Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (18 page)

One axis down, another to go. Do sea anemones have anything analogous to our belly-to-back axis? Sea anemones don’t seem to have anything comparable. Despite this, Martindale and his colleagues took the bold step of searching in the sea anemone for the genes that specify our belly-to-back axis. They knew what our genes looked like, and this gave them a search image. They uncovered not one, but many different belly-to-back genes in the sea anemone. But although these genes were active along an axis in the sea anemone, that axis didn’t seem to correlate with any pattern in how the adult animal’s organs are put together.

Jellyfish relatives, such as sea anemones, have a front and a back as we do, a body plan set up by versions of the same genes.

Just what this hidden axis could be is not apparent from the outside of the animal. If we cut one in half, however, we find an important clue, another axis of symmetry. Called the directive axis, it seems to define two distinct sides of the creature, almost a left and a right. This obscure axis was known to anatomists back in the 1920s but remained a curiosity in the scientific literature. Martindale, Finnerty, and their team changed that.

All animals are the same but different. Like a cake recipe passed down from generation to generation—with enhancements to the cake in each—the recipe that builds our bodies has been passed down, and modified, for eons. We may not look much like sea anemones and jellyfish, but the recipe that builds us is a more intricate version of the one that builds them.

Powerful evidence for a common genetic recipe for animal bodies is found when we swap genes between species. What happens when you swap a body-building gene from an animal that has a complex body plan like ours with one from a sea anemone? Recall the gene
Noggin,
which in frogs, mice, and humans is turned on in places that will develop into back structures. Inject extra amounts of frog Noggin into a frog egg, and the frog will grow extra back structures, sometimes even a second head. In sea anemone embryos, a version of
Noggin
is also turned on at one end of the directive axis. Now, the million-dollar experiment: take the product of Noggin from a sea anemone and inject it into a frog embryo. The result: a frog with extra back structures, almost the same result as if the frog were injected with its own Noggin.

Now, though, as we go back in time, we are left with what looks like a huge gap. Everything in this chapter had a body. How do we compare ourselves with things that have no bodies at all—with single-celled microbes?

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 dyes. It was like looking at creatures made of glass.

During these long hours at the microscope, I was literally watching an animal being built. The earliest embryos would have tiny little limb buds and the cells inside would be evenly spaced. Then, at later stages, the cells would clump inside the limb bud. In successively older embryos, the cells would take different shapes and the bones would form. Each of those clumps I saw during the early stages became a bone.

It is hard not to feel awestruck watching an animal assemble itself. Just like a brick house, a limb is built by smaller pieces joining to make a larger structure. But there is a huge difference. Houses have a builder, somebody who actually knows where all the bricks need to go; limbs and bodies do not. The information that builds limbs is not in some architectural plan but is contained within each cell. Imagine a house coming together spontaneously from all the information contained in the bricks: that is how animal bodies are made.

Much of what makes a body is locked inside the cell; in fact, much of what makes us unique is there, too. Our body looks different from that of a jellyfish because of the ways our cells attach to one another, the ways they communicate, and the different materials they make.

Before we could even have a “body plan”—let alone a head, brain, or arm—there had to be a way to make a body in the first place. What does this mean? To make all of a body’s tissues and structures, cells had to know how to cooperate—to come together to make an entirely new kind of individual.

To understand the meaning of this, let’s first consider what a body is. Then, let’s address the three great questions about bodies: When? How? And Why? When did bodies arise, how did they come about, and, most important, why are there bodies at all?

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 up with a dead human or fish, depending on which cells you remove.

So the thought experiment reveals one of the defining features of bodies: our component parts work together to make a greater whole. But not all parts of bodies are equal; some parts are absolutely required for life. Moreover, in bodies, there is a division of labor between parts; brains, hearts, and stomachs have distinct functions. This division of labor extends to the smallest levels of structure, including the cells, genes, and proteins that make bodies.

The body of a worm or a person has an identity that the constituent parts—organs, tissues, and cells—lack. Our skin cells, for example, are continually dividing, dying, and being sloughed off. Yet you are the same individual you were seven years ago, even though virtually every one of your skin cells is now different: the ones you had back then are dead and gone, replaced by new ones. The same is true of virtually every cell in our bodies. Like a river that remains the same despite changes in its course, water content, even size, we remain the same individuals despite the continual turnover of our parts.

And despite this continual change, each of our organs “knows” its size and place in the body. We grow in the correct proportions because the growth of the bones in our arms is coordinated with the growth of the bones in our fingers and our skulls. Our skin is smooth because cells can communicate to maintain its integrity and the regularity of its surface. Until something out of the ordinary happens, like, for instance, we get a wart. The cells inside the wart aren’t following the rules: they do not know when to stop growing.

When the finely tuned balance among the different parts of bodies breaks down, the individual creature can die. A cancerous tumor, for example, is born when one batch of cells no longer cooperates with others. By dividing endlessly, or by failing to die properly, these cells can destroy the necessary balance that makes a living individual person. Cancers break the rules that allow cells to cooperate with one another. Like bullies who break down highly cooperative societies, cancers behave in their own best interest until they kill their larger community, the human body.

What made all this complexity possible? For our distant ancestors to go from single-celled creatures to bodied ones, as they did over a billion years ago, their cells had to utilize new mechanisms to work together. They needed to be able to communicate with one another. They needed to be able to stick together in new ways. And they needed to be able to make new things, such as the molecules that make our organs distinct. These features—the glue between cells, the ways cells can “talk” to each other, and the molecules that cells make—constitute the toolkit needed to build all the different bodies we see on earth.

The invention of these tools amounted to a revolution. The shift from single-celled animals to animals with bodies reveals a whole new world. New creatures with whole new capabilities came about: they got big, they moved around, and they developed new organs that helped them sense, eat, and digest their world.

DIGGING UP BODIES

Here’s a humbling thought for all of us worms, fish, and humans: most of life’s history is the story of single-celled creatures. Virtually everything we have talked about thus far—animals with hands, heads, sense organs, even body plans—has been around for only a small fraction of the earth’s history. Those of us who teach paleontology often use the analogy of the “earth year” to illustrate how tiny that fraction is. Take the entire 4.5-billion-year history of the earth and scale it down to a single year, with January 1 being the origin of the earth and midnight on December 31 being the present. Until June, the only organisms were single-celled microbes, such as algae, bacteria, and amoebae. The first animal with a head did not appear until October. The first human appears on December 31. We, like all the animals and plants that have ever lived, are recent crashers at the party of life on earth.

The vastness of this time scale becomes abundantly clear when we look at the rocks in the world. Rocks older than 600 million years are generally devoid of animals or plants. In them we find only single-celled creatures or colonies of algae. These colonies form mats or strands; some colonies are doorknob-shaped. In no way are these to be confused with bodies.