YOHO NATIONAL PARK, British Columbia - If the history of Earth is condensed to fit in a single, 24-hour day, life emerges sometime before dawn. Photosynthesis evolves around midmorning, and the atmosphere becomes oxygen-rich right before lunch. But most of the day is utterly boring; all organisms are microscopic and occupied with little more than belching gasses and oozing slime.
It isn't til 9 p.m., about half a billion years before the present, that we see the first complex, multicellular beings. Scientists call this juncture the "Cambrian explosion" - the moment when billions of years of bacteria gave way to the rapidly evolving beings we know as animals. This evolutionary burst is responsible for every elephant, every fly, every bowlegged amphibian and wriggling worm, every complex creature that ever walked, swam, flew or scurried on this Earth. And I'm about to witness it firsthand.
"Ready to go back in time?" asks Ardelle Hynes, a cheerful, ponytailed ranger at Yoho National Park in British Columbia.
It's a drizzly July morning, and I'm huffing in Hynes' wake as we ascend a sheer mountainside in the Canadian Rockies. Our destination, high on the cliff face, is a jumble of 510-million-year-old rocks known as the Burgess Shale.
Formed during the middle part of the Cambrian period, the shale boasts tens of thousands of perfectly preserved fossils from the dawn of the animal kingdom. Many were soft-bodied organisms whose existence in most other places has been lost to the ravages of time. This wealth of small, strange specimens has shaped scientists' understanding of evolution and offered insight into the link between Earth's climate and the life it can support, making the Burgess Shale one of the most precious and important fossil sites in the world.
This remarkable record exists only because of a catastrophic underwater landslide that buried the organisms in a deluge of sediment millions of years ago. The sand was so fine it would have filled the animals' gills and the hinges of their legs, trapping and suffocating them. The high alkalinity of the oceans, combined with the utter absence of oxygen, would have held at bay the bacteria that would otherwise decompose an organism's soft and squishy parts.
"Think about all the factors that had to come together for us to be able to experience this," Hynes says. The animals had to die in a manner that allowed them to fossilize. Those conditions had to persist for millions upon millions of years. The rocks had to be lifted from the bottom of the ocean to the top of the world by the action of tectonics, and then scraped by the slow crawl of glaciers to reveal the treasures they contained. And, finally, an enterprising ape species had to evolve sufficient intelligence to invent the field of geology, hike up this mountain and recognize the significance of what they found. "Aren't we lucky?" Hynes says.
I take a breath of sharp, clean air and survey the spruce forest, the ice-capped mountains cloaked in wisps of fog. Hynes gives me a fossil, a piece of this planet's history, and I feel its heft in my hand. I see what she means.
But then I think of the invasive bark beetles, spurred by a warming climate, that are eating away at this forest. I think of retreating glaciers and vanishing species and all the consequences of unchecked carbon consumption that are still to come.
I think of the United Nations scientists who declared last year that we had just over a decade to get climate change under control, and the officials meeting in Madrid this month who have fallen far short of the commitments needed to make that happen.
Life on Earth has been evolving for nearly 4 billion years. Yet only now, as the geological clock strikes midnight, is there a creature capable of looking back at that history and appreciating it. Only now, as our own actions imperil this extraordinary and singular planet, do humans have a chance to comprehend all that is about to be lost.
What a profound responsibility that is. What a beautiful gift.
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As we hike, Hynes paints a picture of how the landscape would have looked half a billion years ago. The continents were clumped into two large masses, empty but for some slimy microbial mats. Without land plants to prevent erosion, sediment was constantly being swept out to sea, where it settled in thick, silty layers.
It's hard for me to envision. At most points in its history, humans wouldn't recognize our planet at all.
Researchers debate what caused the single-celled microbes of the Proterozoic (or "simple life") eon to evolve into the complex organisms seen in the Burgess Shale. Perhaps it was climate change - Earth was slowly recovering from an intense ice age - or the greater availability of oxygen in the atmosphere. Others have suggested that some key biological innovation, like the development of vision or the rise of predators, set off an evolutionary arms race that resulted in ever-more-complex creatures.
After trudging for 2.5 miles and 2,000 feet of elevation gain, we round a bend in the trail and are suddenly at the quarry. Gray and brown slabs of shale litter a football-field-size expanse of mountainside. Hynes instructs my fellow hikers and me to set aside our trekking poles, which can damage fossils. Then we set out across the rocks.
It's difficult to know where to put my feet; nearly every stone seems to bear at least a fragment of an ancient animal. Hynes points out a fossil resembling a slice of pineapple. These are the mouth parts of Anomalocaris - the bizarre, dog-size apex predator of the Cambrian seas.
Weirdness seems to be the defining characteristic of Burgess Shale organisms. Hynes shows us illustrations of Opabinia, an oddball with five eyes and a vacuum cleaner nozzle for a nose, and the monstrous Hallucigenia, which boasted eight pairs of legs and an equal number of conical spines. The ancestor of all modern vertebrates, including fish, birds and humans, was Pikaia, a wriggling eel-like organism no longer than your big toe. My favorites are the trilobites, distant relatives of today's horseshoe crabs, with jointed legs and shells of overlapping plates that almost look like ribs. They thrived for 300 million years, through the drifting of continents and the rise and fall of sea levels, the flourishing of coal age plants, the invention of the backbone.
