In 1893, the Norwegian zoologist Fridtjof Nansen set off to find the North Pole. He would not use pack dogs to cross the Arctic Ice. Instead, he locked his fate into the ice itself. He sailed his ship The Fram directly into the congealing autumn Arctic, until it became locked in the frozen sea. Nansen was convinced that the ice itself would drift up to the pole, taking him and his crew along for the ride.

For two and a half years they drifted with the pack. It gradually became clear to Nansen that The Fram had stopped moving north and was now traveling east instead, back towards Europe. He leaped out of the ship and tried to sled up to the pole, only to discover that the ice he was now traveling on was moving south. Only four degrees away from true north, he decided to retreat. He bolted back for Franz Josef Land.

The Fram meanwhile continued to drift east. After several months, it broke free of the ice, and the crew sailed the ship south to the island of Spitzbergen. There on the bare flats they saw a giant balloon.

Its pilot was a young Swedish engineer named Salamon Andrée. Andrée had decided that ships like the Fram could never reach the Pole, and that flight offered the only hope. He had convinced the king of Sweden and Alfred Nobel to pay for a balloon which he had brought by ship to Spitzbergen. And there he mixed tons of sulfuric acid and zinc to create hydrogen gas, which filled his silk canopy for four days. But gales hit the island before he was ready to launch the balloon, and then the Fram arrived with stories of how Nansen was racing on sleds towards the pole. Andrée let the canopy fall back to the ground.

When he got back to Sweden, Andrée discovered that Nansen had actually failed and had returned to Norway. He began to plot a second attempt. He returned to Spitzbergen in 1897 and this time he succeeded in launching his balloon. For a few days Andrée floated north with his crew of two, bobbing up and down with the sudden changes in temperature and moisture of the Arctic atmosphere. But as he crossed over the edge of the polar ice, the voyage took a turn for the worse. The balloon became burdened with rain and snow, until the guidelines dragged across the ice, until the gondola bounced like a ball on the ground, until the balloon came to a rest.

For a week the crew huddled in cramped fog. Andrée decided to pack sledges with food and a collapsible boat, which they dragged over the drifting ice. Hauling them across sloshing leads, they hoped, like Nansen, that they could find refuge in Franz Josef Land. But the ice wandered in the wrong direction under their feet, and after two months of this polar treadmill they reached a little hump of Arctic rock called White Island. In 1930 whalers came to the island and discovered their decrepit boat, their journals, and Andrée’s corpse still sitting in the snow.

But in 1897, no one knew where Andrée had gone. His fellow Swedish scientists searched for him by ship in the following summers, first travelling around Spitzbergen, and then heading to Greenland. As the pack ice opened, they traveled for eight weeks along its eastern edge in their sail- and steam-powered ship. They mapped the tentacled coast, and in one fjord along an elephant-backed mountain they named Celsius Berg, the explorers found bones.

They weren’t the bones of Andrée and his crew. They were the bones of fish that had been resting in the Greenland rocks for over 350 millions years.

Other fossils of these fish had been found elsewhere in late Devonian rocks, but to those who studied that era, Greenland was a revelation. It was as if a new continent suddenly appeared on the map: other Devonian rocks were hid for the most part under a woody, bushy carpet in places like England and Pennsylvania, while the mountains of Greenland were mercilessly bare. Unfortunately the new fossils were also so remote that only some greater pretext–like the search for a famous explorer–could get the paleontologists to this far corner of the Arctic.

Another rationale came about thirty years later, when Denmark and Norway began competing in the late 1920s for control of Eastern Greenland, and the oil and minerals that it might hold. The Danes brought Swedish scientists with them, and they found bones of more fish, including lobe-fins, as well as a few things they didn’t know what to make of, simply marking them as “scales of a fish-like vertebrate of uncertain affinities.”

