Living fossils use old body plans that still work today.
The Volkswagen Type 1, with its walleyed headlights and domed roof, is uniquely enduring among automobiles. An enormous commercial success from its first production in 1938 through the 1960s, the iconic car continues to be driven widely and to inspire enthusiasts around the world. By any objective standard, newer models are better—faster, sleeker, safer, and more efficient or maneuverable. And yet the Volkswagen “Bug” maintains a tenacious grip on existence.
Type 1s may have gone into production more than 70 years ago, but they offer features and characteristics that remain attractive to this day, because they solved common problems. Air cooling reduced the size and weight of the engine while also bringing down the number of components that could break. Combining rear-wheel drive with a rear-mounted engine was excellent for driving in poor weather—the engine’s position over the drive wheels weighed them down and offered the vehicle great traction despite its light frame.1 Finally, the legendary Sicherheit of German engineering, combined with a reasonable price, rendered the Type 1 a secure bet for working-class families around the globe. For a while the cars were everywhere. So even though an old Beetle sputtering down the road isn’t much to look at, its original features keep it competitive in a more advanced world.
In the same way, it is their features that set apart the planet’s “living fossils”: organisms that evolved hundreds of millions of years ago and that have succeeded across eons without major changes. Like the Volkswagens of decades past, they tend not to be terribly common now, but they continue to survive in spite of a world that has passed them by. All the ancient organisms surviving to the present day have done so because they possess certain core features that not only ensure their survival but also continue to define them.
Some of these living fossils have always been rare. But some had a rockstar past: they’ve been among the dominant life forms in the oceans, filling the shallows and the deeps and controlling the ecology of early ocean ecosystems. Yet the evolutionary wheel eventually turned, and newer forms came to dominate our seas. When we look at today’s ocean, it seems so normal that it is filled with fast swimming fish, gigantic whales, and leaping dolphins that it might be a shock to discover that prior oceans didn’t look that way.
As the world heated and cooled, as continents broke apart and then came together like gigantic pools of mercury, before the land was colonized by complex life, the trilobites thrived. They could be found in every ocean, filling various ecological niches: predators, prey, scavengers. They were bottom-crawling, multi-legged creatures with strong carapaces and sharp claws. Among them were the eurypterid sea scorpions measuring up to 8 feet long and bearing barbs at the ends of their tails.2 These primitive arthropods were the common ancestor of today’s spiders, scorpions, ticks, and horseshoe crabs. They shared the trilobites’ powerful survival tools and enjoyed similar success.
Trilobites were not Volkswagen Beetles—they were an entire fleet of diverse organisms, a whole production line and not just a single model, that burst upon the ocean scene in the Cambrian, 540 million years ago. By the time of the Burgess Shale (see Chapter 1), they had evolved into a riot of species with specialized spines, legs, and sophisticated eyes.3 Their exquisite preservation in rocks opens up their world to us in a way that few other fossil groups have ever done. There were “predators, mud-grubbers and filter feeders”4 that divided up the early food resources of their ancient seas. They rolled up when disturbed, like a panicky pill bug, making spiny-carapaced species that resembled armored pincushions. They left behind their skeletons when they molted, just like crabs do today, and nineteenth-century paleontologists painstakingly put together growth series from tiny juveniles to full adults.5
Perhaps the most startling adaptation of these early arthropods were their eyes:6 large compound structures containing hundreds of small calcite lenses.7 Calcite is a form of calcium carbonate (the same material as a crab’s shell or the sand on a tropical beach) that is usually as transparent as a brick. But the calcium carbonate in a trilobite’s eyes is transparent, “constructed of calcite (crystals) so precisely oriented crystallographically that they behave optically as if they were made of glass.”8 These animals took an opaque skeleton, made it transparent, and watched the world through these unique lenses for hundreds of millions of years. Then they took this crystallographic magic to the grave with them.
