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The First Organisms to Blaze Trails

Half a billion years ago, a creature slithered across the seafloor for the first time. It may have launched an evolutionary arms race.

By Robert Moor
Oct 6, 2016 5:00 AMNov 12, 2019 5:54 AM
Paleontologists gather around fossil traces left by some of the first creatures to move themselves across Earth’s surface. | Ruth Schowalter


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It is impossible to fully appreciate the value of a trail until you have been forced to walk through the wilderness without one. There is a practical reason why, for more than a thousand years, after the fall of Rome and before the rise of Romanticism, little was more abhorrent to the European mind than the prospect of a “pathless” or “tangled” wilderness. Pathless wildernesses still exist in the modern world, and at least some have retained their power to elicit dread. I have visited one such place. It lay on the northern rim of a glacial fjord called Western Brook Pond, on the island of Newfoundland, in Canada’s easternmost province. If you want to be taught (however harshly) the blessing of a well-marked trail, go there.

To cross the fjord’s stygian waters, I had to hire a ferryboat. On the far side of the fjord, the captain dropped me and four other hikers off at the base of a long ravine, where a series of animal trails led through a dense fern jungle and up a granite cliff face bisected by a waterfall. At the top of the ravine, I found a vast green tableland. The trail I had been following vanished altogether. Soaked in sweat from the hike up, I took a moment to rest, my feet dangling over the cliff’s edge. At the ragged western edge of the tableland, it abruptly dropped hundreds of feet to the fjord’s indigo water.

But this hike was a mere diversion, a side trip. My ultimate destination was a yet more baffling and inaccessible wilderness: the distant past. I was making my way to a rocky outcropping on the island’s southeast corner, where I hoped to find the oldest trails on Earth. These fossil trails, which are roughly 565 million years old, date back to the dimmest dawn of animal life. Now fossilized and faint, each one is roughly a centimeter wide, like a fingertip’s errant brush across the surface of a drying clay pot. I had read all about them, but I wanted to touch them, to trace their runnels like a blind man.

I hoped that encountering them up close would resolve a question I’ve long harbored: Why do we, as animals, uproot ourselves rather than maintaining the stately fixity of trees? Why do we venture into places where we were not born and do not belong? Why do we press forward into the unknown?

Cardinal Crawlers

The world’s oldest trails were discovered one afternoon in 2008 by Alex Liu, now a researcher at the University of Bristol. He and his research assistant were scouting for new fossil sites out on a rocky promontory called Mistaken Point, where a series of well-known fossil beds overlook the North Atlantic. Bordering one surface, Liu noticed, was a small shelf of mudstone that bore a red patina. The red was rust — an oxidized form of iron pyrite, which commonly appears on local Precambrian fossil beds. They scrambled down the bluff to inspect it. There, Liu spotted what many other paleontologists before him had somehow missed: a series of sinuous traces thought to be left behind by organisms of the Ediacaran biota, the planet’s earliest known forms of animal life.

The ancient Ediacarans, which likely went extinct around 541 million years ago, were exceedingly odd creatures. Soft-bodied and largely immobile, mouthless and anusless, some were shaped like discs, others like quilted mattresses, others like fronds. One unfortunate type is often described as looking like a bag of mud.

We can envision them only dimly. Paleontologists don’t know what color they were, how long they lived, what they ate or how they reproduced. We do not know why they began to crawl — perhaps they were hunting for food, fleeing a mysterious predator or doing something else entirely.

Despite all these uncertainties, what Liu’s trails undoubtedly suggest is that 565 million years ago, a living thing did something virtually unprecedented on this planet — it shivered, swelled, reached forth, scrunched up and, in doing so, began to move across the seafloor, leaving a trail behind it.

A Path Less Traveled As a boy, Liu loved dinosaurs, particularly those in Jurassic Park. The romance of those craning beasts, which he never fully outgrew, coupled with his love of fieldwork and knack for geology, drew him to fossil hunting. When he was pursuing his master’s degree at Oxford, he had planned to study ancient mammals, but he found the field crowded; his thesis project was spent studying the teeth of Eocene-era elephants in Egypt. For his Ph.D. work, he turned to the much older and largely unstudied Ediacaran biota. “If I had taken on a mammal project, then I’d have been trying to answer questions that people have looked at for hundreds of years,” he said. “Whereas I knew that Ediacaran stuff was new, uncertain. And that was more enticing, really, because the questions are bigger.”

