Coming Onto the Land

The evolution of fish into walking land animals was one of the greatest chapters in the history of life. Now a remarkable fossil creature shows that all the real excitement happened underwater.

By Carl Zimmer|Thursday, June 01, 1995
When you consider the years paleontologists often spend in daily and intimate contact with their fossils, it’s not very surprising that they come to regard their long-gone animals as pets. Some work on the show dogs of the fossil world; they brag about how fast their velociraptor ran or how efficiently their saber-toothed tiger could sever a spinal cord. But when you listen to Jenny Clack talk about her pet, a fossil creature named Acanthostega that she has been working on for seven years, she sounds like the owner of a sweet, homely mongrel. It wasn’t very smart, she says. It probably spent a lot of its time sitting at the bottom of lagoons, hidden in the muck, waiting for something to come by it could eat. Clack has helped make a reconstruction of it; its salamander-like body has big glassy eyes sitting on top of a flat, Muppet-style head, its mouth permanently caught in a foolish smile. Acanthostega mixes the anatomy of a newt with the charm of a mutt.

But though it may not have their flair, in terms of evolutionary significance Acanthostega can easily go nose to nose with any of its fossilized companions. Velociraptors and sabertooths are both tetrapods-- that is, they have four limbs, along with fingers and toes, hips and shoulders. We humans are also tetrapods, as are iguanas, ravens, bullfrogs, porcupines, and every vertebrate that has ever walked on dry land. Even whales and snakes are tetrapods, although they shed their legs long ago. Acanthostega was a tetrapod, too, but at 360 million years old, it has a special distinction: aside from creatures suggested by a few older fossil fragments, it is the most primitive tetrapod known. That means that Jenny Clack’s sweet, unprepossessing pet holds answers to the great mystery of how our ancestors changed from fish and hauled their bodies out of the water.

Clack, who works at the University of Cambridge’s Museum of Zoology, discovered the bulk of Acanthostega’s skeleton in 1987 and has been carefully reconstructing it ever since with fellow paleontologist Michael Coates. They are just finishing up their monographs on the creature, and some of the conclusions they’ve drawn from its body are surprising other paleontologists. For a long time it was assumed that our limbs and feet, which work so well for walking on land, evolved for that exact purpose. But Acanthostega has convinced Clack and Coates otherwise; tetrapod anatomy evolved while our ancestors lived exclusively underwater-- and it evolved for life underwater. The first vertebrate that walked onto land didn’t crawl on fish fins; it had evolved well-turned legs millions of years beforehand.

Before she found Acanthostega, Clack had studied early tetrapods for ten years, but she never expected to have the privilege of studying their actual origin. It’s not something you build your career on, she says. You may hear some paleontologists bemoan the rarity of dinosaurs, but compared with the first tetrapods, they’re as common as gravel. For most of this century, in fact, only one primitive specimen was known in any detail: a bulky, dog-size beast from Greenland named Ichthyostega.

At the end of the 1800s, Greenland, like most of the Arctic, was still pretty much terra incognita. The quest for the undiscovered North Pole had drawn a number of explorers, but many of them--and whatever knowledge they had gained--were lost in seas of ice. In 1895 an ambitious Swedish engineer named Salomon Andrée decided that instead of traveling over the ice by sled, he would soar to the pole in a hot-air balloon. Two years later he and two companions first sailed as far north as they could, to the island of Spitsbergen, 600 miles south of the pole. There they filled their balloon with hydrogen and rose into the sky. If the winds had been with them, they would have reached the pole in two days. As it turned out, no human ever saw them touch land again.

During the following two summers expeditions were launched to find the lost explorers. The second journey led to the east coast of Greenland, where geologist A. G. Nathorst found a few bones on the side of a mountain he named Celsius Bjerg, after the eighteenth-century Swedish astronomer. The bones had nothing to do with the balloonists; as would be learned in 1930, Andrée had crashed 300 miles northeast of Spitsbergen. These bones were instead the remains of a 360-million-year-old fish.

