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.
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