Of all the manifold questions surrounding these elusive, soft-bodied organisms, the biggest might concern the origins of animal movement. Some paleontologists theorize that the first Ediacaran trail-maker may have set off a series of morphological changes. This led animal life, in fits and starts, from a serene garden of swaying anemone-like creatures to today’s violent, skeletonized kingdom of sprinters, jumpers, fliers, swimmers, diggers and walkers. It is rare in science to run across a big new question, and harder still to answer it, but Liu seemed to have this one by the scruff of its neck.

For a respectable scientist, wading into the murky world of the Ediacarans is a treacherous endeavor. Information about that distant era is extremely limited, and even the most basic assumptions often prove unreliable. For instance, we still do not know for certain which kingdom of life the Ediacarans belonged to. At various times, it has been proposed that they could have been plants, fungi, colonies of single-celled organisms or, according to the trace fossil expert Adolf Seilacher, a “lost kingdom” called Vendobionta. While most Ediacaran researchers tentatively agree that they were animals, recently, some have begun arguing that lumping all the known Ediacaran species into one kingdom or another may be too reductive, and each fossil must instead be re-assessed one by one.

Alien Earth

As I sat next to him at dinner one night in the Newfoundland town of Trepassey, it seemed odd to me that Liu, a soft-spoken and exceptionally careful researcher, was drawn to such a field. Liu told me he first became interested in Ediacarans during a class in his second year at Oxford with a professor named Martin Brasier, who spoke inspiringly about the mysteries of Precambrian fossils. Brasier — who died in a car accident in 2014, at the age of 67 — was a Shiva-like figure among Ediacaran paleontologists, slashing down flimsy theories and widening the domain of that which cannot be definitively stated.

In his 2009 book, Darwin’s Lost World, Brasier briskly disassembled the principle of uniformity, which broadly says that, natural laws being uniform, fossils can be best understood by studying living animals. Uniformitarianism has proved a powerful tool in many fields, Brasier admitted, but it ignores an organism’s profound interdependence with its environment. The theory breaks down in the Precambrian era, when the oceanic ecosystem was radically different.

“The world before the Cambrian was, arguably, more like a distant planet,” Brasier wrote.

To us land dwellers, even the present-day deep sea is foreign, a crushing black space haunted by spectral oddities: glass squids, carnivorous jellyfish, a fever dream of fluorescence. But in the time when Ediacarans thrived, the oceans were stranger still.

The first Ediacaran to begin crawling around would have discovered a world devoid of predatory animals, with a seafloor covered either in thick bacterial mats or toxic sediment and, possibly, a climate thawing from a worldwide glaciation event known as “Snowball Earth.” If that pioneering Ediacaran could see, it would have discovered an underwater desert patchily carpeted with gelatin. Here and there it may have spotted other, non-mobile Ediacarans, which resembled fleshy leaves, many-tendriled sea anemones or low, round blobs: a whole world populated by brainless, jelly-quivering do-nothings.

The mystery Liu was trying to unravel — regarding the origins of animal movement — is central to solving the larger mystery of how that alien planet transformed into the natural world we all know.

Soft-bodied creatures are rare in the fossil record. And that makes Mistaken Point a key site for paleontologist Alex Liu, who’s trying to understand the origins of animal movement. | Alex Liu

Muscular locomotion could have allowed animals to graze on the beefsteak-like bacterial mats and to attack other stationary organisms. The invention of violence might then have kicked off a biological arms race, prompting organisms to evolve hard shells and sharp teeth, the shields and swords that characterize the Cambrian fossil record.

This hardening of animal bodies eventually led to the rise of trilobites and tyrannosaurs and Eocene-era Egyptian elephants — and us.

Before the discovery of Ediacaran fossils, and even for a while afterward, many prominent scientists argued that complex life began at the dawn of the Cambrian Period. Looked at from a certain angle, the fossil record seemed to support this theory. Around 530 million years ago, like a symphony warming up, the fossil record began teeming with a cacophony of different fossil types. Further back than that was nothing: silence. Some scientists, like Roderick Murchison, a geologist and devout Christian, believed that this lack of evidence was geologic proof of a biblical genesis. (“And God said, ‘Let the water teem with living creatures ...’ ”)

Paleontologists gather around fossil traces left by some of the first creatures to move themselves across Earth’s surface. | Ruth Schowalter

Charles Darwin cautioned against this interpretation, writing in On the Origin of Species that, “We should not forget that only a small portion of the world is known with accuracy.” He saw the entire geologic record as a history book stretching across multiple volumes. “Of this history we possess the last volume alone, relating only to two or three countries,” he wrote. “Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines.”

