Spliceosomal introns are prevalent in our genomes and also in our minds as unsolved evolutionary mysteries. Are introns primordial features of eukaryotic genes? Or have they been acquired during eukaryotic evolution? These questions are central to a still-simmering debate among biologists. To describe the phylogenetic pattern of introns across eukaryotes, two general models have emerged: the introns-late view claims that all introns have been gained into preformed genes, with their current-day distributions explained by processes of both gain and loss; whereas introns-early proponents posit that most introns can be explained by frequent loss from intron-rich ancestral genes that predate eukaryotic cells (1–4). But the key questions remain unanswered. Both views agree that intron loss does occur, but the main disagreement concerns what fraction of present-day introns have been gained, and how. Spliceosomal introns are dominant features of most eukaryotic genes and genomes, yet we have little knowledge about their mechanisms of acquisition (1). By using evolutionary comparisons between nematode genes, Coghlan and Wolfe (5), in this issue of PNAS, make major strides in understanding spliceosomal intron gain and provide us with a clearer picture of intron evolution in eukaryotic genomes. They not only demonstrate that 122 introns have been gained recently in Caenorhabditis genes, but also provide solid evidence that 28 of them are actually derived from “donor” introns present in the same genome. Indeed, a few of these new introns apparently derive from other introns in the same gene! Previous phylogenetic interpretations of introns indicate that many, if not most, introns have been gained once without subsequent loss (1, 6). These inferences are powerful when considering the pattern of the vast number of introns known to take residence in eukaryotic genes, but they have been impotent in illuminating the underlying molecular mechanism(s) of intron insertion. Scant …