by Neil Shubin
TRACING HEADS: FROM HEADLESS WONDERS TO OUR HEADED ANCESTORS
Why stop at frogs and sharks? Why not extend our comparison to other creatures, like insects or worms? But why would we do this when none of these creatures has a skull, much less cranial nerves? None of them even has bones. When we leave fish for worms, we get to a very soft and headless world. Bits of ourselves are there, though, if you look closely.
Those of us who teach comparative anatomy to undergraduates usually begin the course with a slide of Amphioxus. Every September, hundreds of Amphioxus slides appear on screens in college lecture halls from Maine to California. Why? Remember the simple dichotomy between invertebrates and vertebrates? Amphioxus is a worm, an invertebrate, that shares many features with backboned animals such as fish, amphibians, and mammals. Amphioxus lacks a backbone, but like all creatures with backbones, it has a nerve cord that runs along its back. In addition, a rod runs the length of its body, parallel to the nerve cord. This rod, known as the notochord, is filled with a jelly-like substance and provides support for the body. As embryos, we have a notochord, too, but unlike Amphioxus’s, ours breaks up and ultimately becomes part of the disks that lie between our vertebrae. Rupture a disk and the jelly-like substance of what was once a notochord can wreak havoc when it pinches nerves or interferes with the ability of one vertebra to move along the next. When we injure a disk, a very ancient part of our body plan is rupturing. Thanks a lot, Amphioxus.
The closest relatives to animals with heads are worms with gill slits. Shown are Amphioxus and a reconstruction of a fossil worm (Haikouella) over 530 million years old. Both worms have a notochord, a nerve cord, and gill slits. The fossil worm is known from over three hundred individual specimens from southern China.
Amphioxus is not unique among worms. Some of the best examples are not in the oceans of today but in ancient rocks of China and Canada. Buried in sediments over 500 million years old are small worms that lack heads, complex brains, or cranial nerves. They may not look like much, being small smudges in the rock, but the preservation of these fossils is incredible. When you look under a microscope, you find beautifully preserved impressions that display their soft anatomy in fine detail, occasionally even with impressions of skin. They show something else wonderful, too. They are the earliest creatures with notochords and nerve cords. These worms are telling us something about the origin of parts of our bodies.
But there is something else we share with these little worms: gill arches. Amphioxus, for example, has them in abundance, and associated with each arch is a little bar of cartilage. Like the cartilages that form our jaws, our ear bones, and parts of our voice box, these rods support the gill slit. The essence of our head goes back to worms, organisms that do not even have a head. What does Amphioxus do with the gill arches? It pumps water through them to filter out little particles of food. From so humble a beginning comes the basic structures of our own head. Just as teeth, genes, and limbs have been modified and their functions repurposed over the ages, so, too, has the basic structure of our head.
CHAPTER SIX
THE BEST-LAID (BODY) PLANS
We 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 cows have bodies that are symmetrical with a front/back, top/bottom, and left/right. Their front ends (corresponding to the top of an upright human) all have heads, with sense organs and brains inside. They have a spinal cord that runs the length of the body along the back. Also like us, they have an anus, which is at the opposite end of their bodies from the mouth. The head is on the forward end, in the direction they typically swim or walk. As you can imagine, “anus-forward” wouldn’t work very well in most settings, particularly aquatic ones. Social situations would be a problem, too.
It is more difficult to find our basic design in really primitive animals—jellyfish, for example. Jellyfish have a different kind of body plan: their cells are organized into disks that have a top and bottom. Lacking a front and back, a head and tail, and a left and right, jellyfish body organization appears very different from our own. Don’t even bother trying to compare your body plan with a sponge. You could try, but the mere fact that you were trying would reveal something more psychiatric than anatomical.
To properly compare ourselves with these primitive animals, we need some tools. Just as with heads and limbs, our history is written within our development from egg to adult. Embryos hold the clues to some of the profound mysteries of life. They also have the ability to derail my plans.
THE COMMON PLAN: COMPARING EMBRYOS
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 inch long. It was exciting watching development progress; as the embryo got bigger, the yolk, its food supply, got smaller and smaller. By the time the yolk was gone, the embryo was usually big enough to hatch.
Watching the process of development brought about a huge intellectual transformation in me. From such simple embryonic beginnings—small blobs of cells—came wonderfully complex birds, frogs, and trout comprising trillions of cells arranged in just the right way. But there was more. The fish, amphibian, and chicken embryos were like nothing I had ever seen before in biology. They all looked generally alike. All of them had a head with gill arches. All of them had a little brain that began its development with three swellings. All of them had little limb buds. In fact, the limbs were to become my thesis, the focus of my next three years’ work. Here, in comparing how the skeleton developed in birds, salamanders, frogs, and turtles, I was finding that limbs as different as bird wings and frog legs looked very similar during their development. In seeing these embryos, I was seeing a common architecture. The species ended up looking different, but they started from a generally similar place. Looking at embryos, it almost seems that the differences among mammals, birds, amphibians, and fish simply pale in comparison with their fundamental similarities. Then I learned of the work of Karl Ernst von Baer.