These expeditions were a bit less brutal than Andrée’s and Nansen’s trips. The scientists still traveled in wooden steamers with three square-rigged masts, and while they could now bring a hydroplane for their surveys they still wore polar bear suits when they flew. In 1931 an energetic 22-year old geologist named Gunnar Säve-Söderbergh was put in charge of the expeditions. For sixteen hours a day he could climb mountains, throwing rocks into his rucksack and sketching out stratigraphy along the way.

He had a book of numbered tags made for the expeditions, P. for fishes, and A. for amphibians–a supremely confident system, considering that no one had ever found a Devonian amphibian. The fossil record of land vertebrates with legs–known as tetrapods–only went back about 300 million years and stopped cold.

That first summer, Säve-Söderbergh made his way around the northern slope of Celsius Berg and found more fish. In the cones of fallen rocks below the mountain’s eastern plateau, he also found more than a dozen scraps of a flat skull that didn’t look like any species of fish he had seen before. Optimistically, he marked them with A. tags.

Back in Stockholm that fall, Säve-Söderbergh slowly worked the bones free of the hard sandstone, painting them with alchohol and balsam to reveal the sutures between the bones. Looking down on the flat roof of the skull, he could see that some of the bones were patterned like the skulls of a group of fish known as lobe-fins–represented today only by lungfishes and coelacanths. Many naturalists argued that tetrapods had evolved from an lobe-fin ancestor. But Säve-Söderbergh could also see that it had some traits–like a long snout–that had only been found in early tetrapod fossils.

Looking at that skull, Säve-Söderbergh realized that he had found the earliest tetrapod. He named it Ichthyostega–”fish plate”–after the top of the animal’s skull.

The discovery was a great hit in Denmark, not only with the politicians who wanted to tighten their grip on Greenland, but with the public as well. In celebration one newspaper cartoonist drew a trout with dog legs carrying a pipe-smoking caveman, as snakes encircled moutain peaks and elephants flapped their wings overhead.

Säve-Söderbergh spent the following few summers mapping more of the region by foot, boat, and Icelandic horse. Fossils practically fell out of the rocks for him–mostly fish but on rare occasion another piece of Ichthyostega. The strange scales that had been found in 1929 turned out to be Ichthyostega’s ribs, massive and overlapping like Venetian blinds of bone. His assistants, particularly a student from the University of Upssala named Erik Jarvik, found more Ichthyostega skulls. One skull, unearthed in 1934, was so handsome the paleontologists brought it back across the Atlantic resting on a blue velvet pillow.

After five years Säve-Söderbergh was appointed a professor at the University of Uppsala, but in that year he was diagnosed with tuberculosis. He lingered in bed, managing to write a few papers about some of the fish he had collected, and died in June 1948 at only 40. The summer of Säve-Söderbergh’s death, the expedition to Greenland finally found the legs and shoulders and tail of Ichthyostega. At last it had most of a body.

In the 64 years since Säve-Söderbergh’s death, scientists have discovered many more early tetrapods and their extinct lobe-fin relatives. They’ve found some of these beasts on return trips to Greenland. But they’ve also found other species in places like Pennsylvania, northern Canada, Latvia, and, most recently, Nevada. Together, these fossils now offer an illuminating look at one of the most crucial transitions in the history of life. Without it, we’d still be fish in the sea.

Despite all the new company Ichthyostega has enjoyed lavish attention ever since its discovery, thanks to the quality and quantity of the fossils it left behind. For decades, Erik Jarvik pored over the fossils, and then, after his death, Jennifer Clack of the University of Cambridge and other paleontologists took a look for themselves.

Ichthyostega’s legs, while short and squat, had the elbows, knees, ankles, wrists, and toes that qualified it as a tetrapod. (Strangely, it had seven digits on its feet.) Its spine was sturdy, its hips and shoulders massive, its skull rigid. Yet Ichthyostega’s rigid skull still shared some traits with the flexible skull of lobe fins. It had a distinctive suture in the skull at the same place where a lobe-fin skull has a hinge. Under the tetrapod palimpsest its ancestry could be seen.