A list of the trilobites from the late Cambrian, 500 million years ago or so, is an impressive collection, but the best would come later, in the Ordovician era 490–445 million years ago. In this period, trilobites occupied the most regions of the sea in “sunlit reefs and in gloomy abysses.”9 Later epochs saw trilobites dwindle as one extinction event after another reduced their diversity and numbers. Eventually, trilobites and sea scorpions died out. They disappeared from the oceans about 250 million years ago at the end of the Permian era, when 96% of the ocean’s species disappeared in the biggest mass extinction of all time. Five genera of trilobites were going strong until the mass extinction, and paleontologist R. M. Owens mused, “But for the extreme stresses that the entire marine biota suffered towards the end of the Permian, trilobites would probably have survived for much longer.”10
Trilobites were not a flash-in-the-pan invention. They were a dominant part of the life of the ocean for 200 million years, 100 times longer than our own species has existed. They were the first mega-successes to climb out of the empty barrel of the Cambrian, and they made an empire of the entire ocean. They may have fed the rise of ammonoids, nautiloids, and sharks, and they may have competed with upstart newcomers like fish. But while they were here, they defined the life in the sea. Yet they were living fossils 250 million years ago, not today, because their line completely died out. Other lines were similarly successful but have provided us a few key living species to remember them by.
The distinctive, spiraling rose-colored shell of the chambered nautilus, Nautilus pompilius, epitomizes the sea’s delicate beauty. It is usually shown cut in half, revealing curved chambers bathed in a pearly glow. Lined with pale lustrous nacre, the shell’s interior is a study in mortal humility. Building from a tiny solid core, the animal secretes a curled tube of material with which to shelter its soft body. As it grows, so must the tube—ever wider and longer, looping back around itself again and again.
Chambered nautilus. Photograph by Chris 73 / Wikimedia Commons.
The animal that makes this shell is not so delicate looking: a hulking, tiger-striped creature stuffed sloppily inside a coil of plaster white. A broad, visor-like mantle protrudes from the shell and sits above a writhing clutch of short tentacles. Two alien eyes twitch on the sides of its head, using primitive pinhole apertures instead of lenses to search for prey and enemies.
Ancient seas held huge numbers of these creatures: nautiloids plus a related group called ammonoids, now extinct. Hundreds of millions of years before the rise of the sharks and fish, these shelled, tentacled creatures were the most advanced predators of their age. They ranged in shell size up to nearly 10 feet across.11 A shell 10 feet wide probably made ammonoids cumbersome, so they most likely preyed on slow bottom-dwelling creatures such as trilobites. Some had shells twisting around in such strange contortions that it’s difficult to imagine how they swam at all. These strange oversized beasts persisted in great variety and huge numbers for hundreds of millions of years.
The heavy shell is the nautilus’s defining feature: a legacy of its ancient family line. The chambers are no simple curiosity, no crude solution to a simple problem of unused shell space. As the animal grows, the shell grows as well, and the animal must keep its position at the opening. So it vacates the space where it used to live, moving forward in the shell, and sealing the old apartment off with a thin film of pearly material, creating the “chambers” for which it is named. The chambers themselves are marvels, laid end-to-end to describe a nearly perfect logarithmic spiral.12
The chambers are crucial to the animal’s survival, necessitated by the shell itself. Thick shells offer security—but at the cost of heavy weight. With such a load, the nautilus should simply sink to the bottom of the ocean. So why doesn’t it? The chambers are the answer. They may be vacant, but they’re not empty.
The nautilus alters its buoyancy the same way a submarine does: by filling or emptying its ballast tanks. Through each chamber runs a fleshy thread called the siphuncle, connecting the chambers like locks in a canal. This thread alternately oozes fluid into the chambers or absorbs it, altering the animal’s buoyancy like a submarine maintaining its ballast tanks.13 The shifts are gradual, but fast enough to let the animal traverse hundreds of feet up and down the water column as day fades to night. During the day, they bide their time on the darkened slopes of deep coral beds nearly 1,000 feet down. At night, they climb into shallower water to feed. This arrangement has a serious weakness, though. Nautilus cannot add gas to pressurize its chambers, and when there is high external pressure—one experiment suggested a threshold around 1,600 feet deep—the shell collapses.14
The nautilus navigates the world in the same way as most other cephalopods: by using an elongated tube called a siphon that can eject water from the mantle cavity. By taking in and expelling water, the mantle and siphon acts as a primitive underwater jet engine (like rolling your tongue into a tube to expel air).
