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On a cool April afternoon, with two of my friends trailing behind me, I climbed the stone stairs of the National Museum of Natural History to meet Grover Krantz. A trip to the museum wasn’t complete without a visit, and I had promised to introduce my companions to the famed and controversial anthropologist. (He published widely in the field, on everything from fossil apes to the evolution of human culture, but he is perhaps most remembered for his insistence that the mythical Bigfoot is real.) The appointment had been easy enough to make. That’s because Krantz had been dead for more than a decade.
No special permit or permission was needed. Encased in glass, the skeleton of Krantz stood motionless at the end of the temporary Written in Bone exhibit, which was focused on the forensics of Chesapeake Bay residents in the days of colonization. Krantz was the most recent thing in the anthropological story, more of a coda than a part of it, and he didn’t exactly fit with the European transplants arrayed in the osteological maze the Smithsonian had constructed. Still, he would have been happy for his inclusion at the end of the skeletal labyrinth. Krantz had made a career out of studying bones and teaching others their secrets. His dying wish was to continue that calling.
In his old age, after he was diagnosed with cancer, Krantz decided that internment in a grave or reduction to ashes wouldn’t be fitting for someone who’d spent decades teaching anatomy. He wanted his bones to speak for him long after he died, and so he started pulling professional strings to find the perfect place to spend his afterlife. Krantz hoped that his gleaming bones would be reassembled for display alongside his three beloved Irish wolfhounds—Clyde, Icky, and Yahoo—whose bones he had been saving as the pups passed away. Macabre, maybe, but humans have had a long tradition of wanting to spend eternity with the ones we love.
The Smithsonian ultimately agreed to take Krantz, though public display seemed like a long shot. There was no place for him in the existing osteology hall, and the museum already had enough old skeletons hanging out in aging alcoves. Krantz would certainly have a home in the anthropological drawers, collections manager David R. Hunt promised, but Krantz’s great rearticulation seemed like a fantasy when the anthropologist passed away from pancreatic cancer on Valentine’s Day in 2002. His remains were sent to the famed Body Farm, the University of Tennessee’s Anthropology Research Facility, and, denuded of flesh, arrived at the Smithsonian in 2003.
The anthropologist’s bones could have stayed in a drawer with his dogs. There are worse fates than that. But the idea for Written in Bone offered the opportunity to revive Krantz—or at least reassemble him—as a showstopper right at the end. So Smithsonian taxidermist Paul Rhymer was tasked with the job of fitting every little divot to its corresponding notch, creating a slightly altered X-ray version of a photo depicting a happy Clyde jumping to greet Krantz. (Rhymer had to change the positioning from that depicted in the photo to avoid giving the impression that Clyde was leaping to rip out Krantz’s throat.) The two formed an excellent study in the vertebrate form. Almost all the bones in Clyde had their correspondence in Krantz, two expressions of more than five hundred million years of vertebrate evolution.
Leaning back to accept Clyde’s canine embrace, Krantz acted as an emissary for the living bone in all of us. Asking any one person to represent humanity is a foolhardy task that contradicts the diversity we treasure, of course, but it’s still helpful to see someone else’s bones if you want to get a feel for what grows inside you. Reduced to osteology, Krantz’s outside now represents your insides. And much like the deceased anthropologist, you have 206 bones, more or less. Without peeling away all the surrounding tissues, either after death or by way of a high-resolution full-body CT scan that would expose you to an unsafe level of radiation, there’s no way to get a totally precise count. Leaving aside differences in the way we’re born, accidents that might remove an appendage, and surgeries that replace our real bones with flexible facsimiles, even complete human skeletons vary in bone count. Each set of clattering bones is as individual as our personalities.
