The All-Important Zipper

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.