Ichthyostega’s tail was a similar mix of tetrapod and fish. Tetrapods have simple tails consisting of a long series of tapering vertebrae encased in flesh (ours has dwindled to a mere sprout, the coccyx). A lobe-fin’s tail, the motor that the animal uses to move through water, is a much more elaborate affair. Each vertebra has two long rods, one on top and one below. Attaching to each of these rods are more slender bones, called radials, and attaching to the radials is a wide fan of fin rays: a completely differerent kind of bone called dermal bone that also makes up scales. This complex anatomy allows a fish to set up waves in its tail either forward or backwards, to let it dart through the water or suddenly brake.

The bottom of Ichthyostega’s tail had a simplified tetrapod form, but the top still retained all the geegaws of a fish. It was, in a sense, still half in the water.

Clack and her colleagues have used the anatomy of Ichthyostega to figure out what it did in life–and, by extension, to get some clues to how the tetrapod body plan evolved. Long before Ichthyostega came on the scene, lobe-fins were already evolving some of the crucial pieces of that body plan–legs and wrists, for example. These ancient relatives lived unquestionably like fish, using gills to breath and depending on water to support much of their weight. It’s clear, in other words, that even though the tetrapod body is very good for getting around on land, it didn’t start evolving on land.

How far had things gotten by the time Ichthyostega showed up 360 million years ago? Clack has found that Ichthyostega’s ear was tuned for hearing underwater. But when she and her colleagues looked at a series of Ichthyostega skeletons, going from young to old, they found a different story. As the animals matured, their shoulders changed shape, providing more space for anchoring arm muscles. It’s possible that they spent a lot of time in the water when they were young and then spent more time on land when they became adults.

In 2005 Clack and her colleagues did a thorough study of Ichthyostega’s trunk–its spine and rib cage. They concluded that the tetrapod was weirdly stiff, unable to bend from side to side. They suggested two possible ways for Ichthyostega to get around on land. It might walk, but without bending its body the way, for examplpe, a salamander does. Or it might mimic an inchworm. It would bend its spine upwards, reach forward with its front legs, and then straighten out, pushing forward with its hind legs.

Today in Nature, Clack offers more clues to this puzzling creature. She collaborated with John Hutchinson of the Royal Veterinary College, an expert on biomechanics, and his postdoctoral researcher Stephanie Pierce. They have brought Ichthyostega back to life through a detailed computer reconstruction.

They started by making high-resolution scans of its fossils, which they then assembled into a virtual skeleton. Hutchinson has, over the past decade, figured out a way to estimate how animals moved based on this kind of reconstruction. By placing virtual muscles on the virtual bones, he can estimate their range of motion. Hutchinson knows his models are reliable, because he can test them on living animals. His estimates for the movements of animals such as otters and alligators are close to how they really move.

Here are a couple videos showing their results. I’ll explain them below.

The whole body:

The hind leg in action:

Simply put, Ichthyostega could not have been very impressive on land. No matter how hard it tried, it could not walk with its back legs. The limbs could move forward and back, but they could not swivel into a position that would allow Ichthyostega to plant its feet on the ground. Its forelimbs were a little more useful. It could bend its elbows. But its shoulders had little range of motion.

Combined with their rigid trunk, these new findings lead Clack and her colleagues to conclude that the best living analogy for Ichthyostega is a mudskipper. Mudskippers are not lobe-fins. Instead, they are ray-finnsed fish, more closely related to goldfish or trout. In an independent transition from the ocean, they evolved the ability to move around on land by crutching along on their front pair of fins. As the delightful video below from David Attenborough shows, mudskippers are quite successful in their peculiar ecological niche, crawling on muddy beaches, sucking up food from the muck, and then swimming through their underwater burrows to care for their young. But they are hardly an inspiring vision of tetrapods emerging on land.

Clack’s new study stands in intriguing contrast to one that I blogged about in December. University of Chicago scientists reported then that lungfish–our closest living aquatic relatives–can walk underwater with their pelvic fins–which correspond to the hind legs of tetrapods. The Chicago team argued that hind-leg-driven walking could have started out long before the tetrapod body evolved. Clack and her colleagues, on the other hand, propose that hind legs came late to the terrestrial party.