There are a few things I can say for sure about your skeleton, though. Your skull is balanced atop your neck. The stacked column of vertebrae runs along your back rather than along your front. All the necessary equipment for you to see, hear, smell, and taste the world is ensconced in your head rather than being distributed in different places across your body like some kind of Guillermo del Toro monster. None of these characteristics make us unquestionably human. The hallmarks of humanity are far more subtle, and mostly noticeable because every other human species that flourished over the past six million years perished and left us with a gap between us and the great apes. It’s easier to make hard categorical divisions when extinction’s removed your ancestors and cousins. But we can leave aside the subtleties that distinguish us as Homo sapiens for the moment. The basic format of where our arms and legs fit onto our bodies, the placement of our spine, the internal hug of our rib cage around our vital organs—all of these characteristics are widely shared with other vertebrate animals, from chimpanzees to swallows to Triceratops and the little red eft, so tiny you can scarcely believe that something so small could possibly have bones. As different as you are from a crocodile or a tuna or a house cat, your skeleton is laid out along the same body plan, and that’s because we’re all relatives descended from creatures that just happened to have such a conformation. They lived during a time before jaws, before spines, before even bone itself. You can find one of them at the Smithsonian several floors below where Krantz’s skeleton stood.
You’d think that a species so integral to our skeletal layout would be enshrined at the center of every great paleontology hall in the world. What’s left of them would rest on a velvet pillow lit from above, with visitors allowed into the darkened tomb alone or in pairs to spend a few moments with the animals to whom we owe the very core of our being. If star treatment is fitting for gems like the Hope diamond, surely part of our own deep past deserves as much respect. At the very least they ought to be given a prominent place, front and center, as an introduction to the great fossil galleries, their meek little forms giving us the essential context for everything that comes after. But there are no such honors for this particular relative of ours. The animals most important to the organization of our bodies just can’t compete with the fossil attractions that draw the crowds. That’s why the dinosaurs are usually tucked away somewhere that requires you to journey through other halls first, so that you might learn something in your dash to stand in the shadows of the great reptiles as our own furball ancestors did for more than 180 million years. The dinosaurs and the other fan favorite exhibits are the museum equivalents of summer blockbusters. Whether our enjoyment of them is high-minded and self-aware or just slack-jawed ecstasy, they’re the attractions that get asses in the seats (or sore feet on marble floors). That makes the animals I’m about to introduce to you the equivalent of independent films—critically praised but lacking the grand spectacle.
Our critical character is tucked away in a quiet place, one where almost no one goes. As you walk up to the National Museum of Natural History’s Sant Ocean Hall, take a left at the right whale dangling overhead to follow the hanging parade of fossil cetaceans back toward a little side room. This is the place where the museum’s less popular fossil stars have aggregated themselves—the buglike trilobites, the coil-shelled ammonites, stalked crinoids bristling with spikes, and various other invertebrates that provide the intellectual backbone for how extinction has repeatedly stripped the planet’s biota and how life has always sprung back. Here you’ll find the animal I’m talking about, a fossilized doodle in a row of oddities. It’s called Pikaia gracilens , and its relationship to us wasn’t always so clear.
Pikaia , as well as the smattering of fossils displayed around it, all came from a spot in British Columbia whose name is known to every paleontologist, whether they’ve worked on the material that has spilled from the site or not: the Burgess Shale. The standard story of the site’s discovery, as it was told to me by a paleontology professor years ago, goes like this: The field season of 1909 was coming to a close. Charles Doolittle Walcott, who had been scouring the ancient shales near the small town of Field for signs of early life, had come up almost entirely empty-handed. His plans for a momentous discovery dashed upon the very stones that were supposed to yield their secrets, he and his wife packed up their camp and started their way down the mountain as the first snowflakes of the season began to fall, confirming that the year’s work really had come to an end. But then Mrs. Walcott’s horse slipped on the chilled slabs of cracked stone, turning over a piece of rock that could have been easily overlooked. Charles spotted something strange on the chip the horse’s hooves had flipped up. Impressed into the ancient sediment was a prehistoric crustacean unlike any the paleontologist had seen before. If not the beginning of the paleontological superstition, then it became the most cherished example—you will always make your best find on the last hour of the last day of fieldwork. That initial clue was all Walcott needed to return the next year and start prying up an entire menagerie of animals that he literally could not make heads or tails of.
