Several hypotheses for evolutionary origins of tRNA introns have been put forward. An ''intron-early'' scenario proposes the existence of discontiguous primitive tRNA genes interrupted by introns harboring, for example, genes encoding splicing enzymes or aminoacyl transferases. Throughout evolution, these introns could have lost their initial functions and gradually acquired their current features [20] . The related ''split early'' hypothesis speculates that tRNA introns derive from flanking sequences of a priori split tRNA genes [21, 22] . The contrasting ''intron late'' or ''split late'' scenarios assume that tRNA introns arose later in evolution and propagated themselves by gene conversion or transposition [23] . Recent analyses of archaeal genome sequences suggest that tRNA introns in at least some archaeal species have been inserted into contiguous tRNA genes, e.g., by transposition [24] . A following separation of the sequences encoding the mature domains could have given rise to today's split tRNA genes [25, 26] .