Wonderful Life: The Burgess Shale and the Nature of History

Stephen Jay Gould, a Harvard paleontologist, uses a single fossil locality to challenge deeply held assumptions about progress and predictability in the history of life. The Burgess Shale, a small quarry in Yoho National Park, British Columbia, preserves soft-bodied animals from roughly 530 million years ago, just after the Cambrian explosion, the geologically sudden appearance of virtually all major groups of modern animals in the fossil record. Because soft tissues almost never fossilize, the Burgess Shale provides a rare window into the full anatomical range of early animal life. Gould argues that the reinterpretation of this fauna overturns the conventional view of evolution as steady progress, replacing it with a vision dominated by contingency: the dependence of outcomes on unpredictable historical sequences.

Gould opens by contrasting two illustrations. A 1942 painting by Charles R. Knight, based on the classifications of Charles Doolittle Walcott, the American paleontologist who discovered the Burgess Shale in 1909, depicts every organism as a member of a familiar modern group. A 1985 reconstruction foregrounds unique creatures with no modern counterparts, among them Anomalocaris, a two-foot predator with a circular jaw, and Opabinia, a five-eyed creature with a frontal nozzle. The shift between these pictures, Gould contends, encapsulates a fundamental change in our understanding of life's history.

Gould identifies two controlling images that have constrained thinking about evolution. The first is the ladder of progress, a linear sequence from simple organisms to humans, traceable from the Great Chain of Being, a pre-evolutionary hierarchy ranking all beings from lowest to highest, through popular cartoons equating evolution with improvement. The second is the cone of increasing diversity, in which life begins simple and narrow and expands steadily upward and outward, so that older organisms are automatically read as simpler and inferior. Both images persist, Gould argues, because they satisfy a desire for a universe whose meaning centers on humanity. He cites writer Mark Twain's analogy: If the Eiffel Tower represented the earth's age, the thin layer of paint on the pinnacle knob would represent human existence, and nobody could argue the tower was built for the paint.

To counter these iconographies, Gould proposes a thought experiment: Rewind the tape of life to the Burgess seas and let evolution run again. If replays resemble the actual outcome, what happened was broadly inevitable. If they yield different results, outcomes like human intelligence were never guaranteed. He introduces contingency as a third alternative to pure determinism and pure randomness: Each step in a historical sequence proceeds for identifiable reasons, but no finale can be predicted because any pathway passes through thousands of improbable stages. Alter any early event, and evolution cascades into a different channel. Gould also distinguishes diversity, the number of species, from disparity, the range of anatomical body plans, noting that the Burgess revision rests on disparity.

Gould recounts how Walcott discovered the fossils in August 1909, quarried the site over multiple field seasons, and brought some eighty thousand specimens to the Smithsonian Institution, but published only preliminary descriptions and never completed a thorough study. Walcott classified every organism into a modern group. Decades later, the Geological Survey of Canada organized expeditions in 1966 and 1967, led by Harry Whittington of Cambridge University. Whittington's key methodological insight was that the fossils retain three-dimensional structure. By dissecting through layers with dental drills, studying specimens in unusual orientations, and reuniting parts with their counterparts on opposing rock surfaces, Whittington reconstructed complete organisms.

Gould structures the resulting reinterpretation as a five-act intellectual drama. In the early acts, Whittington's monographs on common arthropods such as Marrella and Yohoia reveal that neither fits any known group. The pivotal moment is Whittington's 1975 reconstruction of Opabinia. When Whittington first presented a preliminary version at a 1972 meeting of the Palaeontological Association, the audience burst into laughter at the animal's bizarre anatomy. The completed reconstruction shows five eyes, a flexible frontal nozzle with pincers, a backward-facing mouth, lateral lobes with paddle-shaped gills, and no jointed appendages beneath the carapace. Opabinia belongs to no known phylum. Walcott's shoehorn, Gould's term for forcing Burgess animals into modern classifications, has fractured.

The revision expands through the work of two graduate students. Simon Conway Morris searched Walcott's Smithsonian cabinets, uncovering creatures that fit no known phylum, including Hallucigenia, named for its dreamlike appearance. Derek Briggs discovered unanticipated disparity among bivalved arthropods, yet his study of Canadaspis, the second most common Burgess animal, confirmed it as a genuine crustacean, establishing that the fauna includes both members of modern groups and unique designs. The culminating discovery is the reconstruction of Anomalocaris: structures that Walcott had classified separately as a jellyfish, a sea cucumber, and an odd shrimp turned out to be the circular mouth, the body, and one of a pair of feeding appendages of a single predator belonging to no recognized phylum. In total, the Burgess Shale contains at least 20 unique arthropod body plans alongside all four modern arthropod groups, plus organisms fitting no known animal phylum. Gould argues that no evidence suggests the survivors prevailed through anatomical superiority over the groups that perished.

Gould asks why Walcott committed his shoehorn error. Administrative burdens and family tragedies left Walcott no time for sustained study. His deeply conservative worldview treated evolution as divinely guided progress toward human consciousness, making any alternative interpretation psychologically impossible. Walcott's lifelong commitment to explaining the Cambrian explosion as an artifact of an imperfect fossil record demanded that all Burgess creatures fit into modern groups, since new phyla would undermine the long Precambrian pedigree his theory required.

Gould defends historical science as equal in rigor to experimental science, operating through narrative and what 19th-century philosopher of science William Whewell called consilience: the convergence of independent lines of evidence on a single conclusion. He argues that contingency is the essence of historical explanation, since any outcome depends on a sequence of prior events so particular that altering any step would yield a radically different result. He draws a parallel to Frank Capra's film It's a Wonderful Life, in which a guardian angel replays the tape of life without George Bailey, revealing how one person shaped the fate of an entire community.

To demonstrate that the Burgess pattern is not isolated, Gould presents evidence that mass extinctions operate by rules different from those governing normal times. Traits promoting survival in ordinary competition may prove irrelevant during catastrophic events, and survival may depend on incidental features evolved for unrelated reasons. He traces seven scenarios from the origin of complex cells to the emergence of Homo sapiens, arguing that contingency dominates every critical juncture: the symbiotic merger of simple cells into complex ones, the peculiar fin structure that enabled vertebrates to colonize land, and the unpredictable asteroid impact that ended the age of dinosaurs and permitted mammals to diversify.

In an epilogue, Gould reveals the organism he deliberately withheld: Pikaia gracilens, a two-inch creature that Walcott classified as a worm but that Conway Morris identified as a chordate, a member of the phylum that includes all vertebrates. Pikaia is the earliest known representative of the lineage leading to vertebrates, including humans. Rare in the Burgess and absent from other early fossil assemblages, Pikaia had no guaranteed future. If it had not survived the Burgess decimation, no vertebrate would ever have evolved. A major part of the answer to "Why do humans exist?" must therefore be: because Pikaia survived, a response citing no law of nature and no predictable pathway, but only the contingency of "just history."