Shadows of Forgotten Ancestors

by Carl Sagan

  Exactly what happened between the time of the first seas, rich in organic molecules and future prospects, and the time of the first stromatolites is beyond our present ability to reconstruct. Stromatolite-forming microbes could hardly have been the first living things. Before there were colonial forms, there must, it seems, have been individual, free-living, one-celled organisms. And before that, something even simpler. Perhaps before the first photosynthetic organisms, there were little beings that could eat the organic matter littering the landscape: Eating food seems to be a great deal less demanding than manufacturing it. And those little beings themselves had ancestors … and so on, back to the earliest molecule or molecular system able to make crude copies of itself.

  Why did colonial forms develop so early? Maybe it was because of the air. Oxygen, generated today by green plants, must have been in short supply before the Earth was covered by vegetation. But ozone is generated from oxygen. No oxygen, no ozone. If there’s no ozone, the searing ultraviolet light (UV) from the Sun will penetrate to the ground. The intensity of UV at the surface of the Earth in those early days may have reached lethal levels for unprotected microbes, as it has on Mars today. We are concerned—and for good reason—that chlorofluorocarbons and other products of our industrial civilization will reduce the amount of ozone by a few tens of percent. The predicted biological consequences are dire. How much more serious it must have been to have no ozone shield at all.

  In a world with deadly UV reaching the surface of the waters, sunblock may have been the key to survival—as it may become again. Modern stromatolite microorganisms secrete a kind of extracellular glue that helps them to stick together and also to adhere to the ocean floor. There would have been an optimum depth, not so shallow as to be fried outright by unfiltered UV, and not so deep that the visible light is too feeble for photosynthesis. There, partly shielded by sea-water, it would have been advantageous for the organisms to put some opaque material between themselves and the UV. Suppose, in reproducing, the daughter cells of one-celled organisms did not separate and go their individual ways, but instead remained attached to one another, generating—after many reproductions—an irregular mass. The outer cells would take the brunt of the ultraviolet damage; the inner ones would be protected. If all the cells were spread out thinly on the surface of the sea, all would die; if they were clustered together, most of the interior cells would be sheltered from the deadly radiation. This may have been a potent early impetus for a communal way of life. Some died that others might live.

  There are no earlier fossils known, in part because there’s very little of the Earth’s surface surviving from much before 3.6 billion years ago. Almost all the crust from that epoch has been carried deep into our planet’s interior and destroyed. In a rare 3.8-billion-year-old sediment from Greenland, there is some evidence from the kinds of carbon atoms present that life may have been widespread even then. If so, life happened sometime between about 3.8 and maybe 4.0 billion years ago. It could not have arisen much earlier. So—because of the inhospitability of the Hadean Earth, and the need for adequate time to evolve the stromatolite-building microbes—the origin of life must be confined to a comparatively narrow window in the expanse of geological time. Life seems to have arisen very quickly.

  Tentatively, tortuously, the orphan is trying to figure out, to the nearest hundred million years, when the family tree took root. “How” is much harder than “when.” Deadly environmental perils, a kind of huddling together for mutual protection, and the deaths—of course, neither willing nor unwilling—of vast numbers of little beings were characteristic of life almost from the beginning. Some microbes were saving their brethren. Others were eating the neighbors.

  ——

  When life was first emerging, the Earth seems to have been mainly an ocean planet, the monotony broken, here and there, by the ramparts of large impact craters. The very beginnings of the continents date back about 4 billion years. Being made of lighter rock, then as now, they sat high on the moving, continent-sized plates. Then as now, the plates apparently were being extruded out of the Earth, carried across its surface as on a great conveyor belt, until plummeting back into the semifluid interior. Meanwhile, new plates were emerging. Vast quantities of mobile rock were slowly exchanged between the surface and the depths. A great heat engine had been established.