This Paleozoic tale is the closest paleontologists have to a eureka moment in our canon of stories, and I can certainly understand the appeal. No matter how much you prepare, no matter how skilled an eye you have for the first glimmerings of fossils peeking out of the stone, you may be totally hosed if luck isn’t with you. But as poetic paleontologist Stephen Jay Gould wrote in his book on the Burgess Shale, Wonderful Life , the classic Walcott story is fable, not fact. Walcott kept records of almost every day of his field season, including his first fossil finds. He made them on August 30 or 31 of 1909, Gould pointed out, with no sign of inclement weather. And Walcott’s reaction was not wide-eyed wonderment; he simply noted that he found some “interesting fossils.” That’s all. And it’s more fitting with how major discoveries often take shape. Big finds often start small and uncertain, usually with nothing more than a few curious bone fragments or some enigmatic smears on fine-grained stone. That’s precisely what happened with Walcott. The day after his initial find, he located an even better spot, this one boasting three invertebrates completely new to science, and he collected a few more beautiful slabs and specimens before he and the rest of the party packed out on a warm, sunny September day.
The scientific fate of those specimens was just as circuitous. Walcott returned to the Burgess Shale for a second season in 1910, finding even more, but in the end he described only a small fraction of the massive fossiliferous haul he brought back to the Smithsonian. And in trying to understand these organisms, Walcott took what has been half-jokingly called the shoehorn approach, fitting the confounding jumbles of legs and spines and body segments into already-known groups of organisms. The Cambrian seemed to be a time of jellyfish, sponges, and shrimp, much more ancient but—with the exception of extinct groups such as trilobites—not especially different from the reefs that encrust the bottom of some seabeds today. Walcott’s characterization of the fauna stayed in place for decades. But when paleontologists began to return to the Burgess Shale fossils in the 1960s with new ideas as well as new techniques to prepare the fossils in high relief, they found a community far stranger than Walcott could have ever imagined in even his most fevered fossil dreams.
Exhibited at the Smithsonian, Walcott’s creatures present a gallery of alien body shapes, spindly legs, and googly eyes. Animals were still a novelty in the Cambrian, and it would be tens of millions of years before anything worthy of the title giant would evolve in Earth’s oceans. Burgess Shale species seem disproportionately huge in depictions of this time because there’s no scale to compare them to and they’re too otherworldly for us to understand how all the pulsing, flapping, squirming parts relate to each other. They look like prehistoric equivalents of the shape-shifting monster in John Carpenter’s The Thing , although at much smaller scale. Most of them you could hold in the palm of your hand or on the tip of your finger. Among the smallest of all is little Pikaia gracilens , just about an inch and a half in length. Of all the animals in the Burgess Shale, this is the wee beastie that we have kinship with.
Superficially, Pikaia may be the nadir of petrified impressiveness. The fossil looks like little more than a charcoal scribble on gray stone, shorter than the last few words in this sentence. And here, a word of caution is needed. At 530 million years old, Pikaia lived too far back in time for us to directl y link the protovertebrate to ourselves through an unbroken line of descent. Paleontologists are adamant about such caveats. We can be confident that all life is connected in a great family tree that draws back to a single common ancestor—which may or may not have been the first life on Earth—but, to borrow the analogy the geologists Charles Lyell and Charles Darwin employed, we’re missing a great many characters, words, sentences, and paragraphs from Life’s great story. In the less than two hundred years paleontology has existed as a field of study, we’ve only just begun to brush the dust off all the geo-literary fragments that are out there, much less assemble them in their true order. Broad outlines are clear, but the details of family affiliations from parents to offspring species are almost always in contention, and the further back in time we go, the more concentration is needed to pick out who’s who in the fossil record. That’s why paleontologists often speak of transitional fossils, or species with transitional features—those species that help bridge what might seem to be different lineages, like the way the feathered Archaeopteryx connects nonavian dinosaurs with birds or how the mammal Pakicetus helps demonstrate the changes whales underwent as they went from landlubbers to beasts of the sea. Such creatures are part of evolutionary moments that have gained our special attention, often involving major shifts in anatomy and natural history. From the stone pages of the planet, then, some narratives are already clear, and certain archetypes—if not ancestors—have emerged as heroes in the story. Pikaia is one such champion.