Thirty years after they were uncovered, the bones drew a team of Danish and Swedish scientists back to Celsius Bjerg to look for more fossils and to map the stratigraphy of the mountains. The mountains were formed near the end of the Devonian Period, which stretched from 408 million to 360 million years ago. The Devonian was sometimes called the age of fishes because these were the dominant animals of the day; the oldest known fossils of land vertebrates--creatures called anthracosaurs--were only 300 million years old. In 1931 a 23-year-old paleontologist, Gunnar Säve-Söderberg, took over the now-annual expeditions to Celsius Bjerg, and in that year he found a 360-million-year-old skull that he realized was not from a fish. A telltale pattern of bones in the back of the skull identified it as a tetrapod; Säve-Söderberg named the animal Ichthyostega, meaning fish plate, because the roof of its skull was shaped like that of a fish.

In the summers that followed, Säve-Söderberg’s assistants found more Ichthyostega skulls as he busied himself with his stratigraphic studies. He put off a thorough study of the skulls, intending to get to it later--he was, after all, only in his mid-twenties. But in 1937 he suddenly fell ill and became bedridden. World War II then brought the expeditions temporarily to a halt. In 1948 Säve-Söderberg died, at age 38, only a few months before paleontologists returned to his site and discovered the rest of Ichthyostega’s skeleton.

The task of examining the remains of the hundreds of fossil bones fell to Erik Jarvik, one of Säve-Söderberg’s assistants and a paleontologist at the Swedish Museum of Natural History. His first reports hinted that Ichthyostega was an amphibian that still held on to signs of its fishy heritage. It was obviously a tetrapod, with limbs and digits to walk on, and sturdy shoulders to support them. No longer able to depend on a cushion of water, it had developed a sturdy rib cage to hold up its internal organs. But Ichthyostega also had a broad, flat, fishlike head and a small fin on the top of its tail.

Ichthyostega most resembled a group of lobe-finned fish that first appeared 410 million years ago and that includes today’s lungfish. Their fins are unlike those of other fish: rather than being thin and fan- shaped, they are thick and rounded and mittenlike, and some of the bones inside seem crude predecessors of the bones in tetrapod limbs. Like all fish, a lobe-finned fish can breathe underwater using internal gills, which consist of struts of bones behind its mouth on which hang delicate filaments filled with fine blood vessels. As flaps covering the gills pump water past them, these filaments dump accumulated carbon dioxide from the blood and take in fresh oxygen. But unlike other fish, lobe-finned fish aren’t completely dependent on gills. They also have lungs with which they can breathe air; they use them in oxygen-poor swamps or when they are stranded on dry land.

By the time Ichthyostega was found, paleontologists had already begun to suspect that tetrapods were descended from lobe-finned fish. Jarvik’s preliminary description of the animal seemed to seal the case. And for decades that was essentially where tetrapod matters were left. Jarvik made more expeditions to Greenland and became one of the world’s authorities on ancient fish. He continued examining Ichthyostega, writing brief descriptions and detailed reports of parts of the body. His research continued unabated until last year when, at age 88, he suffered a crippling stroke. But by then he had completed the bulk of a 250-page tome about Ichthyostega. It is scheduled to be published by the end of this year, nearly 100 years after the Swedish balloonists disappeared and lured paleontologists to Ichthyostega’s home.

Paleontologists obey a code of behavior: until the discoverer of a fossil has published his full description of it, other researchers who would like to write about the fossil must wait. That proscription made life hard for graduate students fascinated by early tetrapods, even students in the late 1970s, like Jenny Clack. It was a delicate situation, says Clack. Jarvik had come out with a few publications, including one big one that described the tail and very little else. It’s not that he wouldn’t let anyone else look at the fossil, but they weren’t at liberty to publish anything on it, because one respects other people’s territories. And that was regarded as his.

