drive an automobile
straight down, in an hour or two you would find yourself deep inside the upper mantle of the Earth, far beneath the pediments of the continents, approaching an infernal region where the rock becomes a viscous liquid, mobile and red-hot. And if you could drive for an hour straight up, you would find yourself in the near-vacuum of interplanetary space.3 Beneath you—blue, white, breathtakingly vast, and brimming over with life—would stretch the lovely planet on which our species and so many others have grown up. We inhabit a shallow zone of environmental clemency. Compared to the size of the Earth, it is thinner than the coat of shellac on a large schoolroom globe. But earlier, long ago, even this narrow habitable boundary between hell and heaven was unready to receive life.

  ——

  The Earth accumulates in the dark. Although the primitive Sun is ablaze, there is so much gas and dust between the Earth and the Sun that at first no light gets through. The Earth is embedded in a black cocoon of interplanetary debris. There’s an occasional flash of lightning by which you glimpse a ravaged, pockmarked, not quite spherical world. As it gathers up more and more matter, in units ranging from dust to worldlets, it becomes rounder, less lumpy.

  A collision with a hurtling worldlet produces a shattering explosion, and excavates a great crater. Much of the impactor disintegrates into powder and atoms. There are vast numbers of such collisions. Ice is converted to steam. The planet is blanketed in vapor—which holds in the heat from the impacts. The temperature rises until the Earth’s surface becomes entirely molten, a roiling world-ocean of lava, glowing by its own red heat, and surmounted by a stifling atmosphere of steam. These are the final stages of the great gathering in.

  In this epoch, when the Earth is new, the most spectacular catastrophe in the history of our planet occurs: a collision with a sizeable world. It does not quite crack the Earth open, but it does blast a good fraction of it out into nearby space. The resulting ring of orbiting debris shortly falls together to become the Moon.

  The day is only a few hours long. Gravitational tides raised in the Earth’s oceans and interior by the Moon, and in the Moon’s solid body by the Earth, gradually slow the Earth’s rotation and lengthen the day. From the moment of its formation, the Moon has been drifting away from the Earth. Even now, it hovers over us, a baleful reminder that had the colliding world been much bigger, the Earth would have scattered in fragments through the inner solar system—a short-lived, unlucky world like so many others. Then humans would never have come to be. We would be just one more item on the immense list of unrealized possibilities.

  ——

  Shortly after the Earth had formed, its molten interior was churning, great convection currents circulating, a world in a slow boil. Heavy metal was falling to its center, forming a massive molten core. Motions in the liquid iron began to generate a strong magnetic field.

  The time came when the Solar System had pretty well been swept free of gas and dust and rogue worldlets. On Earth, the massive atmosphere—that had kept the heat in—dissipated. Indeed, the collisions themselves helped to drive that atmosphere into space. Convection still carried hot magma up to the surface, but the heat from the molten rock could now be radiated away to space. Slowly the Earth’s surface began to cool. Some of the rock solidified and a thin, at first fragile crust formed, thickened, and hardened. Through blisters and fissures, magma and heat and gases continued to pour out of the interior.

  Punctuated by spasmodic flurries of worlds falling out of the sky, the bombardment slowed. Each large impact produced a great dust cloud. There were so many impacts at first that a pall of fine particles enveloped the planet, prevented sunlight from reaching the surface, and in effect turned off the atmospheric greenhouse effect and froze the Earth. There seems to have been a period, after the magma ocean solidified but before the massive bombardment ended, when the once molten Earth became a frozen, battered planet. Who, scanning this desolate world, would have pronounced it fit for life? What wild optimist could have foreseen that peonies and eagles would one day spring from this wasteland?

  The original atmosphere had been ejected into space by the relentless rain of worldlets. Now a secondary atmosphere trickled up from the interior and was retained. As the impacts declined, global dust palls became more rare. From the surface of the Earth the Sun would have seemed to be flickering, as in a time-lapse movie. So there was a time when sunlight first broke through the dust pall, when the Sun, Moon, and stars could first be noticed had there been anyone there to see them. There was a first sunrise and a first nightfall.

  In sunny intervals, the surface warmed. Outgassed water vapor cooled and condensed; droplets of liquid water formed and trickled down to fill the lowlands and the impact basins. Icebergs continued to fall from the sky, vaporizing on arrival. Torrents of extraterrestrial rain helped form the primeval seas.

