by Neil Shubin
In the 1950s and 1960s a number of biologists, including Edgar Zwilling and John Saunders, did extraordinarily creative experiments on chicken eggs to understand how the pattern of the skeleton forms. This was an era of slice and dice. Embryos were cut up and various tissues moved about to see what effect this had on development. The approach involved very careful microsurgery, manipulating patches of tissue no more than a millimeter thick. In that way, by moving tissues about in the developing limb, Saunders and Zwilling uncovered some of the key mechanisms that build limbs as different as bird wings, whale flippers, and human hands.
They discovered that two little patches of tissue essentially control the development of the pattern of bones inside limbs. A strip of tissue at the extreme end of the limb bud is essential for all limb development. Remove it, and development stops. Remove it early, and we are left with only an upper arm, or a piece of an arm. Remove it slightly later, and we end up with an upper arm and a forearm. Remove it even later, and the arm is almost complete, except that the digits are short and deformed.
Another experiment, initially done by Mary Gasseling in John Saunders’s laboratory, led to a powerful new line of research. Take a little patch of tissue from what will become the pinky side of a limb bud, early in development, and transplant it on the opposite side, just under where the first finger will form. Let the chick develop and form a wing. The result surprised nearly everybody. The wing developed normally except that it also had a full duplicate set of digits. Even more remarkable was the pattern of the digits: the new fingers were mirror images of the normal set. Obviously, something inside that patch of tissue, some molecule or gene, was able to direct the development of the pattern of the fingers. This result spawned a blizzard of new experiments, and we learned that this effect can be mimicked by a variety of other means. For example, take a chicken embryo and dab a little vitamin A on its limb bud, or simply inject vitamin A into the egg, and let the embryo develop. If you supply the vitamin A at the right concentration and at the right stage, you’ll get the same mirror-image duplication that Gasseling, Saunders, and Zwilling got from the grafting experiments. This patch of tissue was named the zone of polarizing activity (ZPA). Essentially, the ZPA is a patch of tissue that causes the pinky side to be different from the thumb side. Obviously chicks do not have a pinky and a thumb. The terminology we use is to number the digits, with our pinky corresponding to digit five of other animals and our thumb corresponding to digit one.
Moving a little patch of tissue called the ZPA causes the fingers to be duplicated.
The ZPA drew interest because it appeared, in some way, to control the formation of fingers and toes. But how? Some people believed that the cells in the ZPA made a molecule that then spread across the limb to instruct cells to make different fingers. The key proposal was that it was the concentration of this unnamed molecule that was the important factor. In areas close to the ZPA, where there is a high concentration of this molecule, cells would respond by making a pinky. In the opposite side of the developing hand, farther from the ZPA so that the molecule was more diffused, the cells would respond by making a thumb. Cells in the middle would each respond according to the concentration of this molecule to make the second, third, and fourth fingers.
This concentration-dependent idea could be tested. In 1979, Denis Summerbell placed an extremely small piece of foil between the ZPA patch and the rest of the limb. The idea was to use this barrier to prevent any kind of molecule from diffusing from the ZPA to the other side. Summerbell studied what happened to the cells on each side of the barrier. Cells on the ZPA side formed digits. Cells on the opposite side often did not form digits; if they did, the digits were badly malformed. The conclusion was obvious. Something was emanating from the ZPA that controlled how the digits formed and what they looked like. To identify that something, researchers needed to look at DNA.
THE DNA RECIPE
That project was left to a new generation of scientists. Not until the 1990s, when new molecular techniques became available, was the genetic control for the ZPA’s operation unraveled.
A major breakthrough happened in 1993, when Cliff Tabin’s laboratory at Harvard started hunting for the genes that control the ZPA. Their prey was the molecular mechanisms that gave the ZPA its ability to make our pinky different from our thumb. By the time his group started to work in the early 1990s, a number of experiments like the ones I’ve described had led us to believe that some sort of molecule caused the whole thing. This was a grand theory, but nobody knew what this molecule was. People would propose one molecule after another, only to find that none was up to the job. Finally, the Tabin lab came up with a novel notion, and one very relevant to the theme of this book. Look to flies for the answer.
