This chapter addresses a question that must be answered if we are to understand life's origin fully: Did the living state arise a priori from preexisting cellular structures, or did cellular life develop only at a later evolutionary stage? Considerable insight into this basic question can be obtained from our knowledge of the self-assembly processes that are fundamental to contemporary cell structure and function. Before going on, it is useful to outline the biophysical principles that guide research in this area and indicate how they can be used to investigate cellular origins. We will particularly emphasize certain properties of lipidlike molecules that are well known to membrane biophysicists but may not be obvious to the general scientific community.

1. Bilayers assemble from a variety of amphiphilic compounds.

Although contemporary cell membranes incorporate phospholipids as the major component of the lipid bilayer, it is not necessary to think that they were required for early cellular life. In fact, simpler amphiphilic molecules can also assemble into bilayer membranes. These include single-chain amphiphiles such as soap molecules, glycerol monooleate, oxidized cholesterol, and even detergents like dodecyl sulfate mixed with dodecyl alcohol. It seems likely that primitive cells incorporated lipidlike molecules from the environment, almost as a nutrient, rather than undertaking the much more complex process of synthesizing complex lipids de novo.

2. Bilayer permeability strongly depends on chain length of the component amphiphilic molecules.

We tend to think of the lipid bilayer as being a nearly impenetrable barrier to ionic solutes and other large, polar molecules.