  By about 3 billion years ago the continents were becoming larger. They were transported halfway around the Earth by the crustal plate machinery, opening one ocean and closing another. Occasionally, continents would crash into each other in exquisite slow motion, the crust would buckle and crinkle, and mountain ranges would be thrust up. Water vapor and other gases spewed out, mainly along mid-ocean ridges and volcanoes at the edges of plates.

  Today we can readily detect the growth of continents, their relative motion over the Earth’s surface (sometimes called continental drift), and the subsequent transport of the ocean floor down into the interior, in a style of motion called plate tectonics. The continents tend to stay afloat even when their underlying plates plunge down to destruction. Still, time takes its toll even on continents. Some old continental crust is always being carried to the depths and only bits and pieces of truly ancient continents have survived to our time—in Australia, Canada, Greenland, Swaziland, Zimbabwe.

  Greenhouse gases and stratospheric fine particles, both generated by volcanoes, can, respectively, warm or cool the Earth. The changing configuration of the continents determines rainfall and monsoon patterns, and the circulation of warming and cooling ocean currents. When the continents are all aggregated together, the variety of marine environments is limited; when they are scattered over the globe, there are many more kinds of environments, especially those near shore, where a surprising number of the fundamental biological innovations seem to have been made. Thus the history of life, and many of the steps that led to us humans, were governed by great sheets and columns of circulating magma—driven by the heat from long-gone worlds that fell together to make our planet, from the sinking of liquid iron to form the Earth’s core, and from the decay of radioactive atoms originally forged in the death throes of distant stars. Had these events gone a little otherwise, a different amount of heat would have been generated, a different pace or style of plate tectonics elicited, and, from the vast array of possible futures, a different course followed in the evolution of life. Not humans, but some very different species might now be the dominant form of life on Earth.

  We know next to nothing about the configuration of the continents over the first 4 billion years. They may many times have been scattered over the oceans and reaggregated into a single mass. For at least 85 percent of Earth history, a map of our planet would have seemed wholly unfamiliar—as if of another world. The earliest well-substantiated reconstruction we can manage dates to as recent a time as 600 million years ago. The Northern Hemisphere then was mostly ocean; in the South, a single massive continent, plus fragments of future continents, drifted across the face of the Earth at about an inch a year—much slower than a snail’s pace. Trees grow vertically faster than continents move horizontally, but if you have millions of years to play with, this is quite sufficient for continents to collide and wholly alter what’s on the maps.

  For hundreds of millions of years, what are now the southern continents—Antarctica, Australia, Africa, and South America—plus India, were joined in a common assemblage that geologists call Gondwana.* What was later to be North America, Europe, and Asia were adrift, sailing in pieces through the world ocean. Eventually, all this floating continental debris gathered itself together into one massive supercontinent. Whether we describe it as a landlocked planet with an immense saltwater lake, or an ocean planet with an immense island is only a matter of definition. It might have seemed a friendly world: At least, you could walk anywhere; there were no distant lands across the sea. Geologists call this supercontinent Pangaea—“all Earth.” It included, but of course was considerably larger than, Gondwana.

>   Pangaea was formed about 270 million years ago, during the Permian Period, a trying time for Earth. Worldwide, conditions had been warming. In some places the humidity was very high and great swamps formed, later to be supplanted by vast deserts. About 255 million years ago Pangaea began to shatter—because, it is thought, of the sudden rise of a superplume of molten lava through the Earth’s mantle from its deep seething core. Texas, Florida, and England were then at the equator North and South China, in separate pieces, Indochina and Malaya together, and fragments of what would later be Siberia were all large islands. Ice ages flickered on and off every 2.5 million years, and the level of the seas correspondingly fell and rose.

