A diagrammatic tree depicting the organisation of most eukaryotes into six major groups. The relationships amongst most of the major groups and the position of the ‘root’ of the tree are shown as unresolved (note however, the grouping of Opisthokonta and Amoebozoa). The arrow shows a possible precise placement of the root, based on gene fusion data. (Simpson and Roger 2004)
Depending on your view of phylogenetics, a recent publication in Nature reporting the discovery of a new kingdom-level branch on the tree of life, basal to Kingdom Fungi, is either a major revision of our current view of taxonomy or completely unsurprising and expected. While we mostly refer to the four kingdoms within Domain Eukarya as Protista, Plantae, Fungi, and Animalia, it’s understood by the scientific community that Protista is essentially a catch-all category, not a true clade, for eukaryotes that don’t quite fit into the other three groups. While this is convenient for organization, it fails to adequately express the diversity of protists. Four kingdoms is a useful system, but there’s no reason why diversity at the kingdom level couldn’t be much higher. A strict cladist could create hundreds, if not thousands of kingdoms from Protista alone.
Hagfish are primitive eel-like chordates make famous for their relative unattractiveness*, profuse production of slime, and charismatic ability to tie themselves in knots. They are perhaps the only ‘fish’ that possesses a skull, but no vertebral column. But the question “What is a hagfish?” goes much deeper than that and it’s answer is fundamental to the evolution of vertebrates and, ultimately, us.