Clack ended up looking at other animals a little farther along the tetrapod line. The fossil record is silent about the 20 million years immediately following Ichthyostega. But it shows that 340 million years ago a number of more developed tetrapods appeared. These animals pose a paleontological puzzle in that they bear no clear relation to later creatures. All we know is that after another 10 million years or so, tetrapods branched off into two groups. One began coating its eggs with a hard shell; these became the reptiles, which eventually gave rise to dinosaurs, birds, and mammals, like us. The other group, which continued to lay its unshelled eggs in water, is the amphibians. Clack immersed herself in the enigma of how these early tetrapod branches took shape, studying an anthracosaur fossil for hints about the origin of reptiles, and later an early amphibian for clues to the ancestry of frogs.

Clack could have spent the rest of her life with these creatures, but she was nagged by the really big question of her field: How had the ancestors of these animals emerged from the water? What allowed them to leave their cushioned aquatic cradle and enter a world of air and gravity, a world that until then was the domain solely of insects and other invertebrates?

Clack’s husband thoroughly understood her fixation. Rob Clack, though a computer programmer by trade, is at heart a fossil hunter, a man happy to spend his free time browsing through quarries for the relics of past lives. Rob and Jenny also share a passion for motorcycles, and so they often spent their vacations in Scotland, riding from Devonian deposit to Devonian deposit and collecting fossils. They half hoped to find an early tetrapod; they found only fish.

By the mid-1980s, they could both hear Greenland calling, loudly. There are Devonian rocks around the world, not only in Scotland but in Pennsylvania, New York, Australia, and Russia. They no doubt have many fossils locked inside them, but most are hidden under forests and fields. The naked mountains of Greenland, where eroded rocks simply pile into undisturbed heaps, were still the best place to find primitive tetrapods. If Jenny could get there, she might be able to find some she could call her own. But who would pay her to go back to Celsius Bjerg, which had been carefully picked over for fossils, or to wander aimlessly over the tundra?

Nevertheless, Rob kept egging her on. He kept saying to me, ‘Come on then, when are we going to Greenland? Let’s go to Greenland!’

I was very keen to do this, Rob admits. To her it’s material to work on, but to me, since I’m outside the academic world, it’s something to fantasize about.

Clack decided that she could at least go visit Peter Friend, a Cambridge geologist who worked across the street from the museum. In the late sixties and early seventies Friend had continued the stratigraphic work around Celsius Bjerg. Clack wanted to find out if he knew of any sites that might be hiding tetrapods. Friend wasn’t sure, but he handed her a stack of reports that she took back to the museum.

As she paged through them, she came across a brief mention that in 1970, on a mountain called Stensio Bjerg, a graduate student of Friend’s named John Nicholson had found tetrapod skulls. This wasn’t what Clack had expected. Suggestions of tetrapods, maybe, but tetrapods themselves? She went back across the street and asked Friend as calmly as she could if there were any specimens left and, if so, could she look at them, please? Friend took her down to the basement and pulled out a drawer where the rocks had sat for 15 years. They held three skulls, one of which had a pair of prongs at the back of the head. The sight of them gave Clack a start. Ichthyostega doesn’t have prongs.

Nicholson had found a different primitive tetrapod, one that had been a baffling footnote in the history of paleontology. In 1933 Säve- Söderberg and Jarvik discovered the roofs of two pronged skulls, each about eight inches long. I still remember Säve-Söderberg sitting for hours in our tent, twisting and turning this specimen with a puzzled face, Jarvik wrote in his monograph. But neither paleontologist ever found another bone of the creature. In 1952 Jarvik named it Acanthostega (spine plate) and set the bones aside. No other Acanthostega fossil turned up until Nicholson, a geologist without the training to recognize the significance of what he had found, came along. Clack looked over his field notes.

Tetrapods, he had written, blithely unaware of what his words might do to the nerves of a paleontologist, are common here.

Devonian tetrapods aren’t common anywhere, says Clack. It was like writing Unicorns in abundance or Wall-to-wall Archaeopteryx.

Clack’s serendipity held. She got wind that a Danish team was going back to the region with the Greenland Geological Survey. She secured permission for her, Rob, and a graduate student named Per Ahlberg to go along. Nicholson’s neglected fossils got her the funding she needed. In the summer of 1987 a helicopter dropped the fossil hunters at the foot of Stensio Bjerg.