In the end, the thing that got them was climate change; trilobites died out during the end-Permian mass extinction, when gigantic volcanic eruptions raised temperatures, acidified the oceans and killed off some 90 percent of life on Earth.
By comparison, our species seems like little more than a hiccup in the steady march of geologic time. Homo sapiens has existed for just 0.06 percent of the time trilobites survived. Given the environmental crisis we've created, it's unclear how much longer we'll be around.
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Back in Washington, I head over to the Smithsonian's National Museum of Natural History, home to tens of thousands of Burgess Shale specimens. Hans Sues, the museum's chair of paleobiology, guides me down dimly lit hallways to the Cambrian collections, where he pulls out drawer after drawer of fossils. He handles each one like it's a relic.
The events of 500 million years ago are just the beginning of the Burgess Shale's story, he explains. What happened after scientists uncovered them is perhaps even more profound.
It was a secretary of the Smithsonian, Charles Doolittle Walcott, who first excavated the fossil site in 1909. The extraordinary find was announced without fanfare; "A most interesting discovery of unique Cambrian fossil," was all Walcott wrote in an initial scientific report.
Walcott spent 15 field seasons at the shale, but he was so busy digging up fossils he didn't have much time to decipher them. It wasn't until decades later that scientists began to realize how unusual and diverse the Burgess Shale specimens truly were. Paleontologist Stephen Jay Gould found the fossils so strange, he believed that many of them couldn't belong to any known animal group. In his 1989 book, "Wonderful Life," he speculated that Cambrian animals were part of an exceptionally experimental period in Earth's history. Far from being "primitive," these creatures and their ecosystem were as complex as anything we see today. If we were to rewind the geologic clock, Gould argued, perhaps evolution would take an entirely different course. In place of humans, the world could be dominated by Hallucigenia's many-legged descendants.
More recent research has shown that Gould's theories weren't quite correct, Sues says; most of the Burgess Shale specimens do fit into existing categories on an evolutionary tree. But the idea still stands that evolution is unpredictable and undirected, that humanity is a fluke outcome rather than the inevitable result of millennia of increasing complexity. "There were all these other worlds out there," Sues says. Someday, "ours is going to be just another one of them."
Still, a few traits have staying power. The most common type of Cambrian creatures were arthropods, or joint-legged invertebrates. This same group, which includes insects, spiders and crustaceans, still accounts for more than 80 percent of all known animal species. There are probably millions more arthropods that remain undiscovered and unnamed.
In other words, Sues says, if the world of the Burgess Shale seems utterly alien, it's only because we haven't been paying enough attention to our own world.
The price of our ignorance about life's current diversity will be a duller, poorer future, because our inattention has led us to undermine the conditions that make Earth's extraordinary variety possible. Recent studies have found that arthropod populations, survivors of so many millions of years of tumult, are in "hyperalarming" decline in the human era. Flying insects have vanished from German nature preserves. Huge numbers of bugs have disappeared from a pristine forest in Puerto Rico. A catastrophic combination of habitat loss and climate change is transforming ecosystems faster than scientists can study them.
"We're losing things we don't even know about," Sues says. "If we don't understand this world, if we don't appreciate how this world came into being, how can we be capable stewards of it?"
Since the moment the Burgess Shale organisms began crawling out of the mud, living things on this planet have never been stagnant. They've been bombarded by asteroids, numbed by ice, eclipsed by competitors, even suffocated by the products of their own metabolisms. Yet, no matter how terrible the transformation, life has always emerged - altered, yet undeterred.
The world we love, the very fact of our existence, is contingent upon that process. Change is why we are here. And change will happen again.
But at this moment, Earth's climate is changing at a pace unmatched in the planet's 4.6 billion-year history. The systems on which species depend are vanishing. Living things as large and charismatic as whales, as delicate as orchids, as anonymous as tiny gray lichen growing on some remote Arctic tree, are dying out at a rate approaching the scale of the biggest extinctions.
The planet is hurtling toward "the point of no return," U.N. Secretary General António Guterres said last weekend at the opening of the COP 25 climate change summit. It is the last such meeting before the Paris climate agreement goes into effect, but global leaders still have not agreed on a mechanism for achieving the emissions reductions needed. The biggest source of cumulative greenhouse gases in history - the United States - refuses to cooperate on climate change mitigation at all. Meanwhile, unprecedented wildfires have burned millions of acres in Australia, Venice is underwater, hundreds of Bahamians are still missing after Hurricane Dorian devastated the island nation in August. Like the creatures of the Cambrian, humans are entering a world utterly unlike the one in which we evolved. Our species may not die out, but life as we know it cannot go on.
Yet, while the trilobites had no hand in their fate, we brought this revolution on ourselves. And we can still shape its course. We already know what must be done to avert the worst effects of warming: Starting next year, global greenhouse gas emissions must fall by 7.6 percent annually, reaching zero by the middle of the century. And although the scale of such action would be unprecedented, we already know how to achieve it: Put a price on carbon, replace fossil fuels with renewable energy sources, restore nature landscapes that act as carbon sinks, equip ordinary people with the tools to adapt to a transformed world. No new technologies need to be invented to meet the terms of the Paris climate agreement. All we are waiting for is the will to change.
Humans are the first species with not just the power to alter the planet on a geologic scale but also the capacity to predict the consequences. We understand the connection between our actions and each of Earth's possible futures.
What a profound responsibility that is. What a beautiful gift.
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Sarah Kaplan is a Washington Post science reporter.