There’s a seafloor in Central Park. It crops out from under fallen ginkgo leaves, in black hunks sparkling with muscovite. This familiar rock was laid down as deep-sea muck half a billion years ago in a strange ocean haunted by alien exoskeletons, and gelatinous things that pulsed and squirmed.
But you can’t find fossils in this Central Park seabed — they were all cooked to schist tens of millions of years later in titanic continental collisions that pushed snowcapped mountains into tropical New England skies. As you can imagine, this was all a very long time ago — but then again, you can’t imagine it.
This is the central insight of geology. The world is old beyond comprehension, and our story on it is short. The conceit of the Anthropocene, the supposed new epoch we’re living in, is that humanity can already make claims to its geological legacy. But if we’re to endure as a civilization, or even as a species, for anything more than what might amount to a thin layer of odd rock in some windswept canyon of the far future, some humility is in order about our, thus far, infinitesimal part in the history of the planet.
Astronomy gets much of the credit for decentralizing the role of humans in the story of the cosmos, but just as Edwin Hubble placed our island universe in deep space, the geologist James Hutton placed us in deep time, gawking in awe in 1788 at the chasms of history that confronted him in the rocks at Siccar Point on the east coast of Scotland.
To grasp the extent of this abyss, the present-day geologist Robert Hazen proposes going for a walk, with each step representing a century back in time. Let’s walk 500 million years back, roughly to the strange age of the Central Park seafloor. With a nod to the space folks, we’ll start out at the American Museum of Natural History’s Hayden Planetarium on the Upper West Side and head west. We can’t even get to the sidewalk before all of recorded history — all of the empires, the holy books, agriculture, the architecture, all of it — is behind us.
But since it is geological time, not human history that we’re after, we keep walking down city streets in a world now populated by woolly mammoths and giant ground sloths. We walk past Broadway to Riverside Park, eventually hitting the Hudson River.
We’ve already put more than a thousand centuries behind us, but we’ve got a long way to go. So we march up the West Side Highway and cross the George Washington Bridge to New Jersey. Despite our sore feet, and having covered untold millenniums over several miles, we’re stupefied to learn that we’ve scarcely gone back a million years — an all but insignificant amount to geologists. In fact, we haven’t even emerged from the pulsating ice age that has waxed and waned for the past 2.6 million years.
The scale of the task dimly dawning on us, we push on, trudging along the rumble strip of Interstate 80 in New Jersey, battered by gusts of passing tractor-trailers. After walking for more than 24 hours we make it clear across the state, stumbling into Pennsylvania. Morale now collapsing, we’re further gutted to learn that walking as the crow flies 300 miles across the Keystone State won’t even bring us back to the age of dinosaurs.
That august period begins in Ohio and, though all of human civilization lasted only those first few dozen footsteps out of the museum, the age of dinosaurs will continue through the rest of the state. Then Indiana. Then Illinois. Then Iowa. It’s not until we reach the middle of the Triassic somewhere in Nebraska (and some 235 million years ago) that the first humble dinosaurs appear. But we’re still nowhere near that ancient sea world entombed in the Manhattan schist.
So we keep going, across prairies, over the Rocky Mountains, through Utah’s Martian wastes, then Nevada’s bleak Basin and Range, as untold millions of years slip past. Finally, scrambling over the Sierras and across the San Joaquin Valley to San Francisco, we arrive at the edge of the continent, more than 100 miles, and tens of millions of years, short of the Cambrian world revealed in Central Park. Having reached the Pacific Ocean, we have covered 10 percent of earth’s history.
It has been cynically observed by some politicians that over this vast scope of time, “Earth’s climate is always changing.” Indeed, in our transcontinental walk through earth history, it’s true. The planet’s climate in those first few miles of our walk, through the freeze-thaw seesaw of the recent ice ages, is, in fact, far different from the carbon-dioxide infused wasteland inferno of the early Triassic, more than a thousand miles later. Over the grand sweep of earth history our planet has been many different worlds — a snowball earth colonized by sponges, a supercontinental broiler ruled by crocodile kin.
But during the brief window of the past few thousand years in which all of civilization has emerged — those first few steps in our journey — we’ve enjoyed an almost miraculously equable interglacial climate, the most stable of the past several hundred thousand years. It’s these pleasant few footsteps that allowed complex societies to blossom. But in the next few footsteps, we’re projected to return to climates last seen hundreds, if not thousands of miles in our past.
In this century alone, a time scale so laughably brief as to effectively not exist to geologists, we could send the planet back to a climate system not seen for many millions of years. One study recently estimated that humanity has the capacity in the next few centuries to make the planet warmer than it has been in at least 420 million years.
The story of life on earth so far isn’t one of a tidy march of progress, culminating in humanity’s “end of history.” Other alien worlds have claimed this planet for unimaginably longer spans, relinquishing their place only under the duress of mind-bending episodes of chaos, like asteroid hits and large-scale volcanic activity.
And contrary to some accounts of our current moment, we’re not even the first, or only, organism to threaten the planet with mass extinction. At the end of the Ediacaran period, 540 million years ago, burrowing animals and filter feeders might have wiped out vast swathes of exotic life clinging to the seafloor. Almost 200 million years later at the end of the Devonian period, the evolution of trees might have driven such convulsions in climate and ocean chemistry that 97 percent of the world’s vertebrates died.
In the next few decades we will decide whether humanity’s legacy will be a sliver of clay in the limestone strata — a geological embarrassment accessible only in remote outcrops to eagle-eyed geologists of the far future — or an enduring new epoch like the reign of dinosaurs. But even if it’s the former, and we collapse almost as soon, in geologic time, as we got started, the record in the rocks of the extinctions we caused will remain, as eternal as the schist in Central Park.