Reaching Nicholson’s site took them a few days of hiking and backtracking. But, says Rob, once we found it, it was absolutely loaded. Devonian rocks had been eroding from a steep outcrop, dropping to the ground below and cracking open to reveal their treasure of Acanthostega fossils. Tetrapods were indeed common. We were just picking up rocks and filling our rucksacks, says Rob. There might be nothing on one side of a rock, and when you turn it over, it’s got a skull on it. It was absolutely fabulous.

Before they left, they explored other mountains nearby and found new Ichthyostega fossils, including a hind foot. All told they collected a thousand pounds of rock, which traveled by helicopter, cargo plane, and truck back to Cambridge. Clack knew she needed help. Ahlberg was leaving to take a job at Oxford University, and she wouldn’t be able to handle the heap alone. She turned to Michael Coates for help.

Like Clack, Coates had gotten his doctorate at the University of Newcastle upon Tyne--his specialty was fossil fishes. But afterward Coates had discovered just how unkind a career in paleontology can be. Shall we say, I spent a few years at home in the human development industry, he says. Clack knew his rare expertise with fish bones would make him particularly skilled at recognizing just how fishlike Acanthostega was, and she asked him if he’d like to help her. The question was almost absurd. Devonian tetrapods are as rare as gold, Coates says, and you know that anything you do with them is going to shake the whole tetrapod tree because you’re working down there at its base.

What happened down there, where the tetrapod tree first sprouted from its piscine roots, has long been the subject of speculation. With only Ichthyostega--and only sketchy details of Ichthyostega at that-- paleontologists couldn’t reconstruct the story with any certainty. But in the 1940s Alfred Romer of Harvard offered a compelling scenario. Many Devonian rocks are bright red, and in 1916 an American geologist named Joseph Barrell persuaded his colleagues that this coloration meant the rocks had been baked dry, like bricks, in arid conditions. Thus the ancestors of tetrapods, Romer argued, were relatives of lungfish living in freshwater pools that suffered seasonal droughts. Like lungfish, they could breathe air if necessary--when oxygen became scarce in the water, or the ponds dried up.

Today’s lungfish, when their ponds disappear, dig a burrow with their teeth and hide in it until the next rain. But some of their ancient relatives, Romer suggested, instead struggled with their fins over the harsh land to another pond. Fish with weak fins died along the way; fish with strong ones lived to reproduce. Gradually fins turned into limbs, which are much better for overland travel. At the same time, many other parts of the body--such as eyes, ears, and skin--changed to better cope with the new environment. Ichthyostega was a product of this evolution, and it was followed by amphibians increasingly comfortable on dry land.

Romer’s vision became the textbook standard (in part because he wrote a lot of the textbooks). But the world he imagined didn’t actually exist. He had relied on Barrell, and Barrell had been only half right; red rocks do sometimes form in droughts, but they form in moist tropical soils as well. In recent decades plant fossils have shown researchers that Greenland was actually fringed with wetlands resembling coastal mangrove swamps. Likewise, Romer’s scenario was well suited to the old view of Devonian land as a brutal environment. But in reality the Devonian world was most likely a kinder place, and tetrapods probably evolved without the threat of immediate death hanging overhead. Perhaps by laying their eggs in isolated muddy ponds, early tetrapods had unheard-of success in the survival of their young. Maybe the land, brimming with newly evolved invertebrates, was a bountiful frontier of food. All the same, even scenarios like these were fundamentally similar to Romer’s old tale: they presented the emergence of tetrapod limbs as a response to the challenge of walking on land.

Collecting the Acanthostega fossils may have been fabulous, but extricating them from the rock was a nightmare. The rock was as tough as concrete and impervious to acid. To remove it, Sarah Finney, a preparator at the museum, had to use diamond-wire saws, dental needles, and pen-size jackhammers; some of the bones needed months of cleaning. But gradually Clack and Coates were able to take stock of what they had: one complete skeleton, two somewhat less complete ones, and remains of a few others.

