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’s 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.