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.
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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