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ultraviolet light. All plants and animals depend on oxygen as part of their life cycles, lonely exceptions being the microscopic nematode worms that get along in the stagnant oxygen-free depths of the Black Sea and the creatures that survive on those deep-ocean geysers.

    The beginning of animallife had to wait about four billion years until atmospheric oxygen began to rise toward present levels. According to Andrew Knoll, complex multicellular organisms and oxygen first appeared in the fossil record some 580 million to 560 million years ago, during the Ediacaran period. “The oxygen increase pushed earth toward its present state, but it didn’t achieve it all in one go,” he told me when I visited with him at Harvard.
    Life didn’t burst forth onto center stage in full and varied forms until the Cambrian period from 542 million to 488 million years ago.
    The Burgess Shale, the famous quarry of Cambrian life discovered in 1909 by Charles Walcott, a paleontologist and former director of the Smithsonian Institution, sits high in the Canadian Rockies on theeastern border of British Columbia. It is perched at about eight thousand feet on the western slope of a ridge connecting Mount Field and Mount Wapta in Yoho National Park, near the tourist destinations of Banff and Lake Louise. The view from the rocky slopes of the Walcott Quarry—surrounded by a thick conifer forest, Emerald Lake below, and the snowcapped Canadian Rockies beyond—is one of the finest on the continent. Walcott’s daughter, Helen, wrote to her brother Benjamin in March 1912 when she was touring Europe, describing castles, fortresses, the Appian Way, and the Roman aqueducts, “but I’d prefer Burgess Pass to anything I’ve seen yet,” she said.
    Our first really good display of what nature was up to during the Cambrian explosion didn’t materialize until 530 million years ago, when mudslides at the Walcott Quarry captured a broad selection of fossil samples reflecting much of the Cambrian’s incredible animal diversity. Among paleontologists, the stature of these finds can only be appreciated when you take into consideration that since this geological period, and over a vast range of time in which life has been through enormous changes and upheavals, no new body designs, no new phyla, have been added to the collection of life displayed at the Burgess.
    The Burgess Shale is a miracle of preservation. Stephen Jay Gould proclaimed in his book Wonderful Life: The Burgess Shale and the Nature of History that mammalian evolution “is a tale told by teeth mating to produce slightly altered descendant teeth.” Which is to say that if it weren’t for teeth, we wouldn’t know as much about our ancestors. Teeth outlast everything else and are the dominant feature in any anthropological collection.
    Butsoft body parts like stomachs and other fleshy bodily organs and appendages in the wild collection of creatures found at the Burgess Shale? You really have to be lucky to get samples of any of those from the distant past. About 20 percent of the 140 or so original species found in the Burgess Shale were skeletonized, and the rest were soft-bodied. Still the earth that captured these creatures clearly displayed their ghostly impressions. This incredible find is preserved in asection of the shale about the height of a man and not quite as long as a city block, and according to Gould it has “more anatomical disparity than in all the world’s seas today.”
    In a burst of evolutionary creativity, all the major body plans suddenly appeared onstage. Although some scientists wonder if the original cast was so varied, Richard Leakey argues that as many as seventy actors were present, displaying the different body plans or phyla of life. But what remains today are perhaps only thirty or so such plans, the others having been cut from evolution’s cast since the Burgess Shale was formed.
    The Smithsonian’s Charles Walcott was of a more conservative opinion. He