Excavate

Of these, the Percolozoa (Heterolobosea) and Euglenozoa appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria. Most jakobids have tubular cristae, like most other protists, while the metamonads are unusual in having lost classical mitochondria - instead they have 'hydrogenosomes', 'mitosomes' or uncharacterised organelles. Excavate relationships are still uncertain; it is possible that they are not a monophyletic group.

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Certain excavates are often considered among the most primitive eukaryotes, based partly on their placement in many evolutionary trees. This could encourage proposals that excavates are a paraphyletic grade that includes the ancestors of other living eukaryotes. However, the placement of certain excavates as 'early branches' may be an analysis artifact caused by long branch attraction, as has been seen with some other groups, for example, microsporidia.

References

• Cavalier-Smith, T. (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology 52: 297-354.
• Alastair G. B. Simpson (2003). "Cytoskeletal organization, phylogenetic affinities and systematics in the contentious taxon Excavata". International Journal of Systematic and Evolutionary Microbiology 53: 1759-1777.