Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (20 page)

The ratios among the various materials define much of the mechanical differences among bone, cartilage, and teeth. Teeth are very hard and, predictably, there is lots of hydroxyapatite and relatively little collagen between the cells in the enamel. Bone has relatively more collagen, less hydroxyapatite, and no enamel. Consequently, it is not as hard as teeth. Cartilage has lots of collagen and no hydroxyapatite, and is loaded with proteoglycans. It is the softest of the tissues in our skeleton. One of the main reasons our skeletons look and work as they do is that these molecules are deployed in the right places in the right proportions.

What does all this have to do with the origin of bodies? One property is common to animals, whether they have skeletons or not: all of them, including clumps of cells, have molecules that lie between their cells, specifically different kinds of collagens and proteoglycans. Collagen seems particularly important: the most common protein in animals, it makes up over 90 percent of the body’s protein by weight. Bodybuilding in the distant past meant that molecules like these had to be invented.

Something else is essential for bodies: the cells in our bones have to be able to stick together and talk to one another. How do bone cells attach to one another, and how do different parts of bone know to behave differently? Here is where much of our bodybuilding kit lies.

Bone cells, like every cell in our bodies, stick to one another by means of tiny molecular rivets, of which there is a vast diversity. Some bind cells the way contact cement holds the soles of shoes together: one molecule is firmly attached to the outer membrane of one cell, another to the outer membrane of a neighboring cell. Thus attached to both cell membranes, the glue forms a stable bond between the cells.

Other molecular rivets are so precise that they bind selectively, only to the same kind of rivet. This is a hugely significant feature because it helps organize our bodies in a fundamental way. These selective rivets enable cells to organize themselves and ensure that bone cells stick to bone cells, skin to skin, and so on. They can organize our bodies in the absence of other information. If we put a number of cells, each with a different kind of this type of rivet, on a dish and let the cells grow, the cells will organize themselves. Some might form balls, others sheets, as the cells sort out by the numbers and kinds of rivets they have.

But arguably the most important connection between cells lies in the ways that they exchange information with one another. The precise pattern of our skeleton, in fact of our whole body, is possible only because cells know how to behave. Cells need to know when to divide, when to make molecules, and when to die. If, for example, bone or skin cells behaved randomly—if they divided too much or died too little—then we would be very ugly or, worse, very dead.

Cells communicate with one another using “words” written as molecules that move from cell to cell. One cell can “talk” to the next by sending molecules back and forth. For instance, in a relatively simple form of cell-to-cell communication, one cell will emit a signal, in this case a molecule. This molecule will attach to the outer covering, or membrane, of the cell receiving the signal. Once attached to the outer membrane, the molecule will set off a chain reaction of molecular events that travels from the outer membrane all the way, in many cases, to the nucleus of the cell. Remember that the genetic information sits inside the nucleus. Consequently, this molecular signal can cause genes to be turned on and off. The end result of all this is that the cell receiving the information now changes its behavior: it may die, divide, or make new molecules in response to the cue from the other cell.

At the most basic level, these are the things that make bodies possible. All animals with bodies have structural molecules like collagens and proteoglycans, all of them have the array of molecular rivets that hold cells together, and all of them have the molecular tools that allow cells to communicate with one another.

We now have a search image to understand the how of body origins. To see how bodies arose, we need to look for these molecules in the most primitive bodies on the planet, and then, ultimately, in creatures that have no body at all.

BODYBUILDING FOR BLOBS

What does the body of a professor share with a blob? Let’s look at some of the most primitive bodies alive today to find the answer.

One of these creatures has the dubious distinction of almost never being seen in the wild. In the late 1880s, a strangely simple creature was discovered living on the glass walls of an aquarium. Unlike anything else alive, it looked like a mass of goo. The only thing we can compare it with is the alien creature in the Steve McQueen movie
The Blob.
Recall that the Blob was an amorphous glop that, after dropping in from outer space, engulfed its prey: dogs, people, and eventually small diners in little towns in Pennsylvania. The Blob’s digestive end was on its underside: we never saw it; we only heard the shrieks of creatures caught there. Shrink the Blob down to between 200 and 1,000 cells, about two millimeters in diameter, and we have the enigmatic living creature known as a placozoan. Placozoans have only four types of cells, which make a very simple body shaped like a small plate. It is a real body, though. Some of the cells on the undersurface are specialized for digestion; others have flagella, which beat to move the creature around. We have little idea of what they eat in the wild, where they live, or what their natural habitat is. Yet these simple blobs reveal something terrifically important: with a small number of specialized cells, these primitive creatures already have a division of labor among their parts.

Much of what is interesting about bodies already exists in placozoans. They have true bodies, albeit primitively organized ones. In searching through their DNA and examining the molecules on the surface of their cells, we find that much of our bodybuilding apparatus is already there. Placozoans have versions of the molecular rivets and cell communication tools we see in our own bodies.

Our bodybuilding apparatus is found in blobs simpler than some of Reginald Sprigg’s ancient impressions. Can we go further, to even more primitive kinds of bodies? Part of the answer lies in a piece of classic kitchenware: the sponge. At first glance, sponges are unremarkable. The body of a sponge consists of the sponge matrix itself; not a living material, it is a form of silica (glassy material) or calcium carbonate (a hard shell-like material) with some collagen interspersed. Right off the bat, that makes sponges interesting. Recall that collagen is a major part of our intercellular spaces, holding cells and many tissues together. Sponges may not look it, but they already have one of the earmarks of bodies.