  Organic molecules are composed of carbon and other atoms. All life on Earth is made from organic molecules. Clearly they had somehow to be synthesized before the origin of life in order for life to arise. Like water, organic molecules came both from down here and from up there. The early atmosphere was energized by ultraviolet light and the wind from the Sun, the flash and crackle of lightning and thunder, auroral electrons, intense early radioactivity, and the shock waves of objects plummeting groundward. When, in the laboratory, such energy sources are introduced into presumptive atmospheres of the primitive Earth, many of the organic building blocks of life are generated, and with astonishing ease.

  Life began near the end of the heavy bombardment. This is probably no coincidence The cratered surfaces of the Moon, Mars, and Mercury offer eloquent testimony to how massive and world-altering that battering was. Since the worldlets that have survived to our time—the comets and the asteroids—have sizeable proportions of organic matter, it readily follows that similar worldlets, also rich in organic matter but in much vaster numbers, fell on the Earth 4 billion years ago and may have contributed to the origin of life.

  Some of these bodies, and their fragments, burned up entirely as they plunged into the early atmosphere. Others survived unscathed, their cargoes of organic molecules safely delivered to the Earth. Small organic particles drifted down from interplanetary space like a fine sooty snow. We do not know just how much organic matter was delivered to and how much was generated on the early Earth, the ratio of imports to domestic manufactures. But the primitive Earth seems to have been heavily dosed with the stuff of life4—including amino acids (the building blocks of proteins), and nucleotide bases and sugars (the building blocks of the nucleic acids).

  Imagine a period hundreds of millions of years long in which the Earth is awash in the building blocks of life. Impacts are erratically altering the climate; temperatures are falling below the freezing point of water when the impact ejecta obscure the Sun, and then warming as the dust settles. There are pools and lakes undergoing wild fluctuations in conditions—now warm, bright, and bathed in solar ultraviolet light, now frozen and dark. Out of this varied and changeable landscape and this rich organic brew, life arises.

  Presiding over the skies of Earth at the time of the origin of life was a huge Moon, its familiar surface features being etched by mighty collisions and oceans of lava. If tonight’s Moon looks about as large as a nickel at arm’s length, that ancient Moon might have seemed as big as a saucer. It must have been heartbreakingly lovely. But it was billions of years to the nearest lovers.

  We know that the origin of life happened quickly, at least on the time scale by which suns evolve. The magma ocean lasted until about 4.4 billion years ago. The time of the permanent or near-permanent dust pall lasted a little longer. Giant impacts occurred intermittently for hundreds of millions of years after that. The largest ones melted the surface, boiled away the oceans, and flushed the air off into space. This earliest epoch of Earth history is, appropriately, called Hadean, hell-like. Perhaps life arose a number of times, only to be snuffed out by a collision with some wild, careening worldlet newly arrived from the depths of space.
Such “impact frustration” of the origin of life seems to have continued until about 4 billion years ago. But by 3.6 billion years ago, life had exuberantly come to be.

  ——

  The Earth is a vast graveyard, and every now and then we dig up one of our ancestors. The oldest known fossils, you might imagine, are microscopic, discovered only by painstaking scientific analysis. Some are. But some of the most ancient traces left by life on Earth are easily visible to the untrained naked eye—although the beings that made them were microscopic. Often meticulously preserved, they’re called stromatolites; not unusual are examples the size of a basketball or a watermelon. A few are half the length of a football field. Stromatolites are big. Their age is read from the radioactive clocks in the ancient basaltic lava in which they are embedded.

  They still grow and flourish today—in warm bays, lagoons, and inlets in Baja California, Western Australia, or the Bahamas. They’re composed of successive layers of sediment generated by mats of bacteria. The individual cells live together. They must know how to get on with the neighbors.

  We glimpse the earliest lifeforms on Earth and the first message conveyed is not of Nature red in tooth and claw, but of a Nature of cooperation and harmony. Of course, neither extreme is the whole truth; and, examining modern stromatolites more closely, we find single-celled microbes freely swimming in and around the mats. Some of them are busily devouring their fellows. Perhaps they too were there from the beginning.

  Some stromatolite communities are photosynthetic; they know how to convert sunlight, water, and carbon dioxide into food. Even today, we humans are unable to build a machine that can perform this transformation with the efficiency of a photosynthetic microbe, much less a liverwort. Yet 3.6 billion years ago the stromatolitic bacteria could do it.