As you have seen from many of the theories on the origin of life, the evolution of biochemistry occurred under localized conditions. That is, the chemical reactions of life could not occur in a dilute aqueous environment with the molecular participants far apart. There had to be a compartment of some sort that brought together and concentrated the compounds involved in these events. Biologists have proposed that initially this compartment may have simply been a tiny droplet of water on the surface of a rock. But another major event in the origin of life was necessary: the evolution of the cell membrane.

Life as we know it is separated from the environment within structurally defined units called cells. The internal contents of a cell are separated from the nonbiological environment by a special barrier—a membrane. The membrane is not just a barrier; it regulates what goes into and out of the cell, as we will describe in Chapter 6. This role of the surface membrane is very important because it permits the interior of the cell to maintain a chemical composition that is different from that of its external environment.