Intron phylogeny: a new hypothesis

The three major classes of intron are clearly of unequal antiquity. Structured (often self-splicing and sometimes mobile) introns are the most ancient, probably dating (at least for group I) from the ancestral (eubacterial) cell 3500 million years ago, and were originally restricted to tRNA. Protein-spliced introns (usually in tRNA) probably evolved from them by a radical change in splicing mechanism in the common ancestor of eukaryotes and archaebacteria, perhaps only about 1700 million years ago. Spliceosomal introns probably evolved from group-II-like self-splicing introns after the origin of the nucleus between 1700 and 1000 million years ago, and were probably mostly inserted into previously unsplit protein-coding genes after the origin of mitochondria 1000 million years ago.