Time is nature’s way of keeping everything from happening at once.
– John Wheeler
At a certain moment in year 2025, I will be 63 years, 153 days, 3 hours, 34 minutes, and 36 seconds old. I plan to celebrate with a bottle of champagne, while my wife rolls her eyes, because at that moment I will have lived exactly two billion seconds. If I’m very lucky, I might live past three billion seconds, not long after my 95th birthday. How about 4560 million seconds? It’s not likely anyone will ever live that long; it’s more than 144 years. I think we can agree: 4560 million is an overwhelmingly large number. The Earth is that old, in years.
Science gives us measurements for the Earth’s age, but not easy comprehension. For that we have analogies. An eight-inch-thick stack of standard printer paper is about 2,000 sheets. If the top sheet represents this year, then Jesus of Nazareth was in his mid-20s eight inches ago. Five thousand sheets ago, the oldest pyramids had not yet risen from Egypt’s sands. A 48-inch-thick stack will take us back to our Neolithic ancestors planting seeds at the dawn of agriculture. And that pretty much covers all of human history; it’s four feet high.
Flip the paper stack onto its side and add enough paper to get to 66 million sheets. You’ll have to send it out the door, down the street, and maybe into the next town. Our paper stack, lying on its side, is now 4.2 miles long. Stand at the far end, 66 million years ago, and watch as an asteroid plummets from the sky to doom Tyrannosaurus rex and his dinosaur brethren. Add 21 more miles. There, 400 million years ago, watch your lobe-finned fish ancestors take awkward steps onto land, breathing Devonian air with primitive lungs. We’re still less than one-tenth of the way to the birth of the planet. 4.56 billion (4,560 million) sheets will stretch our paper stack 288 miles, about the distance from Burlington to Buffalo. One inch of that is American history.
Evolution and plate tectonics are the guiding theories of biology and geology, respectively. Neither generates much drama over human time spans. But over geologic time, they both produce stunning changes. Here are a couple of examples.
THE ANCESTOR’S TALE
When was the first human? The first bird? First fish? As Richard Dawkins points out, such questions, although arising naturally from our curiosity, miss the vital role of geologic time in evolutionary change. Tiny changes, generation to generation, accumulating over the immensity of time, can create spectacular transformations. Evolution has gone from a long-ago bacterium to you, all without ever producing a “first” of anything.
To visualize this, try the following thought experiment (adapted from Dawkins). Put a photo of yourself at one end of a very long bookshelf. Next to it, put a photo of your mother. Next to that, your grandmother, then your great-grandmother, and so on, back through the generations. (Male ancestors will do just as well.) Soon you’ll go back past photography, but never mind, imagine pictures of your ancestors, in an immensely long row, going back many thousands of generations. Wander back in time along the row. Back past 100 thousand years ago, your ancestors, although increasingly different from you, are all still clearly Homo sapiens. How about one million years ago? Now your ancestors, though recognizably humanoid, look quite different from you, with flatter skulls and prominent brow ridges. We classify them as a different species, Homo erectus. But all along the chain of your ancestors, each one was indisputably the same species as its parents, and its children. There never was a “first” Homo sapiens.
Keep traveling along the bookshelf of your ancestors. What did your ancestor 10 million years ago look like? An ape. Your ancestor 100 million years ago? A shrew-like creature hiding in the shadow of dinosaurs. And 400 million years ago? A fish with limb-like fins. One billion years ago? An amoeba-like eukaryotic cell floating in ocean water. Two billion years ago? A bacterium. At no point along the line was there a “first” of anything. Every one of your ancestors, in every generation, belonged to the same species as its immediate predecessor and descendant. Yet every generation also accumulated tiny changes – particularly ones that favored survival and reproduction. The result: dramatic evolution, thanks to the generous scope of geologic time.
DANCING CONTINENTS
To us, continents appear to stay put. Viewed over geologic time, they drift like loose barges across the Earth. The North Atlantic is just five feet wider now than when the Rolling Stones first sang “Satisfaction” – but, as with evolution, tiny changes, accruing over vast spans, produce stunning results.
Imagine a continent traveling a mere one inch per year (slower than your fingernails grow), about equal to the separation rate of North America and Eurasia across the North Atlantic. In 10 million years, that continent will travel 160 miles. In 500 million years, nearly one-third of the way around the Earth. The earliest continents formed more than three billion years ago. At one inch per year, that adds up to two complete trips around the Earth.
Watching animations of the wandering continents on YouTube is a bit like watching kids driving bumper cars at the carnival: the motions seem equally random and chaotic. But there is a pattern. Over time, the continents cluster into great masses called supercontinents, then tear apart, then gather again in new configurations. The pattern is roughly cyclic, like a grand minuet: together, apart, together, apart. As far as we can tell (it becomes harder to reconstruct continental positions further back in time), perhaps six supercontinents have formed and split up over the past three billion years. The most recent, called Pangaea, began breaking up 250 million years ago (nearly 16 miles of paper) as the Atlantic Ocean opened. Some 300 million years from now (about 19 miles of paper), as the Pacific Ocean closes and the two Americas crunch into east Asia and Australia, the next supercontinent will be born.
