The Inner Chaos Of The Head

Head anatomy is not only complicated but hard to see, since, unlike other parts of the body, the tissues of the head are encapsulated in a bony box. We literally have to saw through the cheek, forehead, and cranium to see the vessels and organs. Having thus opened a human head, we find a clump of what looks like tangled fishing lines. Vessels and nerves make curious loops and turns as they travel through the skull. Thousands of nerve branches, muscles, and bones sit within this small box. At...

Longer Walk Through The

As we've seen, our bodies are not put together at random. Here, I use the word random in a very specific sense I mean that the structure of our bodies is definitely not random with respect to the other animals that walk, fly, swim, or crawl across this earth. Some animals share part of our structure others do not. There is order to what we share with the rest of the world. We have two ears, two eyes, one head, a pair of arms, and a pair of legs. We do not have seven legs or two heads. Nor do we...

Dna And The Organizer

At the time when Spemann won the Nobel Prize, the Organizer was all the rage. Scientists sought the mysterious chemical that could induce the entire body plan. But just as popular culture has yo-yos and Tickle Me Elmo dolls, so science has fads that wax and wane. By the 1970s, the Organizer was viewed as little more than a curiosity, a clever anecdote in the history of embryology. The reason for this fall from grace was that no one could decipher the mechanisms that made it work. The discovery...

The Inner Eargels Moving And Hairs Bending

Move through the external ear, go deeper inside, past the eardrum and three middle ear bones, and you end up deep inside the skull. Here you will find the inner ear tubes and some gel-filled sacs. In humans, as in other mammals, the bony tubes take the snail-shell shape that is so strikingly apparent in the anatomy lab. The inner ear has different parts dedicated to different functions. One part is used in hearing, another in telling us which way our head is tilted, and still another in...

Vision

Vl nly once in my entire career have I found the eye of a fossil creature. I wasn't in the field on an expedition, I was in the back room of a mineral shop in a small town in northeast China. My colleague Gao Keqin and I were studying the earliest known salamanders, beautiful fossils collected from Chinese rocks about 160 million years old. We had just finished a collecting trip to some sites Gao knew about. The locations were secret, because these salamander fossils have serious monetary value...

Shark Past Hernias

Our propensity for hernias, at least for those hernias near the groin, results from taking a fish body and morphing it into a mammal. Fish have gonads that extend toward their chest, approaching their heart. Mammals don't, and therein lies the problem. It is a very good thing that our gonads are not deep in our chest and near our heart although it might make reciting the Pledge of Allegiance a different experience . If our gonads were in our chest, we wouldn't be able to reproduce. Slit the...

Of Flies And

Von Baer watched embryos develop, compared one species to another, and saw fundamental patterns in bodies. Mangold and Spemann physically distorted embryos to learn how their tissues build bodies. In the DNA age, we can ask questions about our own genetic makeup. How do our genes control the development of our tissues and our bodies If you ever thought that flies are unimportant, consider this mutations in flies gave us important clues to the major body plan genes active in human embryos. We...

Chapter Seven Adventures In Bodybuilding

W hen I wasn't out in the field collecting fossils, much of my graduate career was spent staring into a microscope, looking at how cells come together to make bones. I would take the developing limb of a salamander or a frog, and stain the cells with dyes that turn developing cartilage blue and bones red. I could then make the rest of the tissues clear by treating the limb with glycerin. These were beautiful preparations the embryo entirely clear and all the bones radiating the colors of the...

Habeas Corpus Show Me The Body

Not every clump of cells can be awarded the honor of being called a body. A mat of bacteria or a group of skin cells is a very different thing from an array of cells that we would call an individual. This is an essential distinction a thought experiment will help us see the difference. What happens if you take away some bacteria from a mat of bacteria You end up with a smaller mat of bacteria. What happens when you remove some cells of a human or fish, say from the heart or brain You could end...

Teeth And Bonesthe Hard Stuff

Ostracoderm

It almost goes without saying that what makes teeth special among organs is their hardness. Teeth have to be harder than the bits of food they break down imagine trying to cut a steak with a sponge. In many ways, teeth are as hard as rocks, and the reason is that they contain a crystal molecule on the inside. That molecule, known as hydroxyapatite, impregnates the molecular and cellular infrastructure of both teeth and bones, making them resistant to bending, compression, and other stresses....

What we discover on our walk through the zoo mirrors how fossils are laid out in the rocks of the world

Let's now return to our problem of how to find relatives of the first fish to walk on land. In our grouping scheme, these creatures are somewhere between the Everythings and the Everythings with limbs. Map this to what we know of the rocks, and there is strong geological evidence that the period from 380 million to 365 million years ago is the critical time. The younger rocks in that range, those about 360 million years old, include diverse kinds of fossilized animals that we would all...

The Essence In Embryos

Arches Embryos

Nobody starts life with a head sperm and egg come together to make a single cell. Between the moment of conception and the third week thereafter, we go from that single cell to a ball of cells, then to a Frisbee-shaped collection of cells, then to something that looks vaguely like a tube and includes different kinds of tissues. Between the twenty-third and twenty-eighth days after conception, the front end of the tube thickens and folds over the body, so the embryo looks as if it's already...

