Getting a Leg up on Land

GETTING A LEG UP in the almost four billion years since life on earth oozed into existence, evolution has generated some marvelous metamorphoses. One of the most spectacular is surely that which produced terrestrial creatures ON bearing limbs, fingers and toes from water-bound fish with fins. Today this group, the tetrapods, encompasses everything from birds and their dinosaur ancestors to lizards, snakes, turtles, frogs and mammals, in- cluding us. Some of these animals have modified or lost their limbs, but their common ancestor had them—two in front and two in back, where LAND fins once flicked instead. Recent fossil discoveries cast The replacement of fins with limbs was a crucial step in this transformation, but it was by no means the only one. As tetrapods ventured onto light on the evolution of shore, they encountered challenges that no vertebrate had ever faced be- four-limbed animals from fish fore—it was not just a matter of developing legs and walking away. Land is a radically different medium from water, and to conquer it, tetrapods BY JENNIFER A. CLACK had to evolve novel ways to breathe, hear, and contend with gravity—the list goes on. Once this extreme makeover reached completion, however, the land was theirs to exploit. Until about 15 years ago, paleontologists understood very little about the sequence of events that made up the transition from fish to tetrapod. We knew that tetrapods had evolved from fish with fleshy fins akin to today’s lungfish and coelacanth, a relation first proposed by American paleontologist Edward D. Cope in the late 19th century. But the details of this seminal shift remained hidden from view. Further- more, estimates of when this event transpired varied wildly, ranging from 400 million to 350 million years ago, during the Devonian period. The problem was that the pertinent fossil record was sparse, consisting of essentially a single fish of this type, Eusthenopteron, and a single Devonian tetrapod, Ichthyostega, which was too advanced to elucidate tetrapod roots. With such scant clues to work from, scientists could only speculate about the nature of the transition. Perhaps the best known of the sce- narios produced by this guesswork was that championed by famed vertebrate paleontologist Alfred Sherwood Romer of Harvard University, who proposed in the 1950s that fish like Eusthenopteron, stranded under UP FOR AIR: Acanthostega, an early arid conditions, used their muscular appendages to drag themselves to a tetrapod, surfaces in a swamp in what new body of water. Over time, so the idea went, those fish able to cover is now eastern Greenland, some 360 million years ago. Although this animal more ground—and thus reach ever more distant water sources—were had four legs, they would not have been selected for, eventually leading to the origin of true limbs. In other words, able to support its body on land. Thus, fish came out of the water before they evolved legs. rather than limbs evolving as an Since then, however, many more fossils documenting this transforma- adaptation to life on land, it seems that tion have come to light. These discoveries have expanded almost expo- they may have initially functioned to help the animal lift its head out of nentially our understanding of this critical chapter in the history of life oxygen-poor water to breathe. Only on earth—and turned old notions about early tetrapod evolution, diver- later did they find use ashore. sity, biogeography and paleoecology on their heads. 100 SCIENTIFIC AMERICAN DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. www.sciam.com SCIENTIFIC AMERICAN 101 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. Finding a Foothold tion. In other words, this animal, though constituting the fin of Eusthenopteron or among the first fossil finds to pave clearly a tetrapod, was primarily an a similar creature. Ordinarily, scientists the way for our modern conception of aquatic creature whose immediate fore- might have dismissed this as an aberrant tetrapod origins were those of a creature runners were essentially fish that had specimen. But a mysterious partial skel- called Acanthostega, which lived about never left the water. The discovery forced eton of Tulerpeton, a previously known 360 million years ago in what is now east- scholars to rethink the sequence in which early tetrapod from Russia, had a six- ern Greenland. It was first identified in key changes to the skeleton took place. digit foot. And specimens of Ichthyo- 1952 by Erik Jarvik of the Swedish Mu- Rather than portraying a creature like stega also found on our expedition to seum of Natural History in Stockholm Eusthenopteron crawling onto land and eastern Greenland revealed that it, too, on the basis of two partial skull roofs. then gaining legs and feet, as Romer pos- had a foot with more than five digits. But not until 1987 did my colleagues and tulated, the new fossils indicated that Findings from developmental biology I finally