Holozoa is a group of organisms that includes animals and their closest single-celled (protist) relatives, but excludes fungi and all other organisms. It is a monophyletic group or clade, a lineage consisting of all descendants of a common ancestor. Among these descendants, the protists are of high interest because of their close relationship to animals: in the search for the genes responsible for animal multicellularity within these protists, they help elucidate the nature of the unicellular ancestor of animals.[5][6][7][8]

Temporal range:
Orange elephant ear sponge, Agelas clathrodes, in foreground. Two corals in the background: a sea fan, Iciligorgia schrammi, and a sea rod, Plexaurella nutans.
Scientific classification e
Clade: Amorphea
Clade: Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
Lang et al., 2002[1]

Definition and subdivisionsEdit

Holozoa is the most inclusive clade containing Homo sapiens (a metazoan), but not Neurospora crassa (a fungus). It is a clade with a branch-based definition: it contains all the closest relatives to animals that aren't fungi, as well as their common ancestor.[4] The clade was first discovered through phylogenetic analyses in 2002.[1] These mostly unicellular relatives are the protist lineages of choanoflagellates, filastereans, ichthyosporeans, and three independent species Corallochytrium, Syssomonas[9] and Tunicaraptor.[2]

  • Choanoflagellata (>250 species)[10] are the protists most closely related to animals. They are free-living unicellular or colonial flagellates that feed on bacteria using a characteristic “collar” of microvilli. This collar strongly resembles the collar cells of sponges;[11] because of this, choanoflagellates were theorized to be related to sponges even in the 19th century. The mysterious Proterospongia is an example of a colonial choanoflagellate that was thought to be related to the origin of sponges.[12] The affinities of the other single-celled holozoans only began to be recognized in the 1990s.[13]
  • Tunicaraptor is a recently discovered lineage whose position within Holozoa has yet to be resolved.[2]

Evolutionary historyEdit

Holozoan phylogenyEdit

Based on phylogenetic and phylogenomic analyses, the cladogram of Holozoa is shown below, with indications of the time divergence of the different clades in millions of years (Mya).[14][15][9][2] The choanoflagellates, animals and filastereans group together as the clade Filozoa.[7] The Pluriformea clade (Corallochytrium and Syssomonas) is the sister group to Filozoa. An alternative hypothesis is the Teretosporea clade, which unites Pluriformea with Ichthyosporea instead.[9]















950 Mya
1100 Mya
1300 Mya

Unicellular ancestor of animalsEdit

The search for the origin of animals, or the nature of the last metazoan common ancestor (or Urmetazoan), requires deciphering the unicellular-to-multicellular transition that took place. Due to the absence of a fossil record, the only way to investigate the nature of the unicellular metazoan common ancestor and its initial steps towards multicellularity is by studying the genes and genetic pathways shared between animals and their closest living relatives.[10]

The sequencing of unicellular holozoan genomes allows us to reconstruct the gene content of this unicellular metazoan ancestor with a high level of detail.[10] Many traits previously considered “specific to animals” are found in their unicellular relatives,[9] indicating that multicellularity appeared not through the acquisition of new genes exclusive to animals, but through the co-option of ancestral genes already present in their unicellular relatives.[10] For example:

  • A considerable portion of animal transcription factors (TF) is already present in unicellular holozoans, including some TF classes previously thought to be animal-specific (such as p53 and T-box). Other classes and families are animal-specific, such as SMAD, Doublesex and IRF.[10]

Additionally, many biological processes seen in animals were already present in their unicellular ancestor, such as sexual reproduction and gametogenesis in the choanoflagellate Salpingoeca rosetta and several types of multicellular differentiation.[10]

Fossil recordEdit

A billion-year-old freshwater microscopic fossil known as Bicellum brasieri is possibly a holozoan that shows two differentiated cell types or life cycle stages. It consists of a spherical ball of tightly packed cells (stereoblasts) enclosed in a single layer of elongated cells. There are also two populations of stereoblasts with mixed shapes, which have been interpreted as cellular migration to the periphery, a movement that could be explained by differential cell-cell adhesion. These occurrences are consistent with extant unicellular holozoans, which are known to form multicellular stages in complex life cycles.[3]


Because Holozoa is a clade including animals and all protists more closely related to animals than to fungi, some authors prefer it as opposed to recognizing paraphyletic groups that mostly consists of Holozoa minus animals,[17] such as the paraphyletic phylum “Choanozoa” adopted by the protozoologist Thomas Cavalier-Smith.[a] The International Society of Protistologists prefers classifying eukaryotes in monophyletic groups, striving away from traditional ranks (kingdom, phylum, etc.). According to this view, the classification of Holozoa is:[4]

  • Holozoa Lang et al. 2002


  1. ^ a b The term "Choanozoa" has been used since 1991 by Cavalier-Smith as a paraphyletic phylum of opisthokont protists,[19] and the terms "Apoikozoa" and "choanimal" were proposed as names for the clade Metazoa+Choanoflagellata. However, these terms have not been formally described or adopted, and were rejected in favour of a renamed Choanozoa to fit the clade Metazoa+Choanoflagellata.[4]
  2. ^ a b c There are two competing phylogenetic hypotheses: Teretosporea (Corallochytrium + Ichthyosporea) and Pluriformea (Corallochytrium + Syssomonas).[4] This article follows the latter hypothesis, because it is strongly supported by the most recent phylogenetic studies.[2]


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  2. ^ a b c d e f Tikhonenkov DV, Mikhailov KV, Hehenberger E, Mylnikov AP, Aleoshin VV, Keeling PJ, et al. (2020). "New Lineage of Microbial Predators Adds Complexity to Reconstructing the Evolutionary Origin of Animals". Current Biology. 30 (22): 4500–4509. doi:10.1016/j.cub.2020.08.061. PMID 32976804.
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