The Middle Earthe Three Ear Bones

Fish Amphibian Reptile Mammal Brain

With hair and milk-producing glands, we can easily be distinguished from other creatures. It surprises most people to learn that some of the most distinctive traits of mammals lie inside the ear. The bones of the mammalian middle ear are like those of no other animal mammals have three bones, whereas reptiles and amphibians have only one. Fish have none at all. Where did our middle ear bones come from Some anatomy recall that our three middle ear bones are known as the...

Chapter Four

T he 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...

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...

Epilogue

As a parent of two young children, I find myself spending a lot of time lately in zoos, museums, and aquaria. Being a visitor is a strange experience, because I've been involved with these places for decades, working in museum collections and even helping to prepare exhibits on occasion. During family trips, I've come to realize how much my vocation can make me numb to the beauty and sublime complexity of our world and our bodies. I teach and write about millions of years of history and about...

This figure says it all Tiktaalik is intermediate between fish and primitive landliving animal

As the discoverers of the creature, Ted, Farish, and I had the privilege of giving it a formal scientific name. We wanted the name to reflect the fish's provenance in the Nunavut Territory of the Arctic and the debt we owed to the Inuit people for permission to work there. We engaged the Nunavut Council of Elders, formally known as the Inuit Qaujimajatuqangit Katimajiit, to come up with a name in the Inuktitut language. My obvious concern was that a committee named Inuit Qaujimajatuqangit...

Our Inner Shark

The take-home message of many a lawyer joke is that lawyers are an especially voracious kind of shark. Teaching embryology during one of the recurring vogues for these jokes, I remember thinking that the joke is on all of us. We're all modified sharks or, worse, there is a lawyer inside each of us. As we've seen, much of the secret of heads lies in the arches, the swellings that gave us the road map for the complicated cranial nerves and key structures inside the head. Those...

The Bestlaid Body Plans

W e are a package of about two trillion cells assembled in a very precise way. Our bodies exist in three dimensions, with our cells and organs in their proper places. The head is on top. The spinal cord is toward our back. Our guts are on the belly side. Our arms and legs are to the sides. This basic architecture distinguishes us from primitive creatures organized as clumps or disks of cells. The same design is also an important part of the bodies of other creatures. Like us, fish, lizards, and...

An Inner Sea Anemone

It is one thing to compare our bodies with those of frogs and fish. In a real sense we and they are much alike we all have a backbone, two legs, two arms, a head, and so on. What if we compare ourselves with something utterly different, for example jellyfish and their relatives Most animals have body axes defined by their direction of movement or by where their mouth and anus lie relative to each other. Think about it our mouth is on the opposite end of the body from our anus and, as in fish...

The development of a limb in this case a chicken wing All of the key stages in the development of a wing skeleton

To study how this pattern emerges, we need to look at embryos and sometimes interfere with their development to assess what happens when things go wrong. Moreover, we need to look at mutants and at their internal structures and genes, often by making whole mutant populations through careful breeding. Obviously, we cannot study humans in these ways. The challenge for the pioneers in this field was to find the animals that could be useful windows into our own development. The first experimental...

The Common Plan Comparing Embryos

Human Blastocyst

I entered graduate school to study fossil mammals and ended up three years later studying fish and amphibians for my dissertation. My fall from grace, if you want to call it that, happened when I started to look at embryos. We had a lot of embryos in the lab salamander larvae, fish embryos, even fertilized chicken eggs. I'd routinely pop them under the microscope to see what was going on. The embryos of all the species looked like little whitish batches of cells, no more than an eighth of an...

How Is The Human Body Like A

My professional introduction to academia happened in the early 1980s, during my college years, when I volunteered at the American Museum of Natural History in New York City. Aside from the excitement of working behind the scenes in the collections of the museum, one of the most memorable experiences was attending their raucous weekly seminars. Each week a speaker would come to present some esoteric study on natural history. Following the presentation, often a fairly low-key affair, the...

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...

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...

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....

Gill Arch Genes

During the first three weeks after conception, whole batteries of genes are turned on and off in our gill arches and throughout the tissues that will become our future brain. These genes instruct cells to make the different portions of our head. Think of each region of our head as gaining a genetic address that makes it distinctive. Modify this genetic address and we can modify the kinds of structures that develop there. For example, a gene known as Otx is active in the front region, where the...

Jellyfish And The Origins Of Eyes And Ears

Just like Pax 6, which we discussed earlier in connection with eyes, Pax 2 in ears is a major gene, essential for proper development. Interestingly, a link between Pax 2 and Pax 6 suggests that ears and eyes might have had a very ancient common history. This is where the box jellyfish enters our story. Well known to swimmers in Australia because they have particularly poisonous venom, these jellyfish are different from most others in that they have eyes, more than twenty of them. Most of these...