For 400 million years nautiloids and their now-extinct ammonoid cousins reigned. The ammonoids died out when the dinosaurs did, about 65 million years ago. The nautiloids also dropped in numbers, finally dwindling to the six species alive today. Like trilobites, their history is one of profligate diversity, ecological diversification, and great fossil longevity. They were sometimes predators and sometimes prey; fossils exist bearing gaping shark bites and other battle scars. Mosasaur bites (large crocodile-like extinct reptiles) have been found.15
Hundreds of millions of years ago, a time when nearly all complex animals were bottom-dwellers, an organism of the Nautilus’ versatility ruled the seas. The Chambered Nautilus survives to this day because of the same adaptations that made it successful in the first place: the jet-producing siphon and the chambered shell.16 Hidden away in the depths of isolated reefs, the nautilus is a king “bounded in a nutshell.”17
Ocean City, Maryland, is a tectonic boundary between nature and artifice. Built on a spit of a sandbar on the sandy mid-Atlantic coast, the city is in constant conflict with the sea. On the western edge, river waters pool in a brackish estuary with marsh grass growing down to the shore. On the eastern edge, the Atlantic Ocean beats relentlessly on silicate beaches crowded with summer tourists. High-rise hotels and apartment buildings crowd the shoreline, daily disgorging hordes of beachgoers. At the southern end stands a glittering entertainment strip, complete with video arcades and a Ferris wheel embodying a particular, well-aged vision of American glory. Few places are more iconic than an aging Atlantic coast boardwalk: parents explaining to their unimpressed kids how great this was, omnipresent stickiness beneath your feet, and storefront tchotchkes appealing to nostalgia and affronting good taste in equal measure.
The boardwalk looks old, though it has been there a scant century. But along the long, sandy beaches is one of the most enduring life forms on the planet. Just beneath the waves, among the sand bars, lives a true “living fossil,” known colloquially as the horseshoe crab. After hundreds of millions of years on the planet, these animals remain true to a body form that we can find in the deep rocks of ancient fossil beds, with hardly any substantial change.
The best way to learn about horseshoe crabs is simply to find one on the beach and turn it over. Be warned—though the animals are harmless, most beachgoers are unprepared for the clawed shock beneath. Ten long, spindly, claw-tipped legs reach and curl like dead calcified fingers. They emerge from a glistening insectoid thorax in front of a heavy abdomen and behind a thin head adorned with tough, claw-tipped feelers and topped by a carapace like an alien helmet. Lacking jaws or teeth, horseshoe crabs grind up food with their legs and shuttle it to the tiny sucking orifice. These creatures look like nothing so much as aliens from another planet.18
Even in the modern world, they are alone—isolated by eons of evolution. Horseshoe crabs are not crabs in the proper sense. They’re more closely related to spiders than crustaceans, properly in the subphylum Chelicerata rather than in Crustacea, and there are only four species left in the world.19 All have a dome-shaped hard carapace sheltering the entire body. It’s less a suit of armor than a rigid tent laid over a separate body. They breathe through leaves of paper-thin respirating tissue, folded over like the pages of a book between the legs and the long nail-like tail. Attached to their back legs, these “book gills” allow horseshoe crabs to breathe without proper gills. These adaptations were common in ages past, but today the horseshoe crab is the only animal on Earth to feature these anachronisms.
The design may be outdated, but there are ways in which horseshow crabs outdo many of its later cousins. For example, the eyes of modern horseshoe crabs are relatively simple: two large primary eyes, and seven others of varying sensitivity and size arrayed across the body. Yet recent research shows the sophistication of vision in these ancient creatures and how their simple brains process complex visual signals. In particular, horseshoe crab eyes are a million times more sensitive to light at night than during the day. Hyper-sensitive eyes ramp up the sensitivity of the retinas at night and turn them back down during the day, perhaps to pick out mates during night-time low tides.20
Despite the primitive features of horseshoe crabs, gills still deliver oxygen to the blood, the ten legs sift sand for food, and males lumber after females to hold them in a fertile embrace. New generations of the same model have been produced down the eons, natural selection is satisfied, and the shape and habits remain unchanged in the four remaining species. Like the Volkswagen Beetle, they stick out like a sore thumb but persist in sturdy practicality.
