by Neil Shubin
Tiktaalik was the lead story in a number of newspapers the day after the find was announced in April 2006, including above-the-fold headlines in such places as The New York Times. This attention ushered in a week unlike any other in my normally quiet life. Though for me the greatest moment of the whole media blitz was not seeing the political cartoons or reading the editorial coverage and the heated discussions on the blogs. It took place at my son’s preschool.
In the midst of the press hubbub, my son’s preschool teacher asked me to bring in the fossil and describe it. I dutifully brought a cast of Tiktaalik into Nathaniel’s class, bracing myself for the chaos that would ensue. The twenty four-and five-year-olds were surprisingly well behaved as I described how we had worked in the Arctic to find the fossil and showed them the animal’s sharp teeth. Then I asked what they thought it was. Hands shot up. The first child said it was a crocodile or an alligator. When queried why, he said that like a crocodile or lizard it has a flat head with eyes on top. Big teeth, too. Other children started to voice their dissent. Choosing the raised hand of one of these kids, I heard: No, no, it isn’t a crocodile, it is a fish, because it has scales and fins. Yet another child shouted, “Maybe it is both.” Tiktaalik’s message is so straightforward even preschoolers can see it.
For our purposes, there is an even more profound take on Tiktaalik. This fish doesn’t just tell us about fish; it also contains a piece of us. The search for this connection is what led me to the Arctic in the first place.
How can I be so sure that this fossil says something about my own body? Consider the neck of Tiktaalik. All fish prior to Tiktaalik have a set of bones that attach the skull to the shoulder, so that every time the animal bent its body, it also bent its head. Tiktaalik is different. The head is completely free of the shoulder. This whole arrangement is shared with amphibians, reptiles, birds, and mammals, including us. The entire shift can be traced to the loss of a few small bones in a fish like Tiktaalik.
Tracing arm bones from fish to humans.
I can do a similar analysis for the wrists, ribs, ears, and other parts of our skeleton—all these features can be traced back to a fish like this. This fossil is just as much a part of our history as the African hominids, such as Australopithecus afarensis, the famous “Lucy.” Seeing Lucy, we can understand our history as highly advanced primates. Seeing Tiktaalik is seeing our history as fish.
So what have we learned? Our world is so highly ordered that we can use a walk through a zoo to predict the kinds of fossils that lie in the different layers of rocks around the world. Those predictions can bring about fossil discoveries that tell us about ancient events in the history of life. The record of those events remains inside us, as part of our anatomical organization.
What I haven’t mentioned is that we can also trace our history inside our genes, through DNA. This record of our past doesn’t lie in the rocks of the world; it lies in every cell inside us. We’ll use both fossils and genes to tell our story, the story of the making of our bodies.
CHAPTER TWO
GETTING A GRIP
Images of the medical school anatomy lab are impossible to forget. Imagine walking into a room where you will spend several months taking a human body apart layer by layer, organ by organ, all as a way to learn tens of thousands of new names and body structures.
In the months before I did my first human dissection, I readied myself by trying to envision what I would see, how I would react, and what I would feel. It turned out that my imagined world in no way prepared me for the experience. The moment when we removed the sheet and saw the body for the first time wasn’t nearly as stressful as I’d expected. We were to dissect the chest, so we exposed it while leaving the head, arms, and legs wrapped in preservative-drenched gauze. The tissues did not look very human. Having been treated with a number of preservatives, the body didn’t bleed when cut, and the skin and internal organs had the consistency of rubber. I began to think that the cadaver looked more like a doll than a human. A few weeks went by as we exposed the organs of the chest and abdomen. I came to think that I was quite the pro; having already seen most of the internal organs, I had developed a cocky self-confidence about the whole experience. I did my initial dissections, made my cuts, and learned the anatomy of most of the major organs. It was all very mechanical, detached, and scientific.
This comfortable illusion was rudely shattered when I uncovered the hand. As I unwrapped the gauze from the fingers—as I saw the joints, fingertips, and fingernails for the first time—I uncovered emotions that had been concealed during the previous few weeks. This was no doll or mannequin; this had once been a living person, who used that hand to carry and caress. Suddenly, this mechanical exercise, dissection, became deeply and emotionally personal. Until that moment, I was blind to my connection to the cadaver. I had already exposed the stomach, the gallbladder, and other organs; but what sane person forms a human connection at the sight of a gallbladder?
What is it about a hand that seems quintessentially human? The answer must, at some level, be that the hand is a visible connection between us; it is a signature for who we are and what we can attain. Our ability to grasp, to build, and to make our thoughts real lies inside this complex of bones, nerves, and vessels.
The immediate thing that strikes you when you see the inside of the hand is its compactness. The ball of your thumb, the thenar eminence, contains four different muscles. Twiddle your thumb and tilt your hand: ten different muscles and at least six different bones work in unison. Inside the wrist are at least eight small bones that move against one another. Bend your wrist, and you are using a number of muscles that begin in your forearm, extending into tendons as they travel down your arm to end at your hand. Even the simplest motion involves a complex interplay among many parts packed in a small space.
