section_path: Phylogeny/Internal phylogeny
section_name: Internal phylogeny
paragraph_text: Internal phylogeny
  
  The most basal animals, the Porifera, Ctenophora, Cnidaria, and Placozoa, have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora, both of which lack hox genes, which are important for body plan development.
  
  Hox genes are found in the Placozoa, Cnidaria, and Bilateria. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development.
  
  Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about the structure at the base of the tree (dashed lines).
  
  An alternative phylogeny, from Kapli and colleagues (2021), proposes a clade Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade Centroneuralia, consisting of Chordata + Protostomia.
  
  Eumetazoa, a clade which contains Ctenophora and ParaHoxozoa, has been proposed as a sister group to Porifera. A competing hypothesis is the Benthozoa clade, which would consist of Porifera and ParaHoxozoa as a sister group of Ctenophora.