The origin and evolution of intron-exon structures continue to be controversial topics. Two alternative theories, the 'exon theory of genes' and the 'insertional theory of introns', debate the presence or absence of introns in primordial genes. Both sides of the argument have focused on the positions of introns with respect to protein and gene structures. A new approach has emerged in the study of the evolution of intron-exon structures: a population analysis of genes. One example is the statistical analysis of intron phases--the position of introns within or between codons. This analysis detected a significant signal of exon shuffling in the DNA sequence database containing both ancient and modern exon sequences: intron phase correlations, that is, the association together within genes of introns of the same phase. The results of this analysis suggest that exon shuffling played an important role in the origin of both ancient and modern genes.