Protein Architecture and the Origin of Introns

Two possibilities have been proposed for the origin of split structures observed in protein-coding eukaryotic genes. One is that most of the introns were positioned in their present locations in primordial genes and have served during evolution as mediators of exon shuffling to yield new functional proteins by allowing different combinations of functional units (Gilbert 1978). The other is that the origin is rather new and introns were created by insertion of such DNA sequences as retroposons into eukaryotic genes (Cavalier-Smith 1985; Sharp 1985).