The truth, it now seems clear, is that Precambrian animals had existed in great numbers but, being soft-bodied, had not lent themselves to fossilization. They crop up exceedingly rarely, in places like Mistaken Point, where the geologic conditions were just right.

Conquerors and Demons

Liu’s plan was to begin our tour at a prominent fossil site called Pigeon Cove, and then work our way forward in time, covering about 10 miles on foot and by car. We would visit each of the area’s most impressive fossil beds, culminating at the surface where Liu had discovered the fossil trails.

Windows open to the hard sea wind, we raced across a landscape of stooped trees and yellowing grass to Pigeon Cove, where we got out and hiked down a dirt path to the seaside. There lay a flat slab of rock, the size and texture of three cracked concrete tennis courts, which sloped down into the sea. Its surface was a swirl of gray, chalkboard green and dusty eggplant. Impressed into it were faint but distinct symbols. One looked like a fleshy frond. Another looked like an arrowhead, but in life probably resembled one of those conical corn snacks sold at gas stations, with its narrow end stuck into the ground. A third, which paleontologists call a “pizza disc,” was just a big, bubbly mess.

A few hours later, we made our way over to the area’s most famous fossil bed, the blandly named E Surface, which cantilevers out high over the ocean. Before we stepped out onto the bedding plane, we removed our shoes and put on polyester booties to protect the fossils from erosion.

The Pigeon Cove surface had held about 50 fossils; E Surface held 4,000. They were everywhere, a vast fossilized garden of fronds and blobs and spirals, some bigger than a large hand. Of course, it was not an actual garden; plants would not appear in the fossil record for another 200 million years. For some reason, I was stuck on this point. They looked like plants, I kept saying.

Oxford postdoc Jack Matthews, the youngest member of Liu’s research group, explained that this was because, this far in the past, the lines between the kingdoms grow fuzzy. We, and every organism currently living on Earth, he said, are at the crown of the tree of life. Down at the base of the tree lie the very first single-celled organisms, from which everything else sprang. So the further down the trunk of the evolutionary tree you look, the more organisms resemble one another.

“That’s when you get into the nitty-gritty definitions of what defines, say, an animal and a fungus,” Matthews said. “They’re actually biologically really close, but they just ‘decide’ to stick their cells together slightly differently. And just because one evolved to stick its cells together differently than another, one mainly just grows on dead trees, and the other has conquered the Earth.”

What, then, makes a conqueror? We have sex. We eat life, not sunlight. We contain multiple cells, which, in turn, contain nuclei, but lack rigid walls. And, in almost every case, we grow muscles.

Muscles, I learned, are a crucial component of Liu’s big question. While many kinds of organisms (even single-celled ones) can swim, reach, float, squirm and even roll, only animals have developed muscle fiber, which has allowed us to move in a wider variety of ways and heave around vastly more weight.

Liu’s trails, then, could help unravel the question of when animal life began. Because if something was big and strong enough to create those trails 565 million years ago, it must have had muscles, which means it must have been an animal. In a neat coincidence, the same summer Liu discovered the fossil trails, he also unearthed a brand-new Ediacaran species with noticeable muscle fibers — at 560 million years old, by far the earliest muscles in the fossil record. While he doesn’t believe it was responsible for making the trails, it does provide evidence that musculature was developed earlier than anyone had previously thought.

The new species was a ghastly looking thing, a webbed, cupped hand reaching up from a slender stalk, as if waiting to trap a passing foot. Liu named it Haootia quadriformis, drawing from the language of the island’s indigenous inhabitants, the Beothuk. Haoot means, simply, “demon.”

Homeward Bound

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.

These trace fossils hold evidence of Earth’s earliest known muscles, which ultimately led to an evolutionary explosion. | Alex Liu

We worked our way along the shelf, inspecting yet more trails. Some were larger than others, but none was 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 some 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 that looked like repeating parentheses: (((((((

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, 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 trail-maker was propelled either by food, sex or imminent danger. I hadn’t accounted for this counterintuitive but perhaps equally primal need: the desire for stability.

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.

From On Trails: An Exploration by Robert Moor. Copyright © 2016 by Robert Moor. Reprinted by permission of Simon & Schuster, Inc.

[This article originally appeared in print as "Trailblazers."]

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