  Towards the end of the Permian Period, the map of the Earth seems to have been violently reworked. Whole oblasts of Siberia were inundated with lava. Pangaea rotated and drifted north, moving mainland Siberia towards its present position, near the North Pole. “Megamonsoons,” torrential seasonal rains on a much larger scale than humans have ever witnessed, drenched and flooded the land. South China slowly crumpled into Asia. Many volcanoes blew their tops together, belching sulfuric acid into the stratosphere and perhaps playing an important role in cooling the Earth.5 The biological consequences were profound—a worldwide orgy of dying, on land and at sea, the likes of which has never been seen before or since.

  The breakup of Pangaea continued. By 100 million years ago South America and Africa, which even today fit together like two pieces of a jigsaw puzzle, were just barely separated by a narrow strait of ocean—receding from one another at about an inch a year. North and South America were then separate continents, with no Isthmus of Panama connecting them. India was a large island headed north away from Madagascar. Greenland and England were connected to Europe. Indonesia, Malaysia, and Japan were part of the mainland of Asia. You might have strolled from Alaska to Siberia. There were great inland seas where none exists today. This time, at a glance from orbit you would have recognized it as the Earth—but with the configuration of land and water strangely altered, as if by a careless, slapdash cartographer. This was the world of the dinosaurs.

  Later, the continents drifted further apart, pulled by their underlying plates. Africa and South America continued to recede from one another, opening up the Atlantic. Australia split off from Antarctica. India collided with Asia, raising the Himalayas high. This is the world of the primates.

  ——

  Each of us is a tiny being, permitted to ride on the outermost skin of one of the smaller planets for a few dozen trips around the local star. The great internal engine of plate tectonics is indifferent to life, as are the small changes in the Earth’s orbit and tilt, the variation in the brightness of the Sun, and the impact with the Earth of small worlds on rogue orbits. These processes have no notion of what has been going on over billions of years on our planet’s surface. They do not care.

  The longest-lived organisms on Earth endure for about a millionth of the age of our planet. A bacterium lives for one hundred-trillionth of that time. So of course the individual organisms see nothing of the overall pattern—continents, climate, evolution. They barely set foot on the world stage and are promptly snuffed out—yesterday a drop of semen, as the Roman Emperor Marcus Aurelius wrote, tomorrow a handful of ashes. If the Earth were as old as a person, a typical organism would be born, live, and die in a sliver of a second. We are fleeting, transitional creatures, snowflakes fallen on the hearth fire. That we understand even a little of our origins is one of the great triumphs of human insight and courage.

  Who we are and why we are here can be glimpsed only by piecing together something of the full picture—which must encompass aeons of time, millions of species, and a multitude of worlds. In this perspective it is not surprising that we are often a mystery to ourselves, that, despite our manifest pretensions, we are so far from being masters even in our own small house.

  ON IMPERMANENCE

  The present life of man, O king, seems to me, in comparison of that time which is unknown to us, like to the swift flight of a sparrow through the room wherein you sit at supper in winter, with your commanders and ministers, and a good fire in the midst, whilst the storms of rain and snow prevail abroad; the sparrow, I say, flying in at one door, and immediately out at another, whilst he is within, is safe from the wintry storm; but after a short space of fair weather; he immediately vanishes out of your sight, into the dark winter from which he had emerged. So this life of man appears for a short space, but of what went before, or what is to follow, we are utterly ignorant.

  THE VENERABLE BEDE Ecclesiastical History8

  * You can occasionally see, on the automobile bumper stickers of geology graduate students, the nostalgic plea, “Reunite Gondwanaland” Except in a metaphorical political sense (and it’s not too likely there either) it is the most hopeless of lost causes—on any but a geological time scale But the breakup and separation of continents can go only so far. On a round Earth, what you run away from on one side you will eventually edge into on the other A few hundred million years from now our remote descendants, if any, may witness the reaggregation of a supercontinent Gondwanaland will at last have been reunited

  * Although not in consequence of some policy of conscious altruism Any individual that goes along with the stromatolitic arrangement is much more likely to find itself safely on the inside rather than perilously on the outside A communal policy benefits most constituent cells—not entirely risk-free, since those on the outside will be fried, but as if a cost-benefit analysis had been performed for the average cell

  Chapter 3

  “WHAT MAKEST THOU?”