Genetic experiments in the 1980s had revealed the wonderful pattern of gene activity that sculpts the body of a fly from a single-celled egg. The body of a fruit fly is organized from front to back, with the head at the front and the wings at the back. Whole batteries of genes are turned on and off during fly development, and this pattern of gene activity serves to demarcate the different regions of the fly.
Tabin didn’t know it at the time, but two other laboratories—those of Andy MacMahon and Phil Ingham—had already come up with the same general idea independently. What emerged was a remarkably successful collaboration among three different lab groups. One of the fly genes caught the attention of Tabin, McMahon, and Ingham. They noted that this gene made one end of a body segment look different from the other. Fly geneticists named it hedgehog. Doesn’t the function of hedgehog in the fly body—to make one region different from another—sound like what the ZPA does in making the pinky different from the thumb? That parallel was not lost on the three labs. So off they went, looking for a hedgehog gene in creatures like chickens, mice, and fish.
Because the lab groups knew the structure of the fly’s hedgehog gene, they had a search image to help them single out the gene in chickens. Each gene has a distinctive sequence; using a number of molecular tools, the researchers could scan the chicken’s DNA for the hedgehog sequence. After a lot of trial and error, they found a chicken hedgehog gene.
Just as paleontologists get to name new species, geneticists get to name new genes. The fly geneticists who discovered hedgehog had named it that because the flies with a mutation in the gene had bristles that reminded them of a little hedgehog. Tabin, McMahon, and Ingham named the chicken version of the gene Sonic hedgehog, after the Sega Genesis video game.
Now came the fun question: What does Sonic hedgehog actually do in the limb? The Tabin group attached a dye to a molecule that would stick to the gene, enabling them to visualize where the gene is active in the limb. To their great surprise, they found that only cells in a tiny patch of the limb had gene activity: the ZPA.
So the next steps became obvious. The patterns of activity in the Sonic hedgehog gene should mimic those of the ZPA tissue itself. Recall that when you treat the limb with retinoic acid, a form of vitamin A, you get a ZPA active on the opposite side. Guess what happens when you treat a limb with retinoic acid, then map where Sonic hedgehog is active? Sonic hedgehog becomes active on both sides—pinky and thumb—just as the ZPA does when it is treated with retinoic acid.
Knowing the structure of the chicken Sonic hedgehog gave other researchers the tools to look for it in everything else that has fingers, from frogs to humans. Every limbed animal has the Sonic hedgehog gene. And in every single animal that we have studied, Sonic hedgehog is active in the ZPA tissue. If Sonic hedgehog hadn’t turned on properly during the eighth week of your own development, then you either would have extra fingers or your pinky and thumb would look alike. Occasionally, when things go wrong with Sonic hedgehog, the hand ends up looking like a broad paddle with as many as twelve fingers that all look alike.
We now know that Sonic hedgehog is one of dozens of genes that act to sculpt our limbs from shoulder to fingertip by turning on and off at the right time. Remarkably, work in chick
ens, frogs, and mice was telling us the same thing. The DNA recipe to build upper arms, forearms, wrists, and digits is virtually identical in every creature that has limbs.
How far back can we trace Sonic hedgehog and the other bits of DNA that build limbs? Is this stuff active in building the skeleton of fish fins? Or are hands genetically completely different from fish fins? We saw an inner fish in the anatomy of our arms and hands. What about the DNA that builds it?
Enter Randy Dahn with his mermaid’s purses.
GIVING SHARKS A HAND
Randy Dahn entered my laboratory with a simple but very elegant idea: treat skate embryos just the way Cliff Tabin treated chicken eggs. Randy’s goal was to perform all the experiments on skates that chicken biologists had performed on chicken eggs, from Saunders and Zwilling’s tissue surgeries all the way to Cliff Tabin’s gene experiments. Skates develop in an egg with a kind of shell and a yolk. Skates even have big embryos, just as chickens do. Because of these convenient facts, we could apply to skates many of the genetic and experimental tools people had developed to understand chickens.
