W ell over a billion years ago, primitive cells evolved a fantastic molecular machine that allowed them to move. Take, for example, the amoeba. Though simple creatures, these single-cell organisms are nevertheless endowed with the ability to move, giving them a tremendous advantage over other cells that lack this capability.
Humans are no exception.
We depend on this motility machinery to power our muscle contractions as well as many other essential but more subtle activities. For example, every time a cell divides, it assembles a tiny musclelike ring around its waist. Contraction of this ring separates the two daughter cells, starting with the division of a fertilized egg and producing billions of cells in an adult human. The most spectacular example of cellular motility is used to wire our brains. Like the myriad circuits in a complicated computer, each nerve cell in our brains is connected to many other nerve cells by slender cellular wires called axons and dendrites.