Pikaia was one of the earliest Burgess Shale species that Walcott named, and he introduced the world to the squashed fossil in a 1911 paper given the unprepossessing title “Middle Cambrian annelids.” His entire treatment of the species lasts five short paragraphs, less than a page in length. Walcott saw little Pikaia as an annelid worm not altogether different from the night crawlers that squirm their way up onto the surface of a lawn when rain floods their burrows. “This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta,” he wrote, which translates to a plain sandworm to you and me. But when paleontologist Simon Conway Morris later looked at the literal handful of Pikaia fossils that had been found, he did not see a worm. The tiny, almost imperceptible segments running down the two-inch animal’s body were not the stacked bands of an annelid worm, but primitive myomeres—packages of fibers arranged in V shapes that are the forerunners of the skeletal muscles in our bodies. Diminutive Pikaia also had a distinct head end, marked by a pair of strange tentacles, but the most stunning revelation of all was preserved as a thin line of Paleozoic sheen along its back. Pikaia had the beginnings of the spine that, given the circumstances of another five hundred million years of transmutation, would come to hold our backs erect. The rod was not surrounded by bone; that stuff would not come into existence until more than a hundred million years later. But, as Conway Morris and his adviser reported in 1979, Pikaia had a notochord—the basic stiffened structure that would form the foundation of the backbone.
I had to smush my nose against the glass to get my nearsighted eyes close enough to Pikaia to see some of these details when I last visited this old friend of mine. But yes, there they were. And how amazing is that? There are relatively few dinosaurs, fossil mammals, and other creatures that even approach this level of preservation. Fossilization favors the sturdy. Pikaia was barely a wisp in the Cambrian seas, yet conditions were just right to entomb a small portion of these creatures in sediment so fine that we know not only the shape their bodies took in life, but the intricate inner workings that connect them to us across deep time. In the wider view of evolution, drawing back from the tiny twig to get a sense of where that small part fits, Pikaia was one of the earliest chordates, the family to which we also belong.
Not that Pikaia was the only early chordate on the scene. As if the Burgess Shale were not stupendous enough, China has its own equivalent, known as the Chengjiang fauna. These rocks, between 520 and 525 million years old, boast their own fossil treasures, including three small cousins of Pikaia . The first two to be discovered, Haikouichthys and Myllokunmingia (try saying that ten times fast), were announced in 1999, with Zhongjianichthys following in 2003, and all look like simplified versions of the guppies I used to bring home from the pet store. They didn’t have the little tentacles of Pikaia , which remain an unexplained oddity, but they still had the V-shaped muscles and bullet-shaped body plan, which probably were not noticed by the invertebrates that slurped them up in those early Cambrian seas.
What makes these protovertebrates so special is easier to see with our gift of hindsight. First off, Pikaia and other early chordates had a head. Not terribly shocking, I know, but it’s still essential to how our bodies came to be the way they are. Your eyes, mouth, and nose, harboring some of your most critical senses, are all bound up in your head, in proximity to your brain. Had the anatomical state of things been otherwise for the early chordates, the sensory centers of vertebrates may have been placed on their rumps, or distributed over their bodies and requiring the evolution of quick nerve networks to communicate through all the disparate parts. But more important than that, the notochord that formed along the back of these species set the basic framework for what would become the backbone and all the parts attached to it. Sharks, emus, tree frogs, wildebeest, and, of course, you owe the basic construction of the skeleton to animals so close to invertebrates that the first of their kind ever found was mistaken for a worm.
The story of Pikaia is essential to everything that follows. These creatures, or creatures like them, established the basis for why we are how we are. Pikaia and its increasing array of relatives, through pure accident, ended up establishing what it is to be a vertebrate. But the point I want to underscore, articulated by Gould decades ago, is that there was nothing special or remarkable about our ancestors back in the Cambrian. In the context of life at the time, the eventual rise of the vertebrates is an underdog tale.
During the time of all those spectacular Burgess Shale species, most of the wonderful forms swimming and skittering about were species far distant from our own predecessors. If you were able to travel back in time to the Cambrian, and had the forethought to bring a submersible with you, the glorious dawn of vertebrate life would seem rather dimmed.
You bob along at the surface, concentrating to hold back the seasickness, carrying out one last check of the instruments before making your descent. Sweat is already starting to dampen your clothes—how could you have forgotten that in this time the Burgess Shale is just south of the equator?—but in a few more moments you close the hatch and start your journey to the bottom. And even though the opening dirge from the Jaws theme starts to play in the back of your mind as you descend to the reef below, you remind yourself that there’s nothing to worry about. Not only are you cozy in your capsule, but it’s over a hundred million years before the dawn of sharks. The most fearsome creatures you’re going to see are only interested in snaffling up worms and other little morsels.
