Recruitment of the exons that code for a functional domain of one protein in the evolution of a gene for a new protein, to produce hybrids with recognisable motifs, such as kringles and metal-binding fingers. A consequence of this hypothesis is the corollary that introns are as old as structural genes; this is upheld by the intron-early school but opposed by the intron-late school, who point to the absence of introns in prokaryotes and some primitive eukaryotes.