On Trails

by Robert Moor

  For billions of years, these single cells were the only living things on Earth. However, a cell benefits from better communication and cooperation with other cells. So some cells may have formed symbiotic relationships with others, then gathered into colonies, and, eventually, bound together into tissues. Interdependence both shackles and strengthens. Despite the restriction of freedom, tissues allow a much greater range of body types, including bilateral symmetry (a distinct front end and back end), which is the structural basis for the wild array of beasts that now stalks the earth and sea and sky. Matthews cheekily summed up this billion-year-long process of evolution from cells to bilaterians thus: “Tissues developed because it’s nice to have muscles and an ass. It’s not good to shit out of your face. It’s just not a very good idea.”

  We tissuey beings define our individual selves as enclosed, tight-knit systems. (“Otherwise,” Matthews said, “your arm would run away.”) But here too our assumptions begin to break down. As we sat eating our picnic lunches on a flat rock overlooking the sea, Stewart mentioned that he had recently been reading an article in a science magazine that asked what truly defines a human being, since our bodies are dependent on an unseen universe of microorganisms to survive. There are, for example, at least as many bacterial cells as human cells in the human body—possibly many more.

  “There are more cells in you that aren’t you,” Matthews said.

  “Yeah,” said Stewart. “Which sort of brings you to a point of ‘What is you? What am I?’ I had an existential crisis while I was reading it.”

  As I chewed my plum down to its wet-furred pit, a druggy feeling overtook me. I suddenly became aware of my own complexity: a riverine inner landscape swimming with cells both native and foreign; varied tissues clinging to and pulling against an architecture of bone; a digestive tract breaking down a bolus of plant material; two feet pressed against the earth; two nostrils sucking and spouting air; and in between, a branching network of nerves flickering with electricity in a furious effort to make sense of it all. Inside the human body lies a realm of perpetual darkness and riotous life, much of it still unexplored. We are, each of us, wild to our marrow.

 

  After lunch, we headed east along the shoreline, ascending through geologic time over younger and younger rocks. We were following a stratigraphic chart of Liu’s, which looked like a multi-tiered ice-cream sandwich. Each layer depicted a stratum of rock; embedded in some, like a sprinkling of chocolate chips, were known fossils. We paused on one surface where the chart indicated there should be an array of disc-shaped fossils, but we couldn’t see them. The angle of the light wasn’t right; certain layers reveal their fossils only when the sun is low and shadows become pronounced. Liu got down on his haunches, his eyes scanning the rock. Then they clicked into focus. “Ooh,” he said, pointing. “There’s one.” I followed his finger. The lines of an ovoid fossil emerged from the pixelated background, like in those Magic Eye books I used to go cross-eyed over as a child.

  “There are loads of them, actually!” Liu said, his hand sweeping across the marbled surface. As his finger passed over them, the outline of a half-dozen other discs rose from the rock.

  I was baffled. When I looked at these rocks, I saw a jumbled code:

  But when Liu looked, a clear picture arose:

  When I asked him how exactly he managed to zero in on the relevant bits of visual information, Liu said that the secret was training your eye. Stewart disagreed. He thought that Liu was born with better eyes than most—not just the physical eye, but the whole perceptual apparatus. “This guy does it to me all the time,” Stewart said. “We’ll go fossil hunting in England, and he’s like ‘There’s one. There’s one.’ I get so frustrated. But there’s a reason he’s doing this for a living.”

  Later, Liu obliged to describe the process in greater detail. The key, he said, lay in “trying to cut out all the noise”—recognizing and eliminating any nonbiological features that might resemble fossils to the untrained eye. Once de-cluttered, the pattern can emerge. You also need a “search image,” he added. “If you know to look for something, you’ll see it. But if you don’t, you can miss it.”

  It struck me that a similar problem plagues all branches of science. Scientists looking for new discoveries appear to be trapped in a logical bind: it is exceedingly difficult to find something when you don’t even know what it is you are looking for. Here is where the imaginative work of hypothesizing enters into play. By extrapolating from known patterns, we can make predictions and envision new phenomena, which we can then begin looking for in the world.

