Early genes that were oligomeric repeats generated a number of divergent domains on their own.

One of the more popular concepts to emerge in recent years is that new proteins evolved by domain exchanges between preexisting proteins. The presence of introns within eukaryotic genes is thought to enhance such exchanges. Yet domain exchanges must necessarily be the secondarily developed process in evolution, for they would have been effective only after multitudes of domains came into being. Many of the proteins with functionally divergent domains were established before the division of prokaryotes from eukaryotes; i.e., soon after the creation of life on this earth. I attribute the extreme innovativeness of early coding sequences to their construction; i.e., being repeats of oligomeric units. The rhodopsin family of proteins--with seven hydrophobic, alpha-helical transmembrane domains, four extracellular domains, and four intracytoplasmic domains--indeed arose before the division of prokaryotes from eukaryotes and later gave rise to muscarinic acetylcholine receptor and beta-adrenergic receptor among others. In this paper, I show that the entire coding sequence for porcine muscarinic acetylcholine receptor is still replete with copies of three heptameric units that are very closely related to each other. Original heptameric units are more stringently conserved in parts encoding the seven transmembrane domains, whereas new repeating units are comingled with the old in parts encoding extracellular and intracytoplasmic domains.