In the 1800s, some natural philosophers looked to embryos to try to find the common plan for life on earth. Paramount among these observers was Karl Ernst von Baer. Born to a noble family, he initially trained to be a physician. His academic mentor suggested that he study chicken development and try to understand how chicken organs developed.
Unfortunately, von Baer could not afford incubators to work on chickens, nor could he afford many eggs. This was not very promising. Lucky for him, he had an affluent friend, Christian Pander, who could afford to do the experiments. As they looked at embryos, they found something fundamental: all organs in the chicken can be traced to one of three layers of tissue in the developing embryo. These three layers became known as the germ layers. They achieved almost legendary status, which they retain even to this day.
Pander’s three layers gave von Baer the means to ask important questions. Do all animals share this pattern? Are the hearts, lungs, and muscles of all animals derived from these layers? And, importantly, do the same layers develop into the same organs in different species?
Von Baer compared the three layers of Pander’s chicken embryos with everything else he could get his hands on: fish, reptiles, and mammals. Yes, every animal organ originated in one of these three layers. Significantly, the three layers formed the same structur
es in every species. Every heart of every species formed from the same layer. Another layer gave rise to every brain of every animal. And so on. No matter how different the species look as adults, as tiny embryos they all go through the same stages of development.
To fully appreciate the importance of this, we need to look again at our first three weeks after conception. At the moment of fertilization, major changes happen inside the egg—the genetic material of the sperm and egg fuses and the egg begins to divide. Ultimately, the cells form a ball. In humans, over about five days, the single-cell body divides four times, to produce a ball of sixteen cells. This ball of cells, known as a blastocyst, resembles a fluid-filled balloon. A thin spherical wall of cells surrounds some fluid in the center. At this “blastocyst stage” there still does not appear to be any body plan—there is no front and back, and certainly there are not yet any different organs or tissues. On about the sixth day after conception, the ball of cells attaches to its mother’s uterus and begins the process of connecting to it so that mother and embryo can join bloodstreams. There is still no evidence of the body plan. It is a far cry from this ball of cells to anything that you’d recognize as any mammal, reptile, or fish, much less a human.
If we are lucky, our ball of cells has implanted in our mother’s uterus. When a blastocyst implants in the wrong place—when there is an “ectopic implantation”—the results can be dangerous. About 96 percent of ectopic implantations happen in the uterine (or fallopian) tubes, near where conception happens. Sometimes mucus blocks the easy passage of the blastocyst to the uterus, causing it to implant improperly in the tubes. Ectopic pregnancy can cause various tissue ruptures if not caught in time. In really rare cases, the blastocyst is expelled into the mother’s body cavity, the space between her guts and body wall. In even rarer cases, these blastocysts will implant on the outside lining of the mother’s rectum or uterus and the fetus develops to full term! Although these fetuses can sometimes be delivered by an abdominal incision, such implantation is generally very dangerous because it increases the risk of maternal death by bleeding by a factor of 90, as compared with a normal implantation inside the uterus.
In any event, at this stage of development we are extremely humble-looking creatures. Around the beginning of our second week after conception, the blastocyst has implanted, with one part of the ball embedded in the wall of the uterus, and the other free. Think of a balloon pushed into a wall: this flattened disk becomes the human embryo. Our entire body forms from only the top part of this ball, the part that is mushed into the wall. The part of the blastocyst below the disk covers the yolk. At this stage of development, we look like a Frisbee, a simple two-layered disk.
How does this oval Frisbee end up with von Baer’s three germ layers and go on to look anything like a human? First, cells divide and move, causing tissues to fold in on themselves. Eventually, as tissues move and fold, we become a tube with a folded swelling at the head end and another at the tail. If we were to cut ourselves in half right about now, we would find a tube within a tube. The outer tube would be our body wall, the inner tube our eventual digestive tract. A space, the future body cavity, separates the two tubes. This tube-within-a-tube structure stays with us our entire lives. The gut tube gets more complicated, with a big sac for a stomach and long intestinal twists and turns. The outer tube is complicated by hair, skin, ribs, and limbs that push out. But the basic plan persists. We may be more complicated than we were at twenty-one days after conception, but we are still a tube within a tube, and all of our organs derive from one of the three layers of tissue that appeared in our second week after conception.