  Hypothesizing is such a powerful tool that it can bear fruit even when the things we think we are looking for are not, in fact, there. Huxley, foreshadowing Kuhn’s theory of paradigm shifts by some fifty years, argued that this speculative process guides all scientific inquiry:

  Man approaches the unattainable truth through a succession of errors. Confronted by the strange complexity of things, he invents, quite arbitrarily, a simple hypothesis to explain and justify the world. Having invented, he proceeds to act and think in terms of this hypothesis, as though it were correct. Experience gradually shows him where his hypothesis is unsatisfactory and how it should be modified. Thus, great scientific discoveries have been made by men seeking to verify quite erroneous theories about the nature of things. The discoveries have necessitated a modification of the original hypotheses, and further discoveries have been made in the effort to verify the modifications—­discoveries which, in their turn, have led to yet further modifications. And so on, indefinitely.

  The open-ended nature of science is either its greatest asset or its fatal weakness, depending on one’s outlook. Those of a mystical or skeptical cast of mind cite its mutability as proof that all scientific knowledge is ultimately shallow and illusory, while a believer in the scientific method finds comfort in the fact that it continually evolves to more tightly fit the contours of the universe’s dark terrain.

 

  Our rise through geologic time ended at the bedding plane that bore Liu’s fossil trails. On a rock wall facing the sea there protruded a waist-high shelf. We hovered over the shelf, looking down. Once again, I saw only a flat expanse of stone until Liu pointed out the trails subtly etched into the rock.

  Here, finally, was what I had come to see: the world’s oldest trails. They were easy to miss; it looked as if someone had lightly dragged a pencil eraser through drying concrete. Matthews opened his canteen and poured some water over the rock, so the trails would stand out in starker relief. Even still, I came to understand how dozens of other paleontologists had failed to notice them. All around were large, distinct body fossils impressed into grand sweeping surfaces. Liu’s trails were like a poem carved onto a handrail in a stairway of the Louvre.

  We worked our way along the shelf, inspecting yet more trails. Some were larger than others, but none were wider than a thumbprint. Most were relatively straight, but one peculiar trail looped back on itself, like a snake in agony. Liu believed that it provided further evidence that the marks were not, as Retallack had argued, produced by a rock or shell being dragged by a current along the seafloor.

  I lightly ran my fingers over the trails. They bore the distinct texture of life. Their surface was patterned with a series of nesting arcs: (((((((. Liu thinks each arc was made by the creature’s circular foot as it inflated with water and extended forward, smearing the front edge of the previous impression. At the end of some of the trails was a small dimple—((((((()—called a “terminal impression,” which might indicate the organism’s final resting place.

  Modern sea anemones creep along the seafloor using a similar system of hydrostatic inflation. And this, Liu thought, could provide a clue as to why the first animals made trails. Many of the Ediacarans found on Mistaken Point were believed to have lived their lives secured to the ground by suction cup–like feet, with their fleshy bodies extending out
into the water column to gather food. Modern animals with similar body types typically prefer to latch on to a hard substrate, like stone or, when available, glass. In his lab, Liu had observed that when sea anemones were forcefully pried loose from the aquarium’s glass, they would creep across the tank’s sandy bottom until they encountered another hard, flat surface.

  Liu’s best guess was that his fossil trails were similarly formed: an Ediacaran was washed from its rock and, mired in loose sediment, it struggled through the muck to regain its perch.

  I had come to Mistaken Point hoping to gain some understanding of why the first animals began to roam. I would have assumed the ur-trail-maker was propelled by either food, sex, or imminent danger. I hadn’t accounted for this counterintuitive but perhaps equally primal need: the desire for stability.

  I thought back to my experience of being lost amid the Tuckamore, how intensely I had yearned for the comfort of a building, or even just a trail—something solid and familiar to which I could cling. Huxley had felt that yearning too; I suspect most people have. There is no sure way of knowing what the ancient Ediacarans felt, or if they even could feel. But here, written in stone, was a clue. In the end—or rather, in the beginning—the first animals to summon the strength to venture forth may simply have wanted to go back home.

