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?
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