Adaptation to Life in Confinement
WITH CELLULARIZATION, for the first time life became a property of discrete, autonomous, individual units capable of diversification. Darwinian competition was the most immediate consequence of this development, as well as the main driving force of its further evolution. In addition, cellularization permitted a number of acquisitions that further enhanced the capacity of the protocells to survive and proliferate as individual units, and, therefore, offered likely catches for natural selection to net. In most cases, these adaptations were the result of modifications of the cell membrane, which, in addition to its functions as boundary, site of controlled passage, and protomnotive machinery, evolved into a sensitive interface capable of exchanging many signals with the environment and initiating appropriate responses. A brief review, not necessarily chronological, of these aids to protocellular life follows.
Protocells probably first "learned" to explore their environment chemically, by "tasting" it, so to speak. Here is how it may have happened. It is known that membrane proteins often have one end sticking out on one side of the membrane, and the other end on the other side, with the hydrophobic transmembrane segment in between. Imagine now that the outer part of such a protein possesses a site complementary to a given substance and, therefore, has the ability to bind this substance. Imagine, further, that this binding induces a conformational change--a coiling, for example, or an uncoiling--of the transmembrane segment, such that the inner part of the protein, in turn, undergoes a change in shape. Imagine, finally, that this alteration of the inner part of the protein causes a specific effect, for example, the opening or closing of a channel, or the activation or inhibition of an enzyme. What you are witnessing, with the appearance of such a protein, is the creation of a link that