The Atlantic horseshoe crab is a relatively new species,21 but horseshoe crabs as a family first appeared on Earth some 450 million years ago.22 Fossils from 445 million years ago have the characteristic domed carapace of modern horseshoe crabs, along with their thick tails.23 Long persistence with little physical change is what makes horseshoe crabs the poster children of living fossils; their detailed body plan has persisted longer than just about any other. They had cousins with slightly different body forms over the ages, and some had been stronger and more successful for short spells. But ultimately all the others died out, succumbing to the march of time.
There are four species of horseshoe crab left, and no trilobites. So even though trilobites ruled the oceans—and horseshoe crabs never did—in terms of survival, horseshoe crabs have won. But there is one type of trilobite left, embodied in the pill-bug-like youngsters of the modern horseshoe crab. Hatched from eggs buried in sand, the “trilobite” larva of the horseshoe crab is the only living vestige of the trilobite dynasty. So horseshoe crabs are actually double living fossils, embodied in their adult forms and in their youngest stages.
Why horseshoe crabs avoided the extinction that consumed their more successful cousins is unknown. But they provide a link back to the ancient seas before fish or whales or modern corals, and they live today on a few protected sandy beaches of a more crowded world of cotton candy and roller coasters.
What would it be like to be the one who discovers that trilobites are still alive? No one can claim the fame, but exactly this drama played out for another iconic living fossil—a familiar animal from the fossil record, for centuries thought extinct and then amazingly rediscovered! It’s the coelacanth, a deep-dwelling rarity whose discovery laid bare the limits of human knowledge.
Coelacanths take their name from the larger order of “lobe-finned” fish to which they belong, an order that appeared a little later than the earliest sharks—about 400 million years ago. There were never many species, but at least one had a large range from North America to Asia.24 Their thick, bony, finger-like “lobe fins” were relatively primitive swimming instruments that were the evolutionary progenitors to vertebrate limbs. Every bird, reptile, and mammal alive today is descended in some way from this ancient stock of marine ancestors. Paleontologists knew this for a long time, and big fossilized coelacanths were immortalized in more than a few museums. They also knew that the last coelacanths died out in the Cretaceous period, some 65 million years ago.25 They knew this until 1938, when a young woman bought a fresh-caught coelacanth in a South African fish market.
Dr. Marjorie Courtenay-Latimer recollected the day the coelacanth returned: “22 December 1938 dawned a hot shimmering summers day. . . . [The] phone rang to say the trawler Nerine had docked and had a number of specimens for me. . . . So I rang for a taxi and went down to the fishing wharf.”26
Latimer was serendipitously handed a creature that would one day bear her name: Latimeria chalumnae, a large, oily fish trawled up from about 200 feet deep in the Indian Ocean, off the coast of South Africa. For ichthyologists, this was like finding a live dinosaur in the Amazon. The specimen was immediately taken to Europe, where it was displayed in front of a reported crowd of 20,000 eager science enthusiasts. Latimer and the scientists with whom she shared her discovery were worldwide celebrities, and the history of marine evolution had to be rethought. In the case of the coelacanth, countless fishermen must have encountered them, especially along the coast of the Comoro Islands near Madagascar, where most specimens have subsequently been found. But between their poor commercial value and relative rarity, European scientists never made the connection.27
The fish themselves seem not much changed from their primordial ancestors. Thick, fleshy fins and a heavy body render the coelacanth slow and cumbersome even in its natural habitat. A tiny brain occupies less than 2% of its skull cavity. The rest is filled with fat for buoyancy. Its flesh is dense, oily, and foul with urea—in fact, the species’ utter lack of commercial value may be one reason it still exists. Coelacanths swim ponderously, like underwater zeppelins, and float quietly in wait of the small fish that are their prey. Broad, flat fins sway back and forth unsteadily as though the fish were about to roll over. If a shark is the evolutionary equivalent of a serrated butcher’s knife, a coelacanth is a wooden club.
Coelacanths remain on the razor’s edge of extinction, sustained probably only through isolation on the periphery of life. These fish likely survived through little more than evolutionary serendipity. They inhabit a narrow zone in cooler water, between 300 and 600 feet beneath the surface along steep volcanic coasts, seldom coming close to the surface unless cold upwelled waters bring them up. They produce just a few, very large eggs—a 40-inch fish will have eggs the size and hue of an orange. Their ancient body plan still makes them an effective predator of more modern fish, but they remain present in only a tiny fraction of the world they once inhabited.