But if there’s one thing that the past couple decades of fossil-hunting has made clear is that the origin of tetrapods was not some linear march of progress. Starting about 380 million years ago, some lobe fins independently evolved tetrapod-like traits in a grand, unplanned experiment. Different species ended up with different combinations of those traits, perhaps adapting them to different ways of getting around underwater or on land. Ichthyostega might be a good model for the ancestor of all living tetrapods. Or it may have been a very weird beachcomber with hind legs that were only good as underwater paddles. To find out, scientists need to build more virtual skeletons of early tetrapods. And they need to head out to find more fossils.

Let’s just hope that they don’t have to follow doomed explorers to find them.

For more information about the discovery of tetrapod evolution, see my book At the Water’s Edge, from which parts of this post were adapted.

[Images: skeleton from Clack's site. Skull from Tree of Life. Images of the Fram and the failed balloon from Wikipedia.]

Why do flu shots only protect us for a single season? Why can’t influenza vaccines be like polio vaccines: get them in childhood and be done with them? Wouldn’t that be the best way to prepare ourselves for the next pandemic?

These are among the questions that will be addressed at next month’s World Science Festival. To lay the groundwork, I’ve written a blog post at the festival web site on where we stand on the road to a universal flu vaccine. At this point, we have good reason to believe that such a vaccine could be invented. Which makes it all the more urgent that we do so. Check it out.

Manta rays spend their lives in the ocean, sweeping up microscopic animals. And yet scientists have found that their well-being depends on forests. Meadows in the northwestern United States are ecologically linked to salmon thousands of miles out at sea. Today, I’ve got a piece in Yale Environment 360 in which I explore the bonds that join land and sea together. Check it out.

We’ve all heard about tapeworms getting into the intestines. That’s bad enough. But sometimes they can also end up in the brain. In my column in the latest issue of Discover, I write about neurocysticercosis, which is shockingly common in some parts of the world, causing an estimated five million cases of epilepsy. Yet neurocysticercosis experts consider the disease as a fairly easy one to wipe out. We have the tools to do it, but not the will. Check it out.

Over at Download the Universe, we’ve added another crop of entertaining reviews about ebooks that you definitely should–or, in some cases, definitely should not–check out:

“When an Autism Diagnosis Comes as a Blessing”: Steve Silberman writes a powerful review about the reality of autism and a Kindle memoir about living with the condition.

“Meandering Mississippi: An early journalism iBook is all wet”: Seth Mnookin reads an account of last year’s Mississippi floods and wonders why newspapers are squandering the opportunities that ebooks are offering them.

“A Lost Explorer Returns: Todd Balf’s Farthest North: David Dobbs revels in a well-told story of an ill-fated scientific voyage across the Arctic.

“Leonardo: The First Great Science Ebook”: I take a look at a lavishly-produced ebook about Leonardo da Vinci’s forgotten work as a pioneer of anatomy. Staggeringly impressive.

“A Time Machine for the Face of Earth”: My review of a coffee-table-like ebook about how humans (and other forces) are changing the surface of the planet.

“Artificial Epidemics: You’re Not Sick, You’re Just Overdiagnosed”: Neuroscience blogger “Scicurious” is unimpressed with an ebook that claims that depression and prostate cancer are all in your head. (Confused? You should be.)

“Titanic: The e-Book Nobody Loved”: Jennifer Ouellette looks at one of the least successful Titanic anniversary tie-ins. Again, a wasted opportunity.

