Giardia:: highly evolved parasite or early branching eukaryote?

The phylogeny of the commonest protozoal agent of intestinal disease, Giardia, is unclear. Although recent intensive research suggests this important human parasite is an early branching eukaryote that evolved before the endosymbiotic origin of mitochondria, there is also evidence to suggest that, as a highly evolved parasite, it has lost many of its ancestral characteristics. In this case, these organisms might have arisen much more recently from aerobic free-living flagellates.