A walk through Deep Time on the Stone Arch Bridge

"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."

— Peter Brannen, author of "The Ends of the World"

Because humans cannot comprehend Deep Time — the geological clock of Earth — we cannot comprehend our place in it. Nor can we fathom how unprecedentedly rapid are the changes we're making.

In a 2018 New York Times opinion piece, "Rambling through Time," science journalist Peter Brannen tried putting Deep Time into perspective by taking a "walk" wherein each step represented a century. He starts on Manhattan's Upper West Side, and by the time he gets to the Pacific Ocean, he's covered just 10% of the earth's history — a tumultuous, violent history pockmarked with a number of near-death experiences that Brannen chronicles in his book "The Ends of the World." (Brannen will deliver a lecture at 7 p.m. Monday at the University of Minnesota's Tate Hall.)

Brannen's geo-time ramble inspired me to create a Deep Time tour right here in Minnesota. So, let's take a walk, north to south, across the iconic Stone Arch Bridge in downtown Minneapolis, and let's imagine that every step we take represents a million years of time.

With the very first step — the first foot actually — you would pass over the entire 300,000-year history of modern humans, Homo sapiens.

The second step would bring you to the time when Homo erectus took advantage of its unprecedentedly modern human physique — shorter arms, longer legs — to become the first human species to stride out of Africa and into Asia.

On the third step back in time you would pass over the beginning of the so-called Ice Age, which included the megafauna memorialized by the animated movie series, as well as the first Homo species, which was living in Africa at the time and was unable to make it to auditions.

Six steps along — less than 1% of the Stone Arch Bridge — would bring you to the first primate capable of walking upright.

Let's stretch out a little. On your 66th step, you would arrive at the End-Cretaceous mass extinction, the most recent of the so-called Big 5 mass extinctions, defined as an event where more than half of the earth's species go extinct in fewer than a million years.

Sixty-six million years ago, an asteroid slammed into Mexico's Yucatán Peninsula, setting the world on fire and choking the atmosphere with dust and gas. The impact also coincided with the activity of a vast volcanic field in India known as the Deccan Traps. This one-two punch ended the Mesozoic Era — "the age of reptiles/dinosaurs" — leaving avian dinosaurs (birds) and mammals to repopulate a wounded planet.

Another 135 steps brings you to the End-Triassic mass extinction 201 million years ago, when the supercontinent of Pangea began breaking apart, and vast amounts of lava — some 4 million square miles of it — spewed from the cracks. With the lava came high levels of carbon dioxide emissions, which warmed the already warm planet and caused the oceans to acidify, stagnate and lose oxygen. The End-Triassic reef collapse was one of the largest in history, and it took another 300,000 years for corals to reappear in the rock record.

Another 51 steps would bring you to the End-Permian die-off, the mother of all mass extinctions. The extinction brought an end to the Paleozoic Era (541 million to 252 million years ago), which was marked by an unprecedented explosion in different life-forms. A rise in temperatures of 10 degrees Celsius led to the extinction of 95% of marine species, nearly all trees, and 70% of land vertebrate species, and may have taken as little as 60,000 years.

The most compelling explanation is the Siberian Traps, a series of massive volcanic eruptions that spewed enough lava to cover the Earth to a depth of about 20 feet. Making matters worse, the Siberian Traps erupted through and burned one of the world's largest coal basins, adding tens of thousands of gigatons of additional carbon dioxide into the atmosphere.

Amble another 122 steps — in which you pass over the center of the Mississippi River — and you'll arrive at the Late Devonian mass extinction. The Devonian Period has been described as "the age of fishes," but its end may have been land-based. A burgeoning population of land plants broke up the soil and allowed nutrients to wash into the ocean, causing algae blooms and massive dead zones. These plants also sucked up so much carbon dioxide that they triggered cooling and massive glaciation, causing sea levels to drop and stagnate. More than 80% of ocean life died.

Another 71 steps and you arrive at the End-­Ordovician mass extinction 445 million years ago. The Ordovician world was mostly a marine one, "the sea without fishes." Two discrete extinction pulses, both linked to massive glaciation of the South Pole, wiped out 85% of life on Earth.

At this point, the bridge widens a bit, and you'll find an educational sign labeled "Changing the Shape of the Falls." Twenty steps north of the sign — about 455 million years ago — was when Platteville limestone was being laid down in a Bahamas-like shallow sea filled with bottom-dwelling, filter-feeding invertebrates. This limestone caps the waterfalls of the Twin Cities, including the now-submerged St. Anthony Falls. Ten steps south of the sign brings one to 485 million years ago, when the magnesium limestone used to build the Stone Arch Bridge (some of it quarried in Mankato and known as "Kasota stone") was being laid down in a shallow sea.

Take another 56 steps and you'll pass over the lock to reach the beginning of the Phanerozoic Eon (541 million years to the present), the most recent of the four "eons" that geologists use to divide up Deep Time.

"Phanerozoic" means "visible life," referring to the appearance of organisms large enough to leave their mark in the fossil record.

By the time you reach the boulder at the south end of the Stone Arch Bridge, 844 steps from where you began, you are into the tail end of the Proterozoic Eon (2.5 billion to 541 million years ago). During the Proterozoic, cyanobacteria and simple plants began using chlorophyll to capture the sun's energy, producing oxygen as a side effect.

With the planet now energized by captured sun power, more complex organisms like eukaryotes — organisms with a nucleus and a cell membrane — could develop.

But we'll need to do a lot more walking up the West River Parkway trail to get to the end of Deep Time and the beginnings of the Earth.

The North Loop Playground marks the end of Archean Eon (4 billion to 2.5 billion years ago), wherein Minnesota's oldest rock — Morton gneiss, named for the town in Renville County where it is has been quarried — formed 3.5 billion years ago.

The Coloplast building marks the end of the very first eon, the Hadean Eon (4.54 billion to 4 billion years ago), and the Minneapolis Park and Recreation Board building marks the moment 4.54 billion years ago when the Earth was birthed out of the magic sauce — a spinning cloud of dust and gas.

Congratulations: You have completed your tour of Deep Time. Now turn around and take a look: It's 4,540 steps back to the present, back to the north end of the Stone Arch Bridge, where modern humans developed in the very last foot of the very last step. In those 12 inches we have used our greatest gift — our brains — to go from making stone tools to making tools that can drill through stone, wherein we found an immense and seemingly endless cache of energy: old sunlight that was captured eons ago and buried.

Now we must wise up to the consequences of our actions, and to the fact that we are releasing carbon dioxide as fast as (or even faster than) the volcanoes did during the worst extinction event in history, the End-Permian. The speed of this change leaves little time for life to adapt, and the only alternative to adaptation is death, extinction. Fossils, anyone?

The lesson of Deep Time is that this extraordinary planet, the one that breathes life into each of us, has gone sour before, and has been transformed into a nearly lifeless, superheated, acidic funeral pyre. We weren't around for the Big 5 mass-extinction events, but we might be witnessing the beginnings of the sixth, the fastest one by far.

Let's make it not so.

Craig Bowron is a physician and writer in St. Paul. He is at craigbowronmd@gmail.com.