The relationship between complexity and humanity within our hands has long fascinated scientists. In 1822, the eminent Scottish surgeon Sir Charles Bell wrote the classic book on the anatomy of hands. The title says it all: The Hand, Its Mechanism and Vital Endowments as Evincing Design. To Bell, the structure of the hand was “perfect” because it was complex and ideally arranged for the way we live. In his eye, this designed perfection could only have a divine origin.
The great anatomist Sir Richard Owen was one of the scientific leaders in this search for divine order within bodies. He was fortunate to be an anatomist in the mid-1800s, when there were still entirely new kinds of animals to discover living in the distant reaches of the earth. As more and more parts of the world were explored by westerners, all sorts of exotic creatures made their way back to laboratories and museums. Owen described the first gorilla, brought back from expeditions to central Africa. He coined the name “dinosaur” for a new kind of fossil creature discovered in rocks in England. His study of these bizarre new creatures gave him special insights: he began to see important patterns in the seeming chaos of life’s diversity.
Owen discovered that our arms and legs, our hands and feet, fit into a larger scheme. He saw what anatomists before him had long known, that there is a pattern to the skeleton of a human arm: one bone in the upper arm, two bones in the forearm, a bunch of nine little bones at the wrists, then a series of five rods that make the fingers. The pattern of bones in the human leg is much the same: one bone, two bones, lotsa blobs, and five toes. In comparing this pattern with the diversity of skeletons in the world, Owen made a remarkable discovery.
Owen’s genius was not that he focused on what made the various skeletons different. What he found, and later promoted in a series of lectures and volumes, were exceptional similarities among creatures as different as frogs and people. All creatures with limbs, whether those limbs are wings, flippers, or hands, have a common design. One bone, the humerus in the arm or the femur in the leg, articulates with two bones, which attach to a series of small blobs, which connect with the fingers or toes. This pattern underlies the architecture of all limbs. Want to make a bat wing? Make the fingers rea
lly long. Make a horse? Elongate the middle fingers and toes and reduce and lose the outer ones. How about a frog leg? Elongate the bones of the leg and fuse several of them together. The differences between creatures lie in differences in the shapes and sizes of the bones and the numbers of blobs, fingers, and toes. Despite radical changes in what limbs do and what they look like, this underlying blueprint is always present.
The common plan for all limbs: one bone, followed by two bones, then little blobs, then fingers or toes.
For Owen, seeing a design in the limbs was only the beginning: when he looked at skulls and backbones, indeed when he considered the entire architecture of the body, he found the same thing. There is a fundamental design in the skeleton of all animals. Frogs, bats, humans, and lizards are all just variations on a theme. That theme, to Owen, was the plan of the Creator.
Shortly after Owen announced this observation in his classic monograph On the Nature of Limbs, Charles Darwin supplied an elegant explanation for it. The reason the wing of a bat and the arm of a human share a common skeletal pattern is because they shared a common ancestor. The same reasoning applies to human arms and bird wings, human legs and frog legs—everything that has limbs. There is a major difference between Owen’s theory and that of Darwin: Darwin’s theory allows us to make very precise predictions. Following Darwin, we would expect to find that Owen’s blueprint has a history that will be revealed in creatures with no limbs at all. Where, then, do we look for the history of the limb pattern? We look to fish and their fin skeletons.
SEEING THE FISH
In Owen and Darwin’s day, the gulf between fins and limbs seemed impossibly wide. Fish fins don’t have any obvious similarities to limbs. On the outside, most fish fins are largely made up of fin webbing. Our limbs have nothing like this, nor do the limbs of any other creature alive today. The comparisons do not get any easier when you remove the fin webbing to see the skeleton inside. In most fish, there is nothing that can be compared to Owen’s one bone–two bones–lotsa blobs–digits pattern. All limbs have a single long bone at their base: the humerus in the upper arm and the femur in the upper leg. In fish, the whole skeleton looks utterly different. The base of a typical fin has four or more bones inside.
In the mid-1800s, anatomists began to learn of mysterious living fish from the southern continents. One of the first was discovered by German anatomists working in South America. It looked like a normal fish, with fins and scales, but behind its throat were large vascular sacs: lungs. Yet the creature had scales and fins. So confused were the discoverers that they named the creature Lepidosiren paradoxa, “paradoxically scaled amphibian.” Other fish with lungs, aptly named lungfish, were soon found in Africa and Australia. African explorers brought one to Owen. Scientists such as Thomas Huxley and the anatomist Carl Gegenbaur found lungfish to be essentially a cross between an amphibian and a fish. Locals found them delicious.
A seemingly trivial pattern in the fins of these fish had a profound impact on science. The fins of lungfish have at their base a single bone that attaches to the shoulder. To anatomists, the comparison was obvious. Our upper arm has a single bone, and that single bone, the humerus, attaches to our shoulder. In the lungfish, we have a fish with a humerus. And, curiously, it is not just any fish; it is a fish that also has lungs. Coincidence?
As a handful of these living species became known in the 1800s, clues started to come from another source. As you might guess, these insights came from ancient fish.