find specimens revealing the tetrapods evolved these features while have helped unravel some of this mystery. postcranial skeleton of Acanthostega. Although in many ways this animal proved to be exactly the kind of anatom- Many of the critical innovations arose while these ical intermediary between fish and full- beasts were still largely aquatic. And the first changes blown tetrapods that experts might have appear to have been related not to locomotion but to imagined, it told a different story from the one predicted. Here was a creature an increased reliance on breathing air. that had legs and feet but that was other- wise ill equipped for a terrestrial exis- they were still aquatic and only later co- We now know that several genes, includ- tence. Acanthostega’s limbs lacked prop- opted them for walking. This, in turn, ing the Hox series and Sonic Hedgehog, er ankles to support the animal’s weight meant that researchers needed to recon- control elements of fin and limb develop- on land, looking more like paddles for sider the ecological circumstances under ment. The same sets of these genes occur swimming. And although it had lungs, which limbs developed, because Acan- in both fish and tetrapods, but they do its ribs were too short to prevent the col- thostega indicated that terrestrial de- different jobs in each. Hoxd 11 and lapse of the chest cavity once out of wa- mands may not have been the driving Hoxd 13, for instance, appear to play a ter. In fact, many of Acanthostega’s fea- force in early tetrapod evolution. more pronounced role in tetrapods, tures were undeniably fishlike. The bones Acanthostega took pride of place as where their domains in the limb bud are of the forearm displayed proportions the missing link between terrestrial ver- enlarged and skewed relative to those in reminiscent of the pectoral fin of Eusthe- tebrates and their aquatic forebears. the fish fin bud. It is in these regions that nopteron. And the rear of the skeleton There was, however, one characteristic of the digits form. How the five-digit foot showed a deep, oar-shaped tail sporting Acanthostega that called to mind neither evolved from the eight-digit one of Acan- long, bony rays that would have provided tetrapod nor fish. Each of its limbs termi- thostega remains to be determined, but the scaffolding for a fin. Moreover, the nated in a foot bearing eight well-formed we do have a plausible explanation for beast still had gills in addition to lungs. digits, rather than the familiar five. This why the five-digit foot became the de- The piscine resemblance suggested was quite curious, because before this fault tetrapod pattern: it may have helped that the limbs of Acanthostega were not discovery anatomists believed that in the make ankle joints that are both stable only adapted for use in water but that transition from fish to tetrapod, the five- enough to bear weight and flexible this was the ancestral tetrapod condi- digit foot derived directly from the bones enough to allow the walking gait that tetrapods eventually invented. Acanthostega also drew attention to Overview/The Origin of Tetrapods a formerly underappreciated part of early ■ The emergence of land-going vertebrates was a cornerstone event in the tetrapod anatomy: the inside of the lower evolution of life on earth. jaw. Fish generally have two rows of teeth ■ For decades, a paltry fossil record obfuscated efforts to trace the steps that along their lower jaw, with a large num- eventually produced these terrestrial tetrapods from their fish ancestors. ber of small teeth on the outer row com- ) ■ Fossils recovered over the past 15 years have filled many of the gaps in the plementing a pair of large fangs and some story and revolutionized what is known about tetrapod evolution, diversity, small teeth on the inner row. Acantho- biogeography and paleoecology. stega showed that early tetrapods pos- ■ These recent finds indicate that tetrapods evolved many of their sessed a different dental plan: a small preceding pages characteristic features while they were still aquatic. They also reveal that number of larger teeth on the outer row ( early members of the group were more specialized and more geographically and a reduction in the size of the teeth and ecologically widespread than previously thought. populating the inner row—changes that probably accompanied a shift from feed- RAÚL MARTÍN 102 SCIENTIFIC AMERICAN DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. TURNING TETRAPOD The evolution of terrestrial tetrapods from aquatic lobe-fi nned the tail fi n disappeared, as did a series of bones that joined fi sh involved a radical transformation of the skeleton. Among the head to the shoulder girdle (skeletons). Meanwhile other changes, the pectoral and pelvic fi ns became limbs the snout elongated and the bones that covered the gills and with feet and toes, the vertebrae became interlocking, and throat were lost (skulls).

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