Category:Excavata

Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota.[1][2][3] Introduced by Thomas Cavalier-Smith in 2002 as a new phylogenetic category, it contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans, including Giardia and Trichomonas.[4] Excavates were formerly considered to be included in the now obsolete Protista kingdom.[5] They are classified based on their flagellar structures,[6] and they are considered to be the most basal Flagellate lineage.[7] Except for Euglenozoa, they are all non-photosynthetic

Cladogram
Here is a proposed cladogram for the positioning of the Excavata, with the Eukaryote root in the excavates, mainly following Cavalier-Smith.[12][13][14][15][16][17][18][19][20]

In this view, excavata is highly paraphyletic, and is proposed to be abandoned.[21] In alternative view, the Discoba are sister to the rest of the Diphoda.[13][22]

Characteristics
Most excavates are unicellular, heterotrophic flagellates. Only the Euglenozoa are photosynthetic. In some (particularly anaerobic intestinal parasites), the mitochondria have been greatly reduced.[4] They often lack "classical" mitochondria—these organisms are often referred to as "amitochondriate", although most retain a mitochondrial organelle in greatly modified form (e.g. a hydrogenosome or mitosome). Among those with mitochondria, the mitochondrial cristae may be tubular, discoidal, or in some cases, laminar. Most excavates have two, four, or more flagella[8] and many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules (with the term "excavate" deriving from the organisms showing clear evidence of this "excavated" feeding groove).[9][5] However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).[5]

The Acrasidae slime molds are the only excavates to exhibit limited multicellularity. Like other cellular slime molds, they live most of their life as single cells, but will sometimes assemble into larger clusters.

Monophyly
Excavate relationships are still uncertain; it is possible that they are not a monophyletic group. The monophyly of the excavates is far from clear, although there seem to be several clades within the excavates that are monophyletic.[11]

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.