  * * *

  I. This, despite the fact that the land had been webbed with native footpaths since long before white people arrived.

  CHAPTER 2

  I RETURNED HOME from Newfoundland with a skull full of new questions. For reasons I couldn’t quite grasp, the fossil trails at Mistaken Point continued to vex me. The more I thought about those inscrutable old scribbles, the more they struck me as curiously inert—and not only because their makers had been dead for half a billion years. Trails tend to possess a certain vital suppleness, a formal litheness or grace, which they altogether lacked.

  It was only later, by studying the invisible trails of ants—arguably the world’s greatest trail-makers—that I finally located the glitch: strictly speaking, the Ediacaran trails were not really trails, they were traces. Ant trails gain their magical efficiency from a very simple feedback mechanism: a trail is left behind by one ant and then followed by another, and another, and another, subtly evolving with each subsequent trip. We have no reason to believe that one Ediacaran would have followed in the footsteps (or rather, foot smears) of another. Their trails were a call without a response.

  The words we English speakers use to describe lines of movement—­trails, traces, tracks, ways, roads, paths—have grown entangled over the years. I am as guilty of this conflation as anyone else, in part because the meanings of these words, much like the things they denote, tend to overlap. But to better understand how trails function, it helps to momentarily tease them apart. The connotations of trail and path, for example, differ slightly: a “path” sounds dignified, august, and a bit tame, while a “trail” seems unplanned, unkempt, unruly. The Oxford English Dictionary editors define a trail, rather sniffily, as “a rude path.” As they point out, trails only ever pass through wild regions, never cultivated ones; it would sound awkward to speak of strolling down a “garden trail.” But why?

  When we take a step back, we find that the key difference between a trail and a path is directional: paths extend forward, whereas trails extend backward. (The importance of this distinction becomes paramount when you consider the prospect of lying down in the path of a charging elephant versus lying down in its trail.) Paths are perceived as being more civilized in part because of their resemblance to other urban architectural projects: They are lines projected forward in space by the intellect and constructed with those noble appendages, the hands. By contrast, trails tend to form in reverse, messily, from the passage of dirty feet.

  Over time, the meaning of the two terms converged in North America in the nineteenth century, when Anglos often found themselves traveling almost exclusively on trails left behind by animals and Native Americans. The word acquired its flavor out west; the OED’s earliest citation of “trail”—meaning a footpath, animal trace, or wagon road—dates back to the Lewis and Clark expedition. Colonel Richard Irving Dodge, in 1876’s Plains of the Great West, drew from his tracking experience to give us this helpful definition: trails are a string of “sign” that can be reliably followed. I like this definition, because it gets us away from the erroneous assumption that a trail is synonymous with a strip of bare dirt, but it requires some explanation. “Sign”—a word, like its synonym “spoor,” that is always written in the singular—refers to the marks left behind by an animal in its passing: footprints, droppings, broken branches, tree trunks rubbed bare by antlers. “A trail is made up of ‘sign;’ but ‘sign’ is, by no means, a trail,” Dodge clarified. “Deer make ‘sign,’ but it may be impossible to trail them.” Trails, in this—albeit, somewhat tautological—­formulation, are simply that which can be trailed.

  Something miraculous happens when a trail is trailed. The inert line is transformed into a legible sign system, which allows animals to lead one another, as if telepathically, across long distances. (These signs can be physical, chemical, electronic, or theoretical. The medium, in this case, is not the message.)

  The truly incredible thing about these sign systems is that they require no special intelligence to create or follow. Some of the animal kingdom’s earliest trail followers were likely marine gastropods (the ancient progenitors of snails and slugs), which emerged during the Ordovician period. Modern-day marine gastropods regularly track one another’s slime trails across the seafloor by tasting the trail’s mucousy surface, a process called “contact chemoreception.” The slime of gastropods primarily speeds their travel, but this slick medium has also evolved into a signaling mechanism, much like how the smooth surface of the highway, as opposed to the bumpiness of the shoulder, signals that a driver hasn’t veered off the road. Certain gastropods, like mud snails, only travel forward on slime trails—following a gradient toward the freshest mucus—which allows them to shadow one another on their herd-like migrations. Limpets, on the other hand, secrete trails that they trace in reverse, groping their way back to the nook-like homes they carve into rocks.