Impatient to start checking species off your list, you scan your field guide of the 150-some-odd species found in this place. Some of them, such as the bacteria, you’re not going to see as anything more than specks in the water column or mats on the seafloor. But at the very least you know you’ll be the only person to ever see a living trilobite—more than 33 percent of the Burgess Shale fossils found in your own era are of these arthropods and their relatives, members of the segmented invertebrates that include everything from grasshoppers to tarantulas and lobsters of the modern world. And after a few moments, you reach a Seussian seascape of strange tubes. Some grow in branching clusters, like underwater saguaro cactus, while others look like hairy cucumbers that have been cut in half. These are early sponges, the chief architects of the Cambrian reef. As your eyes adjust to this nest of pillowy porifera, the otherworldly shapes of the more charismatic animals begin to register in your vision. A priapulid—or “penis worm” that looks like a spiky version of exactly what its common name implies—darts back into its burrow. A trilobite disturbed by the movement in the water rolls up on itself in self-defense, looking like a stouter version of the roly-polys you used to find under the woodpile back home. A Wiwaxia is either too confident or too unaware to be bothered by such disturbances. The living pincushion pushes its spiky self over the muddy bottom in search of who knows what, passing by a wormlike critter walking on tube feet with sharp spines waving in the current—a species so strange that it was named Hallucigenia . And for a moment you pull the submersible up short as something that looks like an alien mothership speeds across the bow, undulating oarlike fins along the side of its body. That must be Anomalocaris on the prowl for something soft to stuff into its shutter-shaped mouth.
You’d likely run out of air and have to return to the surface, and hopefully your own time, before spotting a Pikaia . (And that’s a good thing, for, as time-travel movies remind us, you wouldn’t want to accidentally kill anything related to your ancestry.) Paleontologists who have pored over the thousands and thousands of Burgess Shale fossils at the Smithsonian, the Royal Ontario Museum, and the Geological Survey of Canada have found that early chordates comprised only 2 percent of the Burgess Shale fauna. Arthropods, sponges, algae, and worms were far more abundant than our relatives. Of course, the fact that Pikaia was a soft little thing without a bone in its body brought a slight bias against it being preserved into the fossil record, but regular mudslides that repeatedly buried the reefs caused these wonderful and alien creatures of the Burgess Shale to be extensively and delicately pressed into the rock. These happenstances, while disastrous for the Cambrian creatures, are why we can look back at this time with such extraordinary clarity. The mudslides buried the reef species so quickly and completely that even the most fragile creatures, such as our cousin Pikaia , were preserved. Bad luck for them, but good luck for us. It’s as close as we can get to a Polaroid of these ancient ecosystems, and they make one particular paleontological point especially clear: the Cambrian was very much an invertebrate world, with weirdos far more eye-catching and numerous than Pikaia . If you were able to visit these ancient reefs without any understanding of how the following 508 million years would play out, you’d dismiss Pikaia as a boring little thread that was clearly taking the lazy route while invertebrates were experimenting with every possible body plan their carapaces could morph into.
For seventeen million years after their origin in those Cambrian seas, vertebrate ancestors were rare, marginal animals that had to evade the piercing, crushing mouthparts of their ravenous neighbors. And they were even luckier and more persistent than we previously realized. Back when Gould was writing Wonderful Life , there seemed to be a hard and fast break at the end of the Cambrian—one of the bright, harsh lines in the fossil record that marks a mass extinction. In divvying up the rocks of the world, pioneering geologists often unintentionally identified these catastrophes by noting the drastic differences on either side of a particular layer of stone. After the Cambrian, the bulk of the ludicrous species spread throughout the ancient seas seemed to totally vanish. There were no more living pincushions, nozzle-nosed foragers, boomerang-headed arthropods, or shutter-mouthed monstrosities. A more anatomically subdued collection of animals such as early trilobites and brachiopods made it through, with our own chordate ancestors joining the throng of lucky survivors. This was one of the most critical moments in evolutionary history for our species, Gould wrote, marking a time when life could have gone in a very different direction. If Anomalocaris or Wiwaxia had been spared—if the extinction had been canceled—then the evolutionary routes between Pikaia and the first true vertebrates would have been closed, totally erasing us from history and vastly altering the evolutionary course of what followed. Or so the story went. New finds have rewritten the tale.