And, finally, Seth Mnookin, Annalee Newitz, Maia Szavalitz, and I engaged in a three-day roundtable discussion about ebooks: how people read them, how they get published, and the future of books:

Day 1: Crap futurism, pleasure reading, and DRM

Day 2: Walled gardens, cruftiness, and a race to the bottom

Day 3: Pirates, parties, pulps, and PowerPoint: Part 3 of a Download the Universe roundtable on e-reading

We’ve learned a lot about how the brain works from functional magnetic resonance images. I should clarify: we’ve learned a lot about the human brain. Thousands of people have volunteered to lie down inside fMRI scanners and have the activity in their brains monitored as they perform different kinds of mental tasks, or even just do nothing at all. We must resist the temptation to look at the pretty fMRI images and think they’re just photographs of the mind. They’re actually more like very complex, statistically worked-over graphs. But even with those caveats, there’s a lot to learn from them. But fMRI only works if you hold very, very still. Having been scanned myself for a story a few years back, I can vouch that this experience takes a lot of patience, and a high tolerance for loud buzzing noises and for narrow, confined spaces. Scientists have managed to take fMRI scans of monkeys and rats, but they’ve either been knocked out cold, or restrained so the images of their brains don’t blur. If you can persuade a gorilla to lie peacefully in the bore of a scanner for half an hour and look at pictures of bananas, do let us know.

All of which is to say that it’s delightful to read a paper just published today in PLoS One in which Emory University scientists report the first successful fMRI scan of unrestrained dogs. The dogs they studied were a border collie named McKenzie and a mutt named Callie. The scientists trained the dogs to jump into the fMRI bore and set their chin on a rest, so that they were staring out the open end. A human handler, standing outside the bore, then made a hand signal. If the left hand was raised, that meant the dogs would get a hot dog after the session. If both hands pointed to each other, that meant no treat. Regardless of the signal, the dogs had to stay motionless in the bore for ten seconds. The hanlder would then give the dog a treat if one had been indicated.

It turned out that the dogs kept their heads still enough that the scientists could line up scans of different trials. And that meant that they could do some comparisons of the activity inside the dogs’ brains when the handler indicated a hot dog was on the way and when the handler indicated it wasn’t. A huge amount of research on humans and lab animals has pointed to one region in particular as being important for perceiving these kinds of rewards, known as the ventral striatum. It releases the neurotransmitter dopamine, which heightens attention throughout the brain. So the scientists focused their attention on the ventral striatum of the dogs.

Of course, dog brains are shaped differently than human brains, so finding it took some work. But once they did, they found a satisfying pattern: when the handler made the hot dog sign, the ventral striatum showed higher levels of activity. (In this picture it’s marked CD, standing for the caudate cluster, in which the ventral striatum is located.)  When the handler made the no-hot-dog sign, on the other hand, the ventral striatum remained quiet. Interestingly, McKenzie the border collie had a much stronger response than Callie.

That may be no coincidence, because McKenzie has had lots of agility training in her life. The pathways for learning rewards and responding to them–particularly from humans–may be stronger in her brain. Which raises an important question: what’s the nature of the reward in a dog’s brain? Is it the prospect of the hot dog alone that triggers the response, or does pleasing a human by doing a drill correctly play a part? As I wrote in this Time story in 2009, a new field called canine cognition is taking shape to address these questions. Until now, canine cognition studies have been limited to basic psychological experiments, such as seeing how well dogs can make sense of a pointed hand. Now we can start to see what’s happening inside the canine brain during some of those experiments as well.

Earlier this year, TEDMED took place in Washington DC, showcasing people doing innovative research in medicine. This year’s talks are now being loaded online, and today I was happy to see that cancer and evolution got their due. Franziska Michor of Harvard explained how the threat of cancer is a legacy of our evolution into multicellular animals, and how every case of cancer is a miniature unfolding of evolution within our own bodies. What makes Michor’s work particular exciting is that she is bringing the mathematical precision of population genetics and other aspects of evolution to the treatment of cancer.

I wrote about some of Michor’s work in my 2007 Scientific American article, “Evolved for Cancer?” (carlzimmer.com, sciam.com) I’ve also explored cancer evolution here on the Loom: “Inside Darwin’s Tumor” and “The Mere Existence of Whales.”  And you can find lots of Michor’s papers as free pdf’s on her publication page.

