Жасушалы Эукариоттардың Excavata тобы: Классификация және Эволюциясы

Supergroup of unicellular organisms belonging to the domain Eukaryotaan explanation of very similar terms

Excavata is an extensive and diverse but paraphyletic group of unicellular Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier Smith in 2002. It contains a variety of free living and symbiotic protists, and includes some important parasites of humans such as Giardia and Trichomonas. Excavates were formerly considered to be included in the now obsolete Protista kingdom. They were distinguished from other lineages based on electron microscopic information about how the cells are arranged (they have a distinctive ultrastructural identity). On the basis of phylogenomic analyses, the group was shown to contain three widely separated eukaryote groups, the discobids, metamonads, and malawimonads. A current view of the composition of the excavates is given below, indicating that the group is paraphyletic. Except for some Euglenozoa, all are non photosynthetic.

Euglenozoa and Heterolobosea (Percolozoa) or Eozoa (as named by Cavalier Smith) appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria (Superphylum Discicristata). A close relationship has been shown between Discicristata and Jakobida, the latter having tubular cristae like most other protists, and hence were united under the taxon name Discoba, which was proposed for this apparently monophyletic group.

The conventional explanation for the origin of the Eukaryotes is that a heimdallarchaeian or another Archaea acquired an alphaproteobacterium as an endosymbiont, and that this became the mitochondrion, the organelle providing oxidative respiration to the eukaryotic cell. Caesar al Jewari and Sandra Baldauf argue instead that the Eukaryotes possibly started with an endosymbiosis event of a Deltaproteobacterium or Gammaproteobacterium, accounting for the otherwise unexplained presence of anaerobic bacterial enzymes in Metamonada. The sister of the Preaxostyla within Metamonada represents the rest of the Eukaryotes which acquired an Alphaproteobacterium. In their scenario, the hydrogenosome and mitosome, both conventionally considered "mitochondrion derived organelles", would predate the mitochondrion, and instead be derived from the earlier symbiotic bacterium.

In 2023, using molecular phylogenetic analysis of 186 taxa, Al Jewari and Baldauf proposed a phylogenetic tree with the metamonad Parabasalia as basal Eukaryotes. Discoba and the rest of the Eukaryota appear to have emerged as sister taxon to the Preaxostyla, incorporating a single alphaproteobacterium as mitochondria by endosymbiosis. Thus the Fornicata are more closely related to e. g. animals than to Parabasalia. The rest of the Eukaryotes emerged within the Excavata as sister of the Discoba; as they are within the same clade but are not cladistically considered part of the Excavata yet, the Excavata are in this analysis highly paraphyletic.