The first clue that they had found a truly remarkable creature came in 1989 when Finney brought out Acanthostega’s arm. In all other tetrapods, from us back to Ichthyostega, the two bones of the forearm--the radius and ulna--are about the same size. In Acanthostega, however, the radius was about a third longer than the ulna--a kind of proportion found in lobe-finned fish and a hint that Acanthostega was the most primitive tetrapod yet found. Clack and Coates also recognized that with the radius bearing most of the animal’s weight, the arm was poorly designed for support. Worse, the bone was spatula-shaped: thick at the elbow but flat at the wrist. This is like using a table knife as a pillar, with the blade on the ground, says Coates. If tetrapods had evolved their limbs for walking on land, how could Acanthostega have had such weak arms?

A bigger surprise, however, was waiting at the end of Acanthostega’s arm. Coates discovered that in spite of its feeble wrists, it bore fully evolved tetrapod fingers--eight of them. (Even if Clack and Coates had thrown the rest of the bones into a quarry pit, these eight fingers ensured that Acanthostega would remain a revolutionary animal for paleontology. Five digits had always been thought to be the primordial standard for tetrapods. Yet Acanthostega showed that the five-finger rule that these researchers assumed was absolute was actually very loose.) These eight fingers were sophisticated and multijointed, yet since they were attached to an insubstantial wrist, they were virtually useless for helping Acanthostega walk on land.

How do you explain a land animal’s body in an animal that couldn’t survive on land? One possibility is that the animal had once come onto the land but, like some amphibians, had subsequently returned to a life underwater, where its skeleton had gradually weakened. That scenario seemed unlikely, though, when Clack and Coates found Acanthostega’s gills. Tetrapods simply aren’t supposed to have a fish’s gills. Amphibians like salamanders retain some of the struts of bone in their neck, but these now anchor the tongue muscles. All the amphibians that returned permanently to the water developed external gills, feathery tissue stuck to these struts and extending out from the body. But Acanthostega had a full battery of gill struts in its neck. It even had a sheet of bone along its shoulders that supported the rear wall of an internal gill chamber. The strong implication was that the animal still possessed an internal gill system. In other words, Acanthostega breathed like a fish.

It heard like a fish, too--or at least like a lobe-finned fish. Our aquatic ancestors had extremely crude ears, consisting of a gill support bone that had changed into a plug in the skull. It wasn’t invented for hearing, Clack explains. It appears to be a supportive structure, connecting different bits of the skull. But when underwater sounds hit that bony plug in their skulls, it also turned out to be able to gently vibrate, and nerve endings could detect the vibration. As their tetrapod descendants later explored the land, the skull plug became the stapes (the stirrup) of the middle ear, and other gill supports turned into the other bones that help the stapes amplify airborne sound. Acanthostega, however, had none of these intricate bones. It had only the plug.

Coates also discovered that on land, Acanthostega’s ribs would probably have been too thin and small to hold up its guts; in addition, its spine was loose and soft. It did, however, have rear legs and toes. It even had hips, which its fish ancestors didn’t have. If you found them on their own, Coates points out, you’d say, ‘Gosh, what a lovely set of tetrapod hips!’ Tetrapod hips, as a rule, are held firmly to the spine by ligaments and a group of fused ribs called the sacrum. But, Coates says, you’d be hard-pressed to point to any area on Acanthostega’s hips and say, ‘Insert tab A into socket B,’ because it just ain’t there. With hips only loosely attached to its spine, Acanthostega would probably have flopped about helplessly on dry land.