The names of these three all-important layers are derived from their position: the outer layer is called ectoderm, the inner layer endoderm, and the middle layer mesoderm. Ectodermformsmuch of the outer part of the body (the skin) and the nervous system. Endoderm, the inside layer, forms many of the inner structures of the body, including our digestive tract and numerous glands associated with it. The middle layer, the mesoderm, forms tissue in between the guts and skin, including much of our skeleton and our muscles. Whether the body belongs to a salmon, a chicken, a frog, or a mouse, all of its organs are formed by endoderm, ectoderm, and mesoderm.
Our early days, the first three weeks after conception. We go from being a single cell to a ball of cells and end up as a tube.
Von Baer saw how embryos reveal fundamental patterns of life. He contrasted two kinds of features in development: features shared by every species, and features that vary from species to species. Features such as the tube-within-a-tube arrangement are shared by all animals with a backbone: fish, amphibians, reptiles, birds, and mammals. These common features appear relatively early in development. The features that distinguish us—bigger brains in humans, shells on turtles, feathers on birds—arise relatively later.
Von Baer’s approach is very different from the “ontogeny recapitulates phylogeny” idea you might have learned in school. Von Baer simply compared embryos and noted that the embryos of different species looked more similar to each other than do the adults of those species. The “ontogeny recapitulates phylogeny” approach championed decades later by Ernst Haeckel made the claim that each species tracked its evolutionary history as it proceeded through development. Accordingly, the embryo of a human went through a fish, a reptile, and a mammal stage. Haeckel would compare a human embryo to an adult fish or a lizard. The differences between the ideas of von Baer and Haeckel might seem subtle, but they are not. In the past one hundred years, time and new evidence have treated von Baer much more kindly. In comparing embryos of one species to adults of another, Haeckel was comparing apples to oranges. A more meaningful comparison is one where we can ultimately uncover the mechanisms that drive evolution. For that, we compare embryos of one species to embryos of another. The embryos of different species are not completely identical, but their similarities are profound. All have gill arches, notochords, and look like a tube within a tube at some stage of their development. And, importantly, embryos as distinct as fish and people have Pander and von Baer’s three germ layers.
At four weeks after conception, we are a tube within a tube and have the three germ layers that give rise to all our organs.
All of these comparisons lead us to the real issue at stake. How does the embryo “know” to develop a head at the front end and an anus at the back? What mechanisms drive development and make cells and tissues able to form bodies?
To answer these questions required a whole new approach. Rather than simply comparing embryos as in von Baer’s day, we had to find a new way of analyzing them. The latter part of the nineteenth century ushered in the era, which we first discussed in Chapter 3, when embryos were chopped, grafted, split, and treated with virtually every kind of chemical imaginable. All in the name of science.
EXPERIMENTING WITH EMBRYOS
Biologists at the turn of the twentieth century were grappling with fundamental questions about bodies. Where in the embryo does the information to build them lie? Is this information contained in every cell or in patches of cells? And what form does this information take—is it a special kind of chemical?
Beginning in 1903, the German embryologist Hans Spemann began to investigate how cells learned to build bodies during development. His goal was to find where the body-building information resides. The big question for Spemann was whether all the cells in the embryo have enough information to build whole bodies, or whether that information is confined to certain parts of the developing embryo.
Working with newt eggs, which were easy to obtain and relatively easy to fiddle with in the lab, Spemann devised a clever experiment. He cut off a strand of his infant daughter’s hair and made a miniature lasso out of it. Baby hair is remarkable stuff; soft, thin, and pliant, it made the ideal material for tying up a tiny sphere such as a newt egg. Spemann did exactly that to a developing newt egg, pinching one side off from the other. Manipulating the nuclei of the cells a bit, he let the resulting contrapt
ion develop and watched what happened. The embryo formed twins: two complete salamanders emerged, each with a normal body plan and each entirely viable. The conclusion was obvious: from one egg can come more than one individual. This is what identical twins are. Biologically, Spemann had demonstrated that in the early embryo some cells have the capacity to form a whole new individual on their own.
This experiment was only the beginning of a whole new phase of discovery.
In the 1920s Hilde Mangold, a graduate student in Spemann’s laboratory, started to work with small embryos. The fine control she had of her fingers made her able to do some incredibly demanding experiments. At the stage of development with which Mangold worked, the salamander embryo is a sphere about a sixteenth of an inch in diameter. She lopped off a tiny piece of tissue, smaller than a pinhead, from one part of the embryo and grafted it onto the embryo of another species. What Mangold transplanted wasn’t just any patch, but an area where cells that were to form much of the three germ layers were moving and folding. Mangold was so skilled that the grafted embryos actually continued to develop, giving her a pleasant surprise. The grafted patch led to the formation of a whole new body, including a spinal cord, back, belly, even a head.
Just by moving a small patch of tissue in the embryo, Mangold produced twins.
Why is all this important? Mangold had discovered a small patch of tissue that was able to direct other cells to form an entire body plan. The tiny, incredibly important patch of tissue containing all this information was to be known as the Organizer.