At some of my recent talks, I’ve been running into people who’ve been annoyed that they forgot to bring a book of mine to get signed. You really couldn’t think of a better way to cheer up a writer, and so I feel the need to reciprocate.

So if you’ve gotten a book of mine and want to get it signed, I’ve printed up some bookplates that I can autograph and send to you.

Just to ensure I’m not signing bookplates for alien robots who will take these bookplates to their home planet to…do whatever evil thing alien robots do with bookplates from science writers…please follow these steps:

1. Take your picture with the book.

2. Email it to me, with your mailing address and any special signing request. As in, “To Ken Ham, so that someday he may appreciate transitional fossils….”

Optional step 3. For those on Twitter: instead of emailing me your photo, you can upload it to Twitter (mentioning my Twitter name @carlzimmer). Be sure to email me your address, too, so that I know where to send the bookplate.

So far, I’ve got three bookplates–one for Parasite Rex, one for Science Ink (in matching Goth type), and one for Planet of Viruses. (See below). It’s weirdly easy to produce these things, so I’m happy to take requests for my other books.

Gothamites: please join Florence Williams and myself at the bookstore McNally Jackson in New York on Thursday May 17. Williams is the author of the new book, Breasts: A Natural and Unnatural History, a smart, wry synthesis of evolution, physiology, microbiology, environmental science, and even biomechanics.

Where: McNally Jackson, 52 Prince St., New York, NY (Phone: 212.274.1160)

When: May 17, 7 pm.

More details are here.

Today the Guardian reviews A Planet of Viruses:

Viruses are everywhere: scientists have found them under Antarctic ice; they lurk inside your lungs which until recently were believed to be sterile; and seawater, which was once thought to contain very few, has now been found to be teeming with viruses. In fact, they outnumber all other residents of the ocean by 15 to 1. Even the human genome contains genes that came from viruses which infected our ancestors some 30m years ago, an idea that Zimmer describes as “almost philosophical in its weirdness.” In this succinct yet elegantly written survey, he explores the vital role viruses play in the evolution of life on Earth and how scientists have begun to reveal their often deadly secrets. Smallpox – the only human virus to have been eradicated – killed an astonishing 500m people every century in Europe between 1400 and 1800. From the common cold, first described 3,500 years ago by the Egyptians, to a new type of giant virus discovered in a Bradford water-cooler that mimics bacteria, this book is a fascinating and enlightening introduction.

The tepuis of northern South America–tabletop mountains ringed by sheer cliffs rising up thousands of feet–inspired Sir Arthur Conan Doyle’s novel The Lost World. Doyle envisioned dinosaurs and other primordial creatures surviving on these remote islands in the sky.

It turns out that the tepuis are indeed ancient vestiges. The surrounding land eroded away 70 million years ago. Biologists have long been fascinated by the plants and animals that live on top of them today. In many cases, the species on a tepui are found nowhere else on Earth. Many have argued for the wonderfully-named “Lost World Hypothesis”–the unique species of the tepuis been stranded up there for 70 million years.

In tomorrow’s New York Times, I report on a team of scientists who tested that hypothesis by looking at the DNA of frogs that live on tepuis. And for them, at least, the hypothesis fails. Somehow, those tiny frogs managed to scale walls that strike fear in even the toughest rock climbers. For the full details, check out the story.

Image by Xyrenita on Flickr/via Creative Commons

I recently sat down for two stimulating conversations which are now in print. One was with Ben Lillie for Story Collider, the new magazine that Lillie has launched to complement his wonderful storytelling series. The other was with Ben Goldfarb of Sage, a student-run environmental arts and journalism publication of the Yale University School of Forestry & Environmental Studies. Ben L. and Ben G. asked lots of thoughtful questions about the art of science writing, and the place of the science writer in society. It’s a little painful to read my ramblings taken down verbatim–I want to reach out and edit away the extra verbiage–but you may still find them interesting.