THE INTRON FACTOR. “The reconstructions consistently indicate that ancestral eukaryotes including the LECA and the founders of each supergroup were intron-rich forms, with intron densities higher than those in the genes of most extant eukaryotes and probably only slightly lower than those in the modern organisms with the most complex gene structures, such as mammals.” (LECA = last eukaryotic common ancestor).
http://onlinelibrary.wiley.com/doi/10.1002/bies.201300037/full