Beautiful, powerful, and thrilling, sharks have ever gripped the human imagination. They are, it has been said, eating machines; perfectly honed instruments of death, the Platonic ideal of the cruel predator. In her book Demon Fish, Juliet Eilperin quotes the Greek poet Oppian, describing sharks “rav[ing] for food with unceasing frenzy, being always hungered and never abating the gluttony of their terrible maw.”28 Their bodies are streamlined torpedoes of hard muscle, equally suited for effortless gliding and the explosive speed of the kill. Their mouths are not tools, but weapons: rank upon rank, teeth like serrated knives advance to replace those shattered on the bones of last week’s lunch. Outlandish sensory organs allow them to detect tiny amounts of blood over vast distances and “hear” the electrical pulses of a victim’s heart. Denticles on the skin—specialized scales that resemble nothing so much as tiny sharp teeth—create minuscule low-pressure pockets of water along the body. Like divots on a golf ball, the pockets reduce drag and improve speed as the animals course the ocean vastness between continents. Those flat black eyes, rolling back at the moment of impact, broadcast the shark’s single-minded lethality to the world. We are terrified of sharks, thrilled by them, and yet attracted to the primal savagery they represent. The shark we barely glimpse in the murky water of a deep SCUBA dive represents the darkest umbra of a world we can never completely illuminate. But this kind of hyperbolic “Shark Week Science” is correct for only the smallest fraction of sharks. Most sharks through history had these basic elements (powerful teeth and a predator’s diet) but ranked fairly low on the terror scale, usually grubbing in the muck for small prey.29
Sharks burst on the evolutionary scene 418 million years ago with a hot new innovation—a jaw—that they shared with the ancestors of bony fish. Their skeletons were made of cartilage, lighter and more flexible than bone. A specialized organ called the ampullae of Lorenzini in the head of sharks senses weak electric fields using a set of specialized ciliated cells.30 In lab tests, sharks have been able to find hidden prey fish solely on the basis of the electrical signals the prey emits and have shown exactly the same behavior when the prey’s electric signal is simulated with electrodes.31
All these things help define sharks. But most importantly, there are the teeth: serrated tools absolutely perfect for the task of eating in a world where the other sharpest bites were from the beaks of cephalopods. When sharks evolved 418 million years ago or so, there were beaks and claws and probosci and rasps: but nothing else had teeth. Those teeth were an amazing invention, and they gave sharks a crucial edge.
It was the invention of sharp, replaceable teeth that gave sharks their market share, their biological brand in the new world of predators that evolved in the dawn sea. Innovations are key to the success of new groups—whether it is Larry Page and Sergey Brin creating PageRank, a new way to rank the importance of web pages for Google,32 or a tooth in a world of more and more armored animals. The innovation of the shark tooth is still paying off after 418 million years.
Sharks continuously grow their teeth—a new set every 7–10 days—and discard the old ones like dull razors.33 And they are supremely sharp, featuring a cutting edge just one thousandth of an inch wide. They’re among the sharpest natural structures on Earth.34 Even a fossil shark tooth can cut you if you are not careful with it. These fearsome weapons have carved themselves a long and bloody legacy.
But how does a shark grow something this sharp? When humans make something sharp, we build it thin, and then pound on it or sand it down until the thin edge is thinner still. Biological structures can’t be pounded or sanded this way. They must be grown already sharp, by cells and tissues that are usually far better at growing things that are soft. Hard, sharp structures were a new idea 418 million years ago. And the sharks hit on a design that is a tiny miracle of cellular engineering.