The more Clack and Coates uncovered of Acanthostega, the more they became convinced that not only was this an animal that lived underwater, but this was an animal whose ancestors had never left the water. By the time they got to the tail, the case was decided. Ichthyostega had a small fin on the top of its tail that was essentially a relic of its fishy pedigree. Acanthostega, on the other hand, had a powerful, flexible tail with large fins running along the top and bottom of it. Each vertebra in the tail tapered into a long upper and lower crest, each of which connected to a rod-shaped bone; the crests and rods could bend like a finger. Connecting to the rod, inside each fin, were rays made of dermal bone, the material that forms the scales of fish. Together the crest, the rod, and the ray (and the muscles attached to them all) allowed Acanthostega to use its tail to create underwater waves that could propel it forward or brake its momentum.

Useful as such a tail might have been in water, it was worthless on land. It may even have been dangerous--the bottom fin would have scraped along the ground and become prone to infection. Hence it’s not surprising that amphibians lost their fins once they began spending time on land. And once they lost the complex architecture of their tail, they never reclaimed it. All amphibians that have returned to the water have developed tails consisting of short vertebral crests and a boneless fin--a pale imitation of the tail Acanthostega sported.

The most elegant explanation for such a tail was the same as that for Acanthostega’s gills, ears, hips, and limbs: its ancestors had never left the water. There’s all sorts of jiggery-pokery you have to do to justify saying Acanthostega was secondarily aquatic, says Coates.

If Acanthostega’s ancestors lived underwater, it follows that fish must have evolved into tetrapods for life underwater. This idea has occurred to different scientists at different times. It occurred to evolutionary biologist James Edwards in the 1970s. He was studying the locomotion of salamanders to understand how early amphibians walked. I went to an aquarium, and I saw these strange creatures walking on the bottom; their pectoral fins really had the appearance of tetrapod limbs, says Edwards. The creatures were antennariid anglerfish, best known for the lure they dangle to attract prey. Edwards contacted an expert on anglerfish who confirmed that these particular fish did indeed walk on their fins in the wild.

Edwards was so charmed by the antennariids that he spent the next few years at Michigan State University studying their locomotion. He showed that the fins had evolved into appendages that looked and functioned very much like legs, even bearing toelike tips. He showed that the fish have different tetrapod gaits, like walking, trotting, and galloping (although because they run underwater, they have the world’s slowest gallop, needing about ten seconds to travel an inch). By fashioning legs and toes out of their fins, these anglerfish were able to move slowly over coral reefs, gaining purchase on the rough surfaces as they attracted prey with their lure.

And these weren’t the only fish to turn fins into limbs, Edwards discovered. The sargassum frogfish, which is a close relative of the antennariid anglerfish and lurks in the dense seaweed forests of the Sargasso Sea, grabs seaweed stalks and swings along like a trapeze artist rather than trying to swim through the thicket. A tetrapod could have evolved limbs to move around underwater in these ways, says Edwards. When Clack came to similar conclusions from her fossils, she eagerly got in touch with him. That was very nice, says Edwards. We had a mutual admiration society meeting, talking about how her stuff supported my suggestions.

With models like the antennariids and frogfish, one can craft a new scenario for how Acanthostega came to be. Coastal lagoons were a new ecosystem in the Devonian, full of dangers and opportunity. The plants that grew in them created a rich stew of organic matter that a dense ecological web of animals could enjoy. But as in today’s wetlands, bacteria used so much oxygen that sometimes life became hard for gilled fish. Some of them developed lungs and so could breathe air when their gills couldn’t function.

And some of these fish with lungs developed shoulders, hips, limbs, and digits. They couldn’t support themselves on land, but they could grasp the rotting branches in the water and climb past them, rather than simply trying to wriggle through. They could walk on the bottom of the wetlands, their guts supported by the water. They could paddle their oarlike feet.

With limbs they also became expert ambushers. To remain motionless, a fish generally has to keep its fins in constant motion, kicking up easily detected waves. (It’s as if someone were waiting to ambush you as you’re walking on one side of a fence, and they’re sitting on the other side on a Harley-Davidson, revving the engine, says Coates.) But the first tetrapods could grab on to roots or rocks as they waited for prey to swim by. When it did, they tucked their limbs to their sides and used their powerful fishlike tail to dart out in pursuit.

