For most of its 3.5-billion-year history, life on Earth consisted exclusively of single-celled organisms--bacteria, algae, amoebas, and the like. Then around 1 billion years ago the first multicellular organisms emerged. In fact, they emerged independently many times, presumably because cooperation among cells was such a good idea. Cooperation made possible a division of labor that promoted efficiency; it allowed the individual organism to grow larger and exploit resources no single cell could reach.
But before cells could cooperate, they had to solve a mechanical problem: how to stick together, and how to stick only to their own kind--so that a brain cell, say, would not in the course of embryonic development become attached to cells of the big toe. In vertebrates and in many invertebrates as well, the most important solution to this problem is a set of proteins that protrude from the surface of virtually all tissue cells. The ...
