Our Cousin The Fishapod

8 minute read
J. Madeleine Nash

The v-shaped bone turned out to be the lower jaw of a fish, but not any fish Neil Shubin had ever seen. The University of Chicago paleontologist had been chipping his way through an ancient rock formation in an icy drizzle near Bird Fjord on Canada’s Ellesmere Island last July when one of his colleagues pointed to a wall of red siltstone and exclaimed, “What’s that?”

That, as Shubin and his colleagues reported last week in a pair of articles in Nature, was part of a creature that grew to at least 9 ft. in length and lived some 375 million years ago, just at the point in evolutionary history when fish were giving rise to the four-legged animals known as tetrapods. And indeed, the creature was a little of each, for along with a fish’s scales, fangs and gills, it had anatomical features usually found only in animals that spend at least some of their time on land. It is, in short, exactly the sort of transitional animal Darwinian theory predicts, with new physical traits gradually emerging to help it thrive in a novel environment. And it has become scientists’ Exhibit A in their long-running debate with creationists and other antievolutionists who have been using the lack of such missing-link organisms to argue that Darwin’s theory is wrong.

It will be hard to explain away the “fishapod,” as Shubin and his team nicknamed their find. Unlike a true fish, it had a broad skull, a flexible neck, and eyes mounted on the top of its head like a crocodile. It also had a big, interlocking rib cage, suggesting that it had lungs and did at least part of its breathing through them, as well as a trunk strong enough to support itself in the shallows or on land. And most startling of all, when technicians dissected its pectoral fins, they found the beginnings of a tetrapod hand, complete with a primitive version of a wrist and five fingerlike bones. “This is not some archaic branch of the animal kingdom,” says Shubin. “This is our branch. You’re looking at your great-great-great-great cousin!”

What really fascinates scientists about the fishapod is that it fits so neatly into one of the most exciting chapters in the history of life–when creatures that swam in seas and rivers gave rise to things that walked, ran and crept on land. The fishapod appears to be a crucial link in the long chain that over time led to amphibians, reptiles, dinosaurs, birds and mammals. Indeed, Tiktaalik roseae, the official name bestowed on the fishapod (in the language of the local Inuit, tiktaalik means “large fish in stream”), falls anatomically between the lobe-finned fish Panderichthys, found in Latvia in the 1920s, and primitive tetrapods like Acanthostega, whose full fossil was recovered in Greenland not quite two decades ago.

Together, these fossils have overturned the old picture of the fish-tetrapod transition, which conjured up the image of creatures like the modern lungfish crawling out of water onto land. That picture certainly didn’t fit Acanthostega, whose short, flimsy legs were ill equipped for terrestrial locomotion. Rather, according to University of Cambridge paleontologist Jennifer Clack, Acanthostega was an aquatic creature that used its limbs and lungs to make a living in water. And that scenario makes sense because it sets up conditions for natural selection–the force that powers evolution–to favor transitional life-forms like the fishapod, with its funny wrist and five digits encased in the webbing of a fin.

On land, observes Shubin’s collaborator Ted Daeschler, chair of vertebrate zoology at Philadelphia’s Academy of Natural Sciences, such an appendage would have been worse than useless. But it would have been more than adequate for propping the animal’s head above the water so that it could survey its surroundings or for anchoring it underwater as it waited to ambush its prey. The advantage of being able to gulp air through lungs as well as gills would likewise have been immediate, given that the fishapod made its home in warm, shallow waters that were frequently rendered inhospitable by decaying vegetation.

The fishapod was among the pioneering organisms to take advantage of an ecological frontier–the marshy floodplains of large rivers–that opened between 410 million and 356 million years ago during the Devonian period, known as the Age of Fishes. Early in the Devonian, the continents were mostly masses of bare rock with just a fringe of plants “no taller than your ankle,” as Daeschler puts it, growing along the wet margins of rivers and streams. By the late Devonian, however, thick vegetation had taken hold in marshes, fens and floodplains, and mosses, ferns and trees had coalesced into the world’s first forests.

Those archaic plants prepared the way for the tetrapods. The plants created new aquatic habitats by stabilizing the banks of rivers and streams. They pumped oxygen into the atmosphere, making the earth habitable for large, air-breathing creatures. And they shed organic debris that formed the basis of a new food chain. Bacteria, fungi and small arthropods (the animal group that includes crustaceans and insects) moved in to feed on the debris; small fish moved in to eat the arthropods; bigger fish moved in to eat the small fish. Among them were the fishapod’s lobe-finned ancestors, which found in the vegetation-clogged shallows abundant food and relative safety from predators.

Because characteristics like limb development are governed by powerful families of genes known as Hox genes, the fishapod’s curious mix of features intrigues developmental biologists as much as it does paleontologists. Recent experiments on mice by University of Geneva geneticist Denis Duboule and his colleagues, for example, show that Hox genes control limb development in two stages. “Even though the same genes are involved,” says Duboule, “separate processes govern the development of arms and legs and the development of hands and feet.”

It all depends on how and when the genes are turned on by a segment of DNA that acts like a switch. Fish have a version of that switch too. For example, Zebrafish (ray-finned fish that split off from the lineage that led to lobe-fins early in the Devonian) have only part of the sequence, whereas coelacanths (lobe-fins closely related to lungfish) have a lot more of it. And the fishapod, presumably, had even more.

None of that comes as a surprise to most biologists. Even the scientists who invoke “intelligent design” to explain life’s diversity concede that transitional creatures have been showing up in the fossil record for quite some time. “The argument that there are no transitional forms,” says Kenneth Miller, a Brown University biologist and staunch defender of evolution, “has been untenable for at least two decades.”

But the lack of missing links is still part of the antievolution rhetoric circulating on the Internet. “Some people will never be convinced,” says paleontologist Michael Novacek, provost of science at the American Museum of Natural History in New York City (see box). “But discoveries like this are valuable because there are people who are still undecided about evolution. This gives us an opportunity to educate them.” And if not with this discovery, then maybe with the next one. “The fishapod,” says Miller, “is one more piece of a rapidly filling jigsaw puzzle. And every couple of years, we put another important piece in place.”

[This article consists of a complex illustration. Please see hardcopy of magazine.] Source: Neil Shubin, University of Chicago 380 million years ago BEFORE TIKTAALIK Lobe-finned fish had forelimbs suitable for moving in water but not on land 375 million years ago TIKTAALIK The forelimbs had the beginnings of fingers and a wrist, wrapped inside a fin 360 million years ago AFTER TIKTAALIK Tetrapod forelimbs have wrists and digits used for crawling on land [This article consists of a complex illustration. Please see hardcopy of magazine.]

A LONG WALK The fishapod represents a crucial step in the progression from sea creatures to land animals to, eventually, humans

Source: The Book of Life by Stephen Jay Gould (Norton; 2001) 600 million of years ago – 0

Proterozoic     • First shellfish and corals Cambrian Ordovician     • First fsh Silurian     • First land plants Devonian     • Tiktaalik     • First tetrapods Carboniferous     • First reptiles     • First mammal-like reptiles Permian Triassic     • First dinosaurs     • First mammals Jurassic     • First birds Cretaceous     • First flowering plants Cenozoic     • First horses     • First monkeys     • First apes     • First modern humans

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