  Slime trails function equally well on land, where terrestrial snails and slugs make frequent use of them. Darwin relayed the story of an acquaintance, named Lonsdale, who once placed two Burgundy snails in a “small and ill-provided” garden. The stronger of the two snails climbed over a wall into an adjoining garden where there was more to eat. “Mr. Lonsdale concluded that it had deserted its sickly mate,” Darwin wrote, “but after an absence of twenty-four hours it returned, and apparently communicated the result of its successful exploration, for both then started along the same track and disappeared over the wall.”

  Mr. Lonsdale seems not to have considered the possibility that, if the first snail can leave behind an intelligible trail for its mate to follow, no other form of communication is necessary. The trail provides one of the animal kingdom’s most elegant ways to share information. Each inch is a sign, like a scrawled arrow, reading simply:

  This way . . .

  This way . . .

  This way . . .

 

  The invention of trails provided a powerful new tool of animal communication, a kind of proto-Internet capable of running on a simple binary language—this way and not. No species has exploited this new technology more brilliantly than ants, which routinely use trails to find new sources of food and transport it back to the nest. Scientists now study these tiny, but stunningly efficient, trail systems to learn how to more quickly route bits of information through our own fiber-optic networks.

  For many centuries, it was a mystery how ants were able to organize themselves so deftly. Some believed each ant was possessed of a tiny special intelligence, which afforded it rationality, language, and the ability to learn, as the naturalist Jean Pierre Huber argued in 1810. Put simply, this vi
ew held that ants found their way to food using their wits, and then “spread the word” of their findings throughout the nest. (This highly anthropomorphized notion remains prevalent among folktales and children’s stories, from Aesop’s Fables to T. H. White’s The Once and Future King. In many of these renditions, like White’s, the worker ants are given their marching orders by an all-­powerful “queen.”) Opponents of this theory, following the teachings of Descartes, held that ants were possessed of no intelligence whatsoever—or, in the language of the time, no “soul”—but were mere machines directed by an almighty deity who either manipulated them like marionettes or engineered them like windup toys. The naturalist Jean-Henri Fabre, an unfashionably late proponent of this theory, wrote in 1879, “Can the insect have acquired its skill gradually, from generation to generation, by a long series of casual experiments, of blind gropings? Can such order be born of chaos; such foresight of hazard; such wisdom of stupidity?” Fabre concluded that it could not. “The more I see and the more I observe, the more does this [divine] Intelligence shine behind the mystery of things.”

  On the one side of this debate lie insects blessed with individual wit, on the other, insects cursed with perfect idiocy, but steered by an omniscient hand. It was not until very recently that scientists began to understand that the answer lay somewhere in between: the complex behavior of ants arises not from smart individuals, but from smart systems—a form of wisdom that exists between, as well as inside, living things.

  All animals fall somewhere on the spectrum between internalized and externalized intelligence. At one extreme of this spectrum lies the mountain hermit, thoughts swirling about in his lonely head like moths in a bell jar. At the other end lies the slime mold. As sprawling, single-celled blobs, slime molds are about as stupid as an organism can be: they lack even the most basic rudiments of a nervous system. However, they have nevertheless developed a very effective technique to hunt for food: They extend their tentacle-like pseudopods, grope around, and then retract them whenever they come up empty. As they retract, the pseudopods leave behind a trail—or rather, a kind of anti-trail—of slime indicating where food has not been found. Then, continuing their blind search, they head off in a new, slime-free direction. Using roughly this same trial-and-error method, slime molds can solve surprisingly complex problems. When researchers tasked a slime mold with connecting a series of oat clusters mirroring the location of the major population centers surrounding Tokyo, the slime mold effectively re-created the layout of the city’s railway system. Linger a moment over that fact: A single-celled organism can design a railway system just as adroitly as Japan’s top engineers. Whatever intelligence slime molds have, though, is wholly external. When their enclosure is wiped down evenly with slime—effectively erasing their trails—slime molds will begin to wander aimlessly, as if struck with dementia. They don’t retain any information; the trail does.