The amitochondrial flagellates are considered the most ancient protists and the early descendants of the first eukaryotic cells or organisms. They comprise the Archamoebae, the Metamonada (orders: Retortamonadida, Diplomonadida, Oxymonadida) and Parabasala.
From protozoology books such as Grasse's 1952 and the last classification of Protozoa Levine et al. 1980, zooflagellates (Zoomastigophora) comprise the groups or lineages of flagellates which have no plastid or which do not belong to the groups containing plastids that are named the phytoflagellates (Phytomastigophora). They comprised groups with mitochondria, such as choanoflagellates, kinetoplastids, proteromonads, opalinids, and groups without mitochondria such as the metamonads (retortamonads, diplomonads and oxymonads) and the parabasalids (trichomonads and hypermastigids). The mastigamoebids were linked to the sarcodinids in Grasse's book and omiitted in the Levine et al. classifcation.
In the phylogeny of lower eukaryotes proposed by Taylor ((Brugerolle & Taylor 1977; Taylor 1978) the zooflagellates having mitochondria were attached to the eukaryotic tree and the amitochondrial flagellates such as the metamonads and the parabasalids were set apart and not linked to the tree.
After the endosymbiotic theory of the origin of mitochondria (Margulis 1981) and plastids (Gibbs 1981) gained credibility the idea that the flagellates deprived of mitochondria and chloroplasts could be ancestral was proposed (Stewart & Mattox 1980). This hypothesis was developed by Cavalier-Smith (1983) who created Archezoa and the idea was continuously ememded by this author (1987, 1989, 1991). This concept recieved a considerable support when the diplomonad Giardia (Sogin et al. 1989) and the microsporidian Vairimorpha (Vossbrinck et al. 1987) both deprived of mitochondria were placed at the base of the eukaryotic tree by comparing their rRNA sequences. The postion of the parabasalids Trichomonas was more uncertain (Baroin et al. 1988; Sogin 1989, 1991) and recent data obtained in comparing the sequences of the large rRNA subunit in several trichomonad genea indicate they branched off before or just after the Euglenoza and the other mitochondrial protists (Viscogliosi et al. 1993).
The study of molecular evolution using rRNA sequence comparison gives quantitative data and other advantages which make it the most powerful method of inferring phylogeneties and has supplanted the comparison of phenotypic characteres (Perasso et al. 1989; Schlegel 1991; Sogin 1981).