  Shall the clay say to him that fashioneth it, What makest thou?

  Isaiah 45:9

  The world and everything in it was made for us, as we were made for God:

  For the last few thousand years, and especially since the end of the Middle Ages, this proud, self-confident assertion was increasingly common belief, held by Emperor and slave, Pope and parish priest. The Earth was a lavishly decorated stage set, designed by an ingenious if inscrutable Director, who had managed to round up, from only He knew where, a multitudinous supporting cast of toucans and mealy bugs, eels, voles, elms, yaks, and much, much more. He placed them all before us, in their opening night costumes. They were ours to do with as we pleased: drag our burdens, pull our plows, guard our homes, produce milk for our babies, offer up their flesh for our dinner tables, and provide useful instruction—bumblebees, for example, on the virtues not just of hard work, but of hereditary monarchy. Why He thought we needed hundreds of distinct species of ticks and roaches, when one or two would have been more than sufficient, why there are more species of beetles than any other kind of being on Earth, no one could say. No matter; the composite effect of life’s extravagant diversity could only be understood by postulating a Maker, not all of whose reasons we could grasp, who had created the stage, the scenery, and the subsidiary players for our benefit. For thousands of years, virtually everyone, theologian and scientist alike, found this, both emotionally and intellectually, a satisfying account.

  The man who wrecked this consensus did so with the utmost reluctance. He was no ideologue bent on kicking in the door of the Establishment, no firebrand. If not for a bit of happenstance he would probably have passed his days as a well-liked Church of England parson in a nineteenth-century rural, picture-postcard village. Instead he ignited a firestorm1 that destroyed more of the old order than any violent political upheaval ever had. Through the astonishingly powerful method of science, this gentleman who was known to find lively conversation too taxing, somehow became the revolutionary’s revolutionary. For more than a century, the mere mention of his name has been sufficient to unsettle the pious and rouse the bookburners from their fitful slumbers.

  ——

  Charles Darwin was born at Shrewsbury, England, on February 12, 1809, the fifth child of Robert Waring Darwin and Susannah Wedgwood. The Darwin and Wedgwood families were allied through the close friendship
of their patriarchs, Erasmus Darwin, the noted author, physician, and inventor, and Josiah Wedgwood, who had risen from poverty to found the Wedgwood pottery dynasty. These two men shared radically progressive views, even going so far as to side with the rebellious colonies in the American Revolution. “He who allows oppression,” Erasmus wrote, “shares the crime.”2

  Their club was called The Lunar Society, because it met only during the full moon when the late-night ride home would be well-lit and therefore less dangerous. Among its members were William Small, who had taught Thomas Jefferson science (at the College of William and Mary in Virginia and whom Jefferson singled out as having “probably fixed the destinies” of his life); James Watt, whose steam engines powered the British Empire; the chemist Joseph Priestley, the discoverer of oxygen; and an expert on electricity named Benjamin Franklin.

  The poet Samuel Taylor Coleridge thought Erasmus Darwin “the most original-minded man” he had ever known. Erasmus was also making quite a name for himself as a doctor. George III invited him to become his personal physician. (Erasmus declined the honor out of an unwillingness, he said, to leave his happy home in the countryside, but perhaps the champion of American revolutionaries had political reasons as well.) His real fame, though, stemmed from a string of hit encyclopaedic rhyming poems.

  Erasmus Darwin’s two-volume work, The Botanic Garden, comprising The Loves of the Plants, written in 1789, and its eagerly awaited sequel, The Economy of Vegetation, were runaway best-sellers. They were so successful that he decided to tackle the animal kingdom next. The result was a 2,500-page tome, this one in prose, entitled Zoonomia: or, the Laws of Organic Life. In it he asked this prescient question: