Walking Towards Land

View Images

Tiktaalik: music to my ears.

Tiktaalik is the lilting name of a newly discovered fossil fish with fingers. It lived 380 million years ago in the northern reaches of Canada, back when the northern reaches of Canada were tropical coastal wetlands not far from the equator. Tiktaalik’s discoverers (Ted Daeschler, Neil Shubin, and Farish Jenkins) detailed their discovery in back-to-back papers in today’s issue of Nature. In some ways Tiktaalik is big news. It may prove to be the single most important fossil for telling us how our ancestors changed from fish to land vertebrates complete with legs, arms, fingers, and toes. But in other ways, Tiktaalik is no news at all–and its non-newsworthiness makes it just as important.

The history of research on this transition is one of the most fascinating episodes in science. I may be biased in that judgment, having spent half of my book At the Water’s Edge exploring that history. But judge for yourself. Before Darwin, the divsion between fish and land vertebrates (tetrapods) was considered by many to be the most profound split in nature. It was proof positive that life could not have evolved. There was one fish, however, that raised some troubling questions–the lungfish of Brazil. It had lungs and other features once thought unique to tetrapods. Richard Owen, Britain’s great Victorian anatomist, dispatched the lungfish to the realms of Fish by its nose. Its nostrils were closed off from its mouth–like the nostrils of other fish and not like any known tetrapod. It was a fish “simply by its nose,” he wrote.

But in 1860, a year after the Origin of Species, an Irish anatomist Robert M’Donnell, examined a lungfish from Africa and reported that its nostrils actually did connect to its mouth. It was a confusing blur of tetrapod and fish features. “I know of no animal more calculated leading to the adoption of the theory of Darwin than Lepidosiren [the lungfish],” M’Donnell wrote.

Over the next few decades scientists came to recognize that lungfish and coelacanths were the two surviving members of a large group of fish that were more like tetrapods than they were like shark or trout or other fish. They came to be known as lobe-fins, for their peculiar appendages. Today there’s no doubt that lobe-finned fish are the closest living relatives to tetrapods. Scientists have made many comparisons of the DNA of tetrapods, ray-finned fish, lobe-finned fish, and other vertebrates, and again and again, the lobe-fins turn up as close kin.

But in the late 1800s, before anyone knew what DNA was, it was much harder to make the case. The most compelling evidence came from the fossil record. One particularly important fossil was of a lobe-fin called Eusthenoperon, which had some limb-like bones in its fins. And then, for about a century, the discoveries came very slowly.

In the 1920s, scientists exploring Greenland found a 360-million year old fossil of a barrel-chested, flat-headed tetrapod that had a fish-like tail, complete with slender rays along its top. They named it Ichthyostega. They also found a few fossils of what looked like a smaller tetrapod, but the bones remained a puzzle until the 1980s. That’s when Cambridge paleontologist Jenny Clack organized a trip of her own back to Greenland and found bones from several individuals, which came to be known as Acanthostega.

I got interested in Clack’s work in the mid-1990s, as she and Michael Coates of the University of Chicago began publishing the results of their painstaking work. Acanthostega was wonderfully mind-blowing. For starters, it had eight fingers, rather than the standard five that everyone thought was the rule for tetrapods. It’s now clear that the developmental genes that controlled digit growth were not tightly locked into the five-fingered rule when tetrapods first evolved. Perhaps even more remarkable was the fact that Acanthostega would have been a miserable land-walker, despite having nicely formed feet and digits. It didn’t have the hips or shoulders to support its weight, and it had rays on the top and bottom of its tail. It also had bones that likely supported gills. It suggested that the tetrapod body adapted first for life underwater and then was adapted for life on land.

If I remember correctly, my article about Acanthostega, which appeared in Discover way back in 1995, was the first major magazine feature about the beast. I decided to make it a central part of At the Water’s Edge. But then other fossils started turning up.They were just scraps–a jaw here, a shoulder bone there–but they were helping to fill in the transition. Most remarkable I thought was an isolated fin of a lobe fin that was not all that closely related to Acanthostega and land tetrapods. Sauripterus, found by Daeschler and Shubin, had lots of finger-like elements in its fin. It was proof that a lot of independent experiments in limb-like fins were going on around 360 to 380 million year ago.

Writing books about science is massive fun, but it is always followed by an unpleasant aftertaste, as you watch all your work become quaint and dated. But after At the Water’s Edge came out in 1998, I thought the tetrapod section held up pretty well. Acanthostega remained the best known fossil from the transition from lobe-fins to tetrapods. Further down the tree, it was mostly scraps until the Eusthenopteron branch.

Well, that’s all over now, thanks to Tiktaalik. Deschler, Shubin, and Jenkins found three individuals of this species (its name comes for the word for a large freshwater fish in the Inuktitut language). Tiktaalik was a fair-sized creature, perhaps three feet long. It was far more fish-like than Acanthostega or any other tetrapod. It had scales across its back, rays in its fins, and a lot of less visible but no less important traits seen in lobe-fins but not tetrapods. But it also has surprising number of features found in tetrapods, such as a neck on which its head could turn, throat bones that could pump air into its lungs, and some bones inside its fishy fins that were remarkably tetrapod-like.

The scientists dedicate an entire paper just to those limb bones, because the fossils are so good that they can determine how the bones articulated and moved around each other. Unlike Sauripterus, which was a separate experiment in limbs, Tiktaalik’s bones share a clear common ancestry with our own arms and legs. More primitive lobe-fins had shoulder bones that only allowed them to paddle back and forth in the water. Tiktaalik could support itself, bending its wrists to lay its hand-like bones flat on the water bottom. Its powerful ribs and spine gave it more support, and with its eyes stuck right on top of its head, it could look at prey (or predators) swimming overhead.

View Images

Tiktaalik’s fins are important not just for what they say about how our fish ancestors lived, but about how any sort of new structure evolves. If you compare your hand to the fin of a fish that’s studied a lot by biologists–zebrafish, for example–it’s hard to see much in common. The fin rays are not made of skeletal bones, and the few skeletal bones they do have don’t correspond in any clear way to our own limbs. So even quite recently some scientists saw the tetrapod limb as a major innovation. But zebrafish are separated from our own ancetry by 400 million years or more of diverging evolution. Now Tiktaalik makes clear that the evolution of arms and legs was not such a big transformation after all. The fins of fish like Titaalik had already evolved much of the limb equipment–down to individual wrist bones–that would emerge in later tetrapods as so important for moving around on land.

So why is Tiktaalik big news and not news at all? It is big news because it blurs the distinction between fish and tetrapod more spectacularly than ever before. It is no news at all, because it is just the sort of creature that one would predict from previously discovered fossils. Its place on our family tree has been cleared and waiting for some time now. And now it’s filled.

View Images