They are, in the opinion of no less an authority than the paleontologist Stephen Jay Gould, “the world’s most important animal fossils”—not Tyrannosaurus rex, not Lucy, but a collection of marine invertebrates mostly a few inches in size, dating from the very dawn of complex life on earth more than 500 million years ago. Their very names—Hallucigenia, Anomalocaris—testify to their strangeness. For decades they have fired the passions of researchers, fueling one of the great scientific controversies of the 20th century, a debate about the nature of life itself.

The discovery of the Burgess Shale fossils, high on a mountainside in the Canadian Rockies, is shrouded in legend. It was late August 1909, and an expedition led by the Smithsonian’s longtime Secretary, Charles D. Walcott, was about to pack up. One tale is that a horse ridden by Walcott’s wife, Helena, slipped, overturning a slab of rock that revealed the first astonishing specimens. Whether or not it happened that way—Gould argued against it—Walcott knew he had found something special, and returned the following year, assembling the nucleus of a collection now numbering some 65,000 specimens representing about 127 species. Some were well known, such as the segmented arthropods known as trilobites, others completely novel. They include Opabinia, a five-eyed creature with a grasping proboscis, whose presentation at a scientific conference was regarded at first as a practical joke; Hallucigenia, a marine worm that earned its name when it was originally reconstructed upside-down, so that it appeared to ambulate on seven pairs of stiltlike spines; and Pikaia, an inch-and-a-half-long creature with a spinal rod called a notochord, the earliest known chordate—the group of animals that would later evolve into vertebrates.

This was the full flowering of the “Cambrian explosion,” the sudden appearance of a vast new panoply of life-forms—creeping, burrowing and swimming through seas that had held nothing like them in the previous three billion years. Cambrian fossils are known from many sites, but usually only from remains of shells and other hard parts; here, owing to some accident of geology, entire organisms were preserved with eyes, tissue and other soft parts visible.

How to classify this trove has been a contentious question. Walcott conservatively tried to place the creatures into groups that were known from other fossils, or living descendants. But decades later, when the Cambridge geologist Harry Whittington and his colleagues took another look, they realized that the Burgess Shale contained not just unique species, but entire phyla (the broadest classification of animals) new to science. The first European to see a kangaroo could not have been more surprised.

What made the creatures seem new is they have no living descendants. They represent entire lineages, major branches on the tree of life, left behind by evolution, most likely in one of the mass extinctions that punctuate the natural history of this planet. Other lineages did survive, including that of the humble Pikaia, which qualifies as at least a collateral ancestor of the vertebrates, including us.

And that raises the profound, almost beautiful mystery that Gould saw in the Burgess Shale, the subject of his book Wonderful Life: Why us? Obvious as the dominance of big-brained mammals may seem, nothing in the Burgess Shale suggests that Pikaia’s offspring were destined for greatness, or even survival, compared, say, with the presumed top predator of those oceans, the two-foot-long shrimplike Anomalocaris. The proliferation of wildly different body plans and the apparently random process by which some thrived while others went defunct brought to Gould’s mind a lottery, in which the lineage leading to human beings just happened to have held a winning ticket. If one could somehow turn the clock back to the Cambrian and run the game again, there is no reason to think the outcome would be the same. These little creatures, entombed in rock for a half-billion years, are a reminder that we are so very lucky to be here.

A science writer and author of the book High Rise, Jerry Adler is a frequent contributor to Smithsonian. He wrote about the role of fire in shaping human evolution in our June issue.