Gould’s underlying point still holds: what exists now relies on innumerable past events that opened particular evolutionary options while closing or constraining others. This is called contingency, and it’s the same as when you wonder what would have happened to your life if you had screwed up the courage to ask your first crush on a date back in middle school, if you had taken that gap year in college like you wanted, or if you had avoided that questionable gas station burrito for lunch—just on a larger scale. The most striking example is when an asteroid smacked the planet sixty-six million years ago to spark a mass extinction that booted the dinosaurs from their dominance and eradicated various other forms of life, shaking things up enough to give mammals a chance at proliferating. The entire history of life on Earth pivoted on this single moment, and if it hadn’t happened, the world would probably still be ruled by toothy, feathered saurians of every shape and size. Almost every slice of deep time has critical moments like this—some of which we can detect, others that are too small to sense.
There was no evidence of any asteroid impact, massive volcano belching greenhouse gases into the atmosphere, or other stark extinction trigger at the end of the Cambrian. The leading explanation for what happened was more a matter of Darwinian calculus. Paleontologists thought that the Cambrian menagerie simply lost the race for life, supplanted by novel species that had an edge over these more archaic forms. Newer, sleeker, more advanced organisms beat them into permanent submission. This was a concept that went all the way back to Darwin’s On the Origin of Species —that descendant species will be better adapted to the prevailing conditions than the species that spawned them, and so the new will usually overtake the old. Paleontologists referred to this particular changing of the guard as the Ordovician radiation. It’s when the seas started to take on a more modern appearance. Snails grazed among gardens of coral while the early cousins of sea stars, squid, and clams mixed with some Cambrian leftovers that managed to hold on, such as the trilobites.
But now we know the closing chapter of the Cambrian does not record a vast extermination of the weird, as was once thought. In 2010, a multi-institution team of paleontologists announced that they had described a vibrant community of Burgess Shale creatures found in rocks 443 to 485 million years old, long after the close of the Cambrian. The “extinction” at the end of the Cambrian wasn’t because the animals disappeared. It was because the proper sort of sedimentology to preserve soft-bodied organisms became much rarer and suitable examples had not yet been found. In fact, the newly discovered deposit seemed to show something of a mix in which the ancient met the then-modern—there were the snails and nautiloids and crinoids of the Ordovician, yes, but there were also reef-building sponges, worms, soft-shelled arthropods, armor-covered animals similar to Wiwaxia , and enormous relatives of the truly bizarre Anomalocaris . No one had expected to find an entire community of animals that looked like they were straight out of the Burgess Shale mixed in with the species that were supposed to have utterly obliterated the competition. The old had survived alongside the new.
So what does this have to do with our little chordate friends? Well, in Wonderful Life , Gould argues that without a mass extinction of weird Cambrian species, our protovertebrate ancestors would have needed to adapt and adjust in different ways. This would have vastly altered evolutionary history in ways that would make the origin of Homo sapiens unlikely, if not impossible. Yet we now know that the end-Cambrian extinction really has been canceled, which makes our ancestry all the more peculiar. The protovertebrates continued to eke out an existence in an invertebrate-dominated world, twitching their little myomeres to escape the rapacious appetites of their neighbors. The ripples of contingency can still be felt. Had the early chordates gone extinct—or if they had their notochord along their belly, say, or led with their tails rather than their heads—evolutionary history would have been wildly different, more so than we’re probably capable of imagining. But thanks to the fortuitous survival of so many Cambrian creatures, our backstory has just gotten that much deeper. To say that the likes of Pikaia were simply lucky now reads as an insult. They were survivors in their own right. Something else, something not yet fully understood, was going on as life flowered in the ocean realm, the dawn of the chordates taking a slower rise against the busy world of the invertebrates. And it’s against that background that another chance event opened up even greater possibilities. The world was about to see bone for the first time.