Shark teeth start deep: they start their lives deep in the mouth, beginning as a low hard ridge in the soft tissue of the throat. Ridge after ridge grow in succession and move up toward the mouth like waves rolling onto shore.35 Individual teeth first form as an amorphous lump of tissue. Next, the tooth heightens through a thin line of cells at the crest of the ridge throwing up a narrow fan of fibers. Those thin fibers define the sharp edge of the tooth, and keeping the fibers in a dense narrow line rather than a clump is what makes the difference between a truly sharp tooth and a blunt one. Once they grow in their precise regiments, the tiny gaps between fibers are then filled in with a bone-like material, accreting slowly into nature’s finest knives.36 Finally, an even harder layer of sealing “enameloid” is applied to the cutting surfaces for extra strength. Slender structures tend to be fragile, so serration is produced by introducing undulations in the narrow ridges of fibers, thereby strengthening the ridge without thickening it. By now this row of teeth has moved into the mouth, ranking behind the those currently in use, ready for deployment if the teeth in front should crack on bone, stone, or steel.
Shark teeth 418 million years ago were very sharp but only an eighth of an inch long.37 The sharks that made these teeth are harder to see in the fossil record, and our first glimpse of an early shark body comes from a 9-inch specimen 409 million years ago.38 The treasure trove of shark body fossils is 370 million years old, preserved almost whole (along with their stomach contents) when ancient proto-sharks called Cladoselache were snatching up primitive fish using rounded grab-and-pull teeth.39 The basic shark body plan was present but unrefined. Cladoselache looked like a nerdy prepubescent shark, combining a skinny body with oversized fins. This was just an early model.
A goblin shark’s snout. Photographed at the Shinagawa Aquarium in Japan. Photograph by Hungarian Snow.
A few sharks continue to look like this today. The goblin shark inhabits the deep sea, and has a huge, elongated “nose” above a scraggle-toothed mouth. Its underslung jaws with ice-pick teeth are on an elastic ligament like a rubber band. The band is stretched tight with the mouth in its retracted position, where it is held in place until prey draw near. When released, the mouth shoots forward to grab soft-bodied bottom-dwelling prey and retracts just as rapidly back into the skull.40
The frilled shark also has been called a living fossil because of its sinuous long body and ancient jaw structures. Though later work seems to put it in a more modern shark group, it shows how older sharks may have lived, with a 6-foot eel-like body and needle-like teeth that can strike quickly and secure small, quick prey.41
Rows of needle-like teeth on the frilled shark, used for catching soft-bodied prey like squid. Photographed at the Shimonoseki Marine Science Museum, Kaikyoukan Marine Aquarium, Japan. Photograph by User:OpenCage.
The sharks of modern seas are not much like those that first appeared 400 million years ago. Unlike the horseshoe crab, the basic body plan of sharks has not been stable over evolutionary time. Instead, the evolution of sharks from predators of bottom-dwelling invertebrates to the killing machines of modern seas is a story of extinction and refinement.
Two hundred and fifty million years ago, Earth experienced the most devastating mass die-off ever recorded. Dubbed the Permian-Triassic Extinction,42 it wiped out a staggering 96% of marine species.43 Rapidly shifting ecological or climate changes could be responsible, and scientists have put forth theories ranging from impacts to massive volcanic events.44 Early sharks died, but they gave rise to a small group called the Neoselachii that is the ancestor of modern sharks. They came of age in a black and empty ocean that took 5–10 million years to recover.45 Conditions were brutal; prey were scarce. Yet the Neoselachii survived, using more powerful bodies and ever-improving dentition. They diversified, grew in size, and eventually evolved the modern sharks we thrill to today.
Restoration of fossil shark Carcharodon megalodon. Reconstruction by Bashford Dean in 1909 at the American Museum of Natural History, reported to be somewhat oversized.
For example, the first great-white-style sharks of order Lamniformes came into being about 65 million years ago. They perfected their dentition and changed their mouth structures to push their jaws out.46 While attacking, those hinges would open like flower petals to expose the ferocious teeth inside. The awe-inspiring Carcharodon megalodon grew nearly 40 feet long with a body mass equal to eight elephants (77 tons).47 Its jaws, up to 6 feet across, could deliver more than 40,000 pounds of force. The biggest of its 276 serrated teeth were 6.5 inches long.48 Megalodon probably fed on large baleen whales and evolved about when they did, about 20–30 million years ago.49 It packed all of modern shark evolution into one body—fast, powerful, and predaceous on species that were themselves extreme in size. They disappeared in the middle of the Ice Ages 2 million years ago, for reasons so far unknown.
