Gunter Wagner (and team) set out to investigate novel phenotypes/cell types - in particular, mammalian pregnancy. How did carrying developing young in the womb evolve?
Wagner and team identified 1500 genes that were unique to the uterus of mammals and involved in placental development. The big kicker? The expression of these genes is under control by transposons. Essentially, large-scale morphological changes (aka - the emergence of mammalian pregnancy) was not due to mutations within key genes, but rather is attributable to the establishment of a novel gene regulatory network (modulated by transposons).