One of the first of these fossils came from the shores of the Gaspé Peninsula in Quebec, in rocks about 380 million years old. The fish was given a tongue-twister name, Eusthenopteron. Eusthenopteron had a surprising mix of features seen in amphibians and fish. Of Owen’s one bone–two bones–lotsa blobs–digits plan of limbs, Eusthenopteron had the one bone–two bones part, but in a fin. Some fish, then, had structures like those in a limb. Owen’s archetype was not a divine and eternal part of all life. It had a history, and that history was to be found in Devonian age rocks, rocks that are between 390 million and 360 million years old. This profound insight defined a whole new research program with a whole new research agenda: somewhere in the Devonian rocks we should find the origin of fingers and toes.
In the 1920s, the rocks provided more surprises. A young Swedish paleontologist, Gunnar Save-Soderbergh, was given the extraordinary opportunity to explore the east coast of Greenland for fossils. The region was terra incognita, but Save-Soderbergh recognized that it featured enormous deposits of Devonian rocks. He was one of the exceptional field paleontologists of all time, who throughout his short career uncovered remarkable fossils with both a bold exploring spirit and a precise attention to detail. (Unfortunately, he was to die tragically of tuberculosis at a young age, soon after the stunning success of his field expeditions.) In expeditions between 1929 and 1934, Save-Soderbergh’s team discovered what, at the time, was labeled a major missing link. Newspapers around the world trumpeted his discovery; editorials analyzed its importance; cartoons lampooned it. The fossils in question were true mosaics: they had fish-like heads and tails, yet they also had fully formed limbs (with fingers and toes), and vertebrae that were extraordinarily amphibian-like. After Save-Soderbergh died, the fossils were described by his colleague Erik Jarvik, who named one of the new species Ichthyostega soderberghi in honor of his friend.
The fins of most fish—for example, a zebrafish (top)—have large amounts of fin webbing and many bones at the base. Lungfish captured people’s interest because like us they have a single bone at the base of the appendage. Eusthenopteron (middle) showed how fossils begin to fill the gap; it has bones that compare to our upper arm and forearm. Acanthostega (bottom) shares Eusthenopteron’s pattern of arm bones with the addition of fully formed digits.
For our story, Ichthyostega is a bit of a letdown. True, it is a remarkable intermediate in most aspects of its head and back, but it says very little about the origin of limbs because, like any amphibian, it already has fingers and toes. Another creature, one that received little notice when Save-Soderbergh announced it, was to provide real insights decades later. This second limbed animal was to remain an enigma until 1988, when a paleontological colleague of mine, Jenny Clack, who we introduced in the first chapter, returned to Save-Soderbergh’s sites and found more of its fossils. The creature, called Acanthostega gunnari back in the 1920s on the basis of Save-Soderbergh’s fragments, now revealed full limbs, with fingers and toes. But it also carried a real surprise: Jenny found that the limb was shaped like a flipper, almost like that of a seal. This suggested to her that the earliest limbs arose to help animals swim, not walk. That insight was a significant advance, but a problem remained: Acanthostega had fully formed digits, with a real wrist and no fin webbing. Acanthostega had a limb, albeit a very primitive one. The search for the origins of hands and feet, wrists and ankles had to go still deeper in time. This is where matters stood until 1995.
FINDING FISH FINGERS AND WRISTS
In 1995, Ted Daeschler and I had just returned to his house in Philadelphia after driving all through central Pennsylvania in an effort to find new roadcuts. We had found a lovely cut on Route 15 north of Williamsport, where PennDOT had created a giant cliff in sandstones about 365 million years old. The agency had dynamited the cliff and left piles of boulders alongside the highway. This was perfect fossil-hunting ground for us, and we stopped to crawl over the boulders, many of them roughly the size of a small microwave oven. Some had fish scales scattered throughout, so we decided to bring a few back home to Philadelphia. Upon our return to Ted’s house, his four-year-old daughter, Daisy, came running out to see her dad and asked what we had found.
In showing Daisy one of the boulders, we suddenly realized that sticking out of it was a sliver of fin belonging to a large fish. We had completely missed it in the field. And, as we were to learn, this was no ordinary fish fin: it clearly had lots of bones inside. People in the lab spent about a month removing the fin from the boulder—and
there, exposed for the first time, was a fish with Owen’s pattern. Closest to the body was one bone. This one bone attached to two bones. Extending away from the fin were about eight rods. This looked for all the world like a fish with fingers.
Our fin had a full set of webbing, scales, and even a fish-like shoulder, but deep inside were bones that corresponded to much of the “standard” limb. Unfortunately, we had only an isolated fin. What we needed was to find a place where whole bodies of creatures could be recovered intact. A single isolated fin could never help us answer the real questions: What did the creature use its fins for, and did the fish fins have bones and joints that worked like ours? The answer would come only from whole skeletons.
Our tantalizing fin. Sadly, we found only this isolated specimen. Stipple diagram used with the permission of Scott Rawlins, Arcadia University. Photo by the author.