Some sharks once thought to retain very primitive features, like the frilled shark, are now known to be advanced families that have re-evolved the body plans of their deep ancestors. Even shark teeth have evolved from having a single enameloid layer to having three.50 Unlike horseshoe crabs and the nautilus, sharks also remain supremely successful in the modern oceans. These evolutionary advances are no less than those made by mammals in their climb up the rungs of ecological importance, from small beginnings and marsupial ancestors, to become the dominant large animals on land.
So, are sharks legitimate living fossils? Their basic structure and body plan hasn’t changed in 409 million years. The way they grow and replace teeth, and the electrical senses we cannot easily duplicate mechanically, defined them long ago and still do. Probably their greatest consistency is in their steady predatory presence in the oceans since nearly the dawn of the age of animals. Since even before the land held much more than millipedes, during the heyday of trilobites and ammonoids, during the first age of the ocean when nothing else had teeth, the sharks patrolled. As the very continents have moved, the sharks have swum their margins, searching for prey in seas alternating between abundance and extinction-caused scarcity. Living fossils? Living wonders.
Living fossils are more similar than their disparate anatomies would suggest: ancient organisms clinging fanatically to their ecological niches, succeeding through a handful of highly refined biological features. Certainly, speciation and adaptation in these groups have occurred over the eons. But what has changed pales in comparison to what has not. Few species manage to exist for more than a few million years. For a body plan to endure that timespan a hundred times over, fundamentally unchanged, through apocalyptic extinction events and the rise of human civilization, is astonishing—but the proof is right in front of our eyes.
Despite the contrary popular conception, evolution does not lead to progress. If anything, evolution rewards short-term success and is devoid of long-term planning. If a species successfully produces the next generation, then evolution gives it at least a passing grade. In this view, why shouldn’t a successful body plan succeed for a long time? Two answers emerge: first, environments change. Second, coevolution with competitors or prey favors continuous innovation. For living fossils, long-term success seems to go hand in hand with a retreat from change. Up in the shallows, waves of mass extinctions have passed across the oceans—five so far, though humans are currently engineering the sixth.51 These game changers reset the flow of life and cataclysmically changed the environments in which most marine species live. But some environments were less perturbed. Deep ocean environments are among the world’s most stable—cold, deep, quiet, and enormous. They may simply be stable enough and big enough for some ancient forms like the coelacanth and the nautilus to survive.
And then there is the coevolutionary race among species. Much of the evolutionary play is a script that depends on the action of different players— predators consume prey, competitors exclude the weakest. When one player evolves a successful new strategy, it imposes pressure on the others to likewise evolve. Some will possess the genetic toolbox to respond, and these will continue to thrive and to change. Some will not have the toolbox, either through bad luck, small populations, or lack of genetic variation that fuels evolutionary change. Again, living fossils are likely to be those that for some reason have fallen into a way of life where few of these coevolutionary races are run.
The coelacanth and the nautilus occupy ecological niches that isolate and protect them. Two species of coelacanth are currently known, in tiny pockets separated by thousands of miles of water.52 There are only four species of horseshoe crab, confined to the Atlantic and East Asian coasts. Perhaps six species of nautilus cruise the Indian and Pacific oceans, restricted to deep region in the tropics.53 At the same time, and despite aggressive fishing by human beings, more than 400 species of shark ply Earth’s oceans. From docile reef dwellers the size of cats to the nightmarish and diamond-rare goblin shark, these fish—for their combined evolutionary longevity and sustained diversity—might be the most successful multicellular organisms alive on our planet today.
All of which leads, we might hope, to some humility. The “living fossils,” sharks excepted, are not high-profile creatures. They are too odd, too cloistered in their habitats and ecological niches to frequently intrude on human activity. It’s possible that their survival is an odd accident of nature, and the world would barely miss them should they vanish. But their peculiarity is joined hand-in-hand with fragility, as niche organisms that rely heavily on their environments. The Atlantic horseshoe crab declines every year because of shoreline development on the eastern seaboard of the United States, and its decline has repercussions up and down the food chain—even as far as affecting migrating seabirds.54 The human race is a young species, a new pack of primates who have existed for an evolutionary heartbeat. We are simply passing through history. The nautilus, the shark, the awkward coelacanth with its lobed fins—are history.