What could we learn by comparing the development of a shark fin to that of a chicken leg? Even more relevant, what could we learn about ourselves from all this?
Chickens, as Saunders, Zwilling, and Tabin showed, are a surprisingly good proxy for our own limbs. Everything that was discovered by Saunders and Zwilling’s cutting and grafting experiments and by Tabin’s DNA work applies to our own limbs as well: we have a ZPA, we have Sonic hedgehog, and both have a great bearing on our well-being. As we saw, a malfunctioning ZPA or a mutation in Sonic hedgehog can cause major malformations in human hands.
Randy wanted to determine how different the apparatus is that builds our hands. How deep is our connection to the rest of life? Is the recipe that builds our hands new, or does it, too, have deep roots in other creatures? If so, how deep?
Sharks and their relatives are the earliest creatures that have fins with a skeleton inside. Ideally, to answer Randy’s question, you would want to bring a 400-million-year-old shark fossil into the laboratory, grind it up, and look at its genetic structure. Then you’d try to manipulate its fossil embryos to learn whether Sonic hedgehog is active in the same general place as in our limbs today. This would be a wonderful experiment, but it is impossible. We cannot extract DNA from fossils so old, and, even if we could, we could never find embryos of those fossil animals on which to do experiments.
Living sharks and their relatives are the next best thing. Nobody would ever confuse a shark fin with a human hand: you couldn’t ask for two more different kinds of appendages. Not only are sharks and humans very distantly related, but also the skeletal structures of their appendages look nothing alike. Nothing even remotely similar to Owen’s one bone–two bones–lotsa blobs–digits pattern is inside a shark’s fin. Instead, the bones inside are shaped like rods, long and short, thin and wide. We call them bones even though they are made of cartilage (sharks and skates are known as cartilaginous fish, because their skeletons never turn into hard bone). If you want to assess whether Sonic hedgehog’s role in limbs is unique to limbed animals, why not choose a species utterly different in almost every way? In addition, why not choose the species that is the most primitive living fish with any kind of paired appendage, whether fin or limb? Sharks fit both bills perfectly.
Our first problem was a simple one. We needed a reliable source for the embryos of sharks and skates. Sharks proved difficult to obtain with any degree of regularity, but skates, their close relatives, were another matter. So we started with sharks and used skates as our supply of sharks dwindled. We found a supplier who would ship us every month or two a batch of twenty or thirty egg cases with embryos inside. We became a virtual cargo cult as we waited each month for our shipment of precious egg cases.
Work by Tabin’s group and others gave Randy important clues to begin his search. Since Tabin’s work in 1993, people had found Sonic hedgehog in a number of different species, everything from fish to humans. With the knowledge of the structure of the gene, Randy was able to search all the DNA of the skate and shark for Sonic hedgehog. In a very short time he found it: a shark Sonic hedgehog gene.
The key questions to answer were Where is Sonic hedgehog active?, and, even more important, What is it doing?
The egg cases were put to use as Randy visualized where and when Sonic hedgehog is active in the development of skates. He first studied whether Sonic hedgehog turns on at the same time in skate fin development as it does in chicken limbs. Yes, it does. Then he studied whether it is turned on in the patch of tissue at the back end of the fin, the equivalent of our pinky. Yes again. Now he did his vitamin A experiment. This was the million-dollar moment. If you treat the limb of a chicken or mammal with this compound, you get a patch of tissue that has Sonic hedgehog activity on the opposite side, and this result is coupled with a duplication of the bones. Randy injected the egg, waited a day or so, and then checked whether, as in chickens, the vitamin A caused Sonic hedgehog to turn on in the opposite side of the limb. It did. Now came the long wait. We knew that Sonic hedgehog was behaving the same way in our hands and in skates’ and sharks’ fins. But what would the effect of all this be on the skeleton? We would have to wait two months for the answer.
