The origin of recent introns: transposons?

The origin of recent introns: transposons?: "Previous large-scale studies have failed to find a single convincing case of intron gain since the divergence of humans and mice [3] or a single case of convincing sequence homology between introns in the same genome for a range of taxa [4], and although some other cases of recent intron insertion have been discovered, the sources of these introns remain unknown. Yet, all characterized metazoan species and most other eukaryotes harbor multiple introns per gene, requiring hundreds of thousands, if not millions, of individual intron gains to have occurred throughout eukaryotic evolution"