The Romeresque blend of analogy, geology, paleontology, and ecology in this scenario has intrigued other experts in the field-- particularly because it turns some traditional ideas upside down. Yet some are wary of drawing so many conclusions from one animal. As Keith Thomson, a student of Romer’s and now president of the Academy of Natural Sciences in Philadelphia, points out, It’s hard to interpret what these animals are doing on a day-to-day basis.

Still, the main point is that when Romer put together his scenario, evolving a tetrapod body and coming onto the land were one and the same piece of history. But Clack and Coates (as well as a number of other paleontologists) think they should be pulled apart. For whatever reasons, tetrapods first evolved their limbs underwater, and only later did they begin to walk on land, and for completely different reasons. However tetrapods became truly terrestrial animals, says Clack, it certainly didn’t happen all at once.

Determining just what finally pushed or pulled tetrapods onto the land, and when, will take some time. But some new fossils will help get at the answers. Since Clack went to Greenland, paleontologists have found fragments from five more tetrapods, all of which were roughly contemporaries of Acanthostega and some of which were more advanced and thus closer to a terrestrial life.

Just last year, for example, Thomson and Ted Daeschler, also of the Philadelphia academy, reported the discovery of a shoulder bone from a tetrapod in Pennsylvania that’s about 5 million years older than Ichthyostega and Acanthostega. Significantly, a broad scoop in the bone showed that it had developed powerful, massive front leg muscles. Without the rest of the skeleton, which Daeschler is now frantically looking for, it’s hard to say whether the animal was already up and running around on land.

Ichthyostega has pretty impressive shoulders of its own, but Coates, who has begun looking at the hind limb that Clack found in Greenland, doubts that it could walk well. You have a massive pair of shoulders and a pair of hind limbs that are like flippers, like a seal. It shows an intermediate existence, one that’s primarily aquatic but that can enable the animal to cope on land. It’s possible that primitive tetrapods came ashore for the same reasons seals do--to mate or reproduce, and occasionally to escape attackers. And just this year a tantalizing clue surfaced that another contemporary relative of Acanthostega had become a true land walker. Researchers reported over 150 footprints that are preserved in southern Ireland. Judging by the shape and spacing of the prints, they seem to have been made by a three-foot-long creature that used all four feet on dry land.

Having finished their study, Clack and Coates are each preparing to embark on new research, but in both cases Acanthostega will remain their muse. Coates, who is now at University College, London, is returning to his first love, fossil fish. Insights gained from Acanthostega’s eight fingers into how tetrapod digits form may help him deduce how different kinds of fins develop as well. These discoveries may help biologists find the genetic commands underlying the development of all vertebrates. Clack meanwhile is going forward, to the puzzling tetrapods that came after Acanthostega and before the reptile-amphibian split. The complete skeleton of the primitive Acanthostega may finally give her the anatomic clues she needs to clean up the story of what happened next.

Most important, Clack and Coates also take with them a deeper respect for the waywardness of evolution, the habit it has of avoiding what looks to us like the simple path. Open an evolution textbook and you’re likely to find phrases like The Conquest of Land, as if it were manifest destiny that our ancestors came ashore and evolved our anatomy. Just because we use our limbs this way now, we can’t assume that that’s how they were used in the first place, says Coates. There’s a kind of horrible foresight to that kind of thinking: ‘Better grow myself a limb because my children are going to need it.’ That’s why we suggest that limbs evolved in the water for use in the water, and then they were hanging around on land, and they were useful there too.

This principle of evolution is sometimes called preadaptation. There’s no foresight involved, though--simply the lucky coincidence that a feature that evolved to do one thing may turn out later to do another thing even better. Bone, for example, probably began as a place where animals could store extra phosphorus; only later did it support their bodies. Acanthostega, loping around underwater with a body prepared from head to foot for life on land, may be one of the strongest demonstrations that we humans owe our existence to preadaptation’s unpredictable nature.

One gets the impression when reading popular accounts that there was some kind of imperative, as if tetrapods felt they had to do it, to embark on the Long March Toward Man, says Clack. It was much more accidental than that.
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