The embryos were developing inside an opaque egg case. All we could tell was whether the creature was alive; the inside of the fin was invisible to us.
The end result was a stunning example of similarity among us, sharks, and skates: a mirror-image fin. The dorsal fins duplicated their structures in a wonderful front-to-back pattern, the same kind we saw with experiments in limbs. Limbs duplicate a limb structure. Shark fins duplicate a shark fin structure as do skates. Sonic hedgehog has a similar effect in even the most different kinds of appendage skeletons found on earth today.
One effect of Sonic hedgehog, you may recall, is to make the fingers distinct from one another. As we saw with respect to the ZPA, what kind of digit develops depends on how close the digit is to the source of Sonic hedgehog. A normal adult skate fin contains many skeletal rods, which all look alike. Could we make these rods different from one another, like our digits? Randy took a small bead impregnated with the protein made by Sonic hedgehog and put it in between these identical skeletal rods. The key to his experiment is that he used mouse Sonic hedgehog. So now we have a real contraption: a skate embryo with a bead inside that is gradually leaking mouse Sonic hedgehog protein. Would that mouse protein have any effect on a shark or a skate?
There are two extreme outcomes to an experiment like this. One is that nothing happens. This would mean that skates are so different from mice that Sonic hedgehog protein has no effect. The other extreme outcome would present a stunning example of our inner fish. This outcome would be that the rods develop differently from one another, demonstrating that Sonic hedgehog does something similar in skates and in us. And let’s not forget that since Randy is using the protein from a mammal, it means that the genetic recipe would be really, really similar.
Not only did the rods end up looking different from one another, they responded to Sonic hedgehog, much as fingers do, on the basis of how close they were to the Sonic hedgehog bead: the closer rods developed a different shape from the ones farther away. To top matters off, it was the mouse protein that did the job so effectively in the skates.
Normal fins (left) and Randy’s treated fins. The treated fins showed a mirror-image duplication just as chicken wings did. Photographs courtesy of Randall Dahn, University of Chicago.
The “inner fish” that Randy found was not a single bone, or even a section of the skeleton. Randy’s inner fish lay in the biological tools that actually build fins. Experiment after experiment on creatures as different as mice, sharks, and flies shows us that the lessons of Sonic hedgehog are very general. All appendages, whether they are fins or limbs, are built by similar kinds of genes. What does this mean for the problem we looked at in the first two chapters—the transitio
n of fish fins into limbs? It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes.
But there is a deeper beauty to these experiments on limbs and fins. Tabin’s lab used work in flies to find a gene in chickens that tells us about human birth defects. Randy used the Tabin lab discovery to tell us something about our connections to skates. An “inner fly” helped find an “inner chicken,” which ultimately helped Randy find an “inner skate.” The connections among living creatures run deep.
CHAPTER FOUR
TEETH EVERYWHERE
The tooth gets short shrift in anatomy class: we spend all of five minutes on it. In the pantheon of favorite organs—I’ll leave it to each of you to make your list—teeth rarely reach the top five. Yet the little tooth contains so much of our connection to the rest of life that it is virtually impossible to understand our bodies without knowing teeth. Teeth also have special significance for me, because it was in searching for them that I first learned how to find fossils and how to run a fossil expedition.
The job of teeth is to make bigger creatures into smaller pieces. When attached to a moving jaw, teeth slice, dice, and macerate. Mouths are only so big, and teeth enable creatures to eat things that are bigger than their mouths. This is particularly true of creatures that do not have hands or claws that can shred or cut things before they get to the mouth. True, big fish tend to eat littler fish. But teeth can be the great equalizer: smaller fish can munch on bigger fish if they have good teeth. Smaller fish can use their teeth to scrape scales, feed on particles, or take out whole chunks of flesh from bigger fish.