In the early 1900s, H.V.P. Wilson showed just how amazing sponges really are. Wilson came to the University of North Carolina as its first professor of biology in 1894. There he went on to train a cadre of American biologists who were to define the field of genetics and cell biology in North America for the next century. As a young man, Wilson decided to focus his life’s research on, of all things, sponges. One of his experiments revealed a truly remarkable capability of these apparently simple creatures. He ran them through a kind of sieve, which broke them down to a set of disaggregated cells. Wilson put the now completely disaggregated, amoeba-like cells in a dish and watched them. At first, they crawled around on the surface of the dish. Then, something surprising happened: the cells came together. First, they formed red cloudy balls of cells. Next, they gained more organization, with cells becoming packed in definite patterns. Finally, the clump of cells would form an entire new sponge body, with the various types of cells assuming the appropriate positions. Wilson was watching a body come together almost from scratch. If we were like sponges, then the Steve Buscemi character who gets minced in the woodchipper in the Coen brothers’ movie
Fargo
would have been just fine. In fact, he might have been invigorated by the experience, as his cells might have aggregated to form many different versions of him.

It is the cells within sponges that make them useful in understanding the origin of bodies. The inside of the sponge is usually a hollow space that can be divided into compartments, depending on the species. Water flows through the space, directed by a very special kind of cell. These cells are shaped like goblets with the cup part facing the inside of the sponge. Tiny cilia extending from the rim of the goblet beat and capture food particles in the water. Also extending from the goblet part of each of these cells is a large flagellum. The concerted action of the flagella of these little beater cells moves water and food through the pores of the sponge. Other cells on the inside of the sponge process the particles of food. Still others line the outside and can contract when the sponge needs to change its shape as water currents change.

A sponge seems a far cry from a body, yet it has many of the most important properties of bodies: its cells have a division of labor; the cells can communicate with one another; and the array of cells functions as a single individual. A sponge is organized, with different kinds of cells in different places doing different things. It is a far cry from a human body with trillions of precisely packaged cells, but it shares some of the human body’s features. Most significantly, the sponge has much of the cell adhesion, communication, and scaffolding apparatus that we have. Sponges are bodies, albeit very primitive and relatively disorganized ones.

Like placozoans and sponges, we have many cells. Like them, our bodies show a division of labor among parts. The whole molecular apparatus that holds bodies together is also present: the rivets that hold cells together; the various devices that help cells signal to one another; and many of the molecules that lie between cells. Like us and all other animals, placozoans and sponges even have collagen. Unlike us, they have very primitive versions of all these features: instead of twenty-one collagens, sponges have two; whereas we have hundreds of different types of molecular rivets, sponges have a small fraction of that number. Sponges are simpler than we and have fewer kinds of cells, but the basic bodybuilding apparatus is there.

Placozoans and sponges are about as simple as bodies get nowadays. To go any further, we have to search for the things that build our bodies in creatures that have no bodies at all: single-celled microbes.

How do you compare a microbe to an animal with a body? Are the tools that build bodies in animals present in single-celled creatures? If so, and if they are not building bodies, what are they doing?

The most straightforward way to begin to answer these questions involves looking inside the genes of microbes to search for any similarities to animals. The earliest comparisons between animal and microbial genomes revealed a striking fact: in many single-celled animals, much of the molecular machinery for cell adhesion, interaction, and so on is just not there. Some analyses even suggested that more than eight hundred of these kinds of molecules are found only in animals with bodies while they are absent in single-celled creatures. This would seem to support the notion that the genes that help cells unite to make bodies arose together with the origin of bodies. And at first glance, it seems to make sense that the tools to build bodies should arise in lockstep with bodies themselves.

The story turned upside down when Nicole King, of the University of California at Berkeley, studied the organisms called choanoflagellates. King’s choice of subject was no accident. From work on DNA, she knew that choanoflagellates are likely the closest microbe relatives of animals with bodies, placozoans, and sponges. She also suspected that hidden in the genes of choanoflagellates are versions of the DNA that make our bodies.

Nicole was aided in her search by the Human Genome Project, an enterprise that has succeeded in mapping all the genes in our bodies. With the success of the Human Genome Project came many other mapping studies: we’ve had the Rat Genome Project, the Fly Genome Project, the Bumblebee Genome Project—there are even ongoing projects to sequence the genomes of sponges, placozoans, and microbes. These maps are a gold mine of information because they enable us to compare the bodybuilding genes in many different species. They also gave Nicole the genetic tools to study her choanoflagellates.

Choanoflagellates look remarkably like the goblet-shaped cells inside a sponge. In fact, for a long time, many people thought that they were just degenerate sponges—sponges without all the other cells. If this were the case, then the DNA of choanoflagellates should resemble that of a bizarre sponge. It doesn’t. When parts of the DNA of choanoflagellates were compared with microbe and sponge DNA, the similarity to microbe DNA turned out to be extraordinary. Choanoflagellates are single-celled microbes.