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Temporal range:
Holozoan diversity. Top half displays representatives of holozoan protists. Bottom half displays representatives of metazoans.
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Amorphea
Clade: Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
Lang et al., 2002[1]

Incertae sedis

  • Choanofila Cavalier-Smith, 2009[5] (plus animals)

Holozoa (from Ancient Greek ὅλος (holos) 'whole', and ζῷον (zoion) 'animal') is a clade of organisms that includes animals and their closest single-celled relatives, but excludes fungi and all other organisms. Together they amount to more than 1.5 million species of purely heterotrophic organisms, including around 300 unicellular species. It consists of various subgroups, namely Metazoa (or animals) and the protists Choanoflagellata, Filasterea, Pluriformea and Ichthyosporea. Along with fungi and some other groups, Holozoa is part of the Opisthokonta, a supergroup of eukaryotes. Choanofila was previously used as the name for a group similar in composition to Holozoa, but its usage is discouraged now because it excludes animals and is therefore paraphyletic.

The holozoan protists play a crucial role in understanding the evolutionary steps leading to the emergence of multicellular animals from single-celled ancestors. Recent genomic studies have shed light on the evolutionary relationships between the various holozoan lineages, revealing insights into the origins of multicellularity. Some fossils of possible metazoans have been reinterpreted as holozoan protists.



Holozoa is a clade that includes animals and their closest relatives, as well as their common ancestor, but excludes fungi. It is defined on a branch-based approach as the clade encompassing all relatives of Homo sapiens (an animal), but not Neurospora crassa (a fungus).[4] Holozoa, besides animals, primarily comprises unicellular protist lineages of varied morphologies such as choanoflagellates, filastereans, ichthyosporeans, and the distinct genera Corallochytrium, Syssomonas, and Tunicaraptor.[6][2]

  • Choanoflagellata, with around 250 species,[7] are the closest living relatives of animals. They are free-living unicellular or colonial flagellates that feed on bacteria using a characteristic "collar" of microvilli. The collar of choanoflagellates closely resembles sponge collar cells,[8] leading to theories since the 19th century about their relatedness to sponges.[9] The mysterious Proterospongia is an example of a colonial choanoflagellate that was thought to be related to the origin of sponges.[10] The affinities of the other single-celled holozoans only began to be recognized in the 1990s.[11]
  • Tunicaraptor unikontum is the newest discovered clade, whose position within Holozoa has yet to be resolved. It is a flagellate with a specialized "mouth" structure absent in other holozoans.[2]


The first sequenced unicellular holozoan genome was that of Monosiga brevicollis, a choanoflagellate. It measures around 41.6 mega–base-pairs (Mbp) and contains around 9200 coding genes, making it comparable in size to the genome of filamentous fungi. Animal genomes are usually larger (e.g. human genome, 2900 Mbp; fruit fly, 180 Mbp), with some exceptions.[15]



Holozoa, along with a clade that contains fungi and their protist relatives (Holomycota), are part of the larger supergroup of eukaryotes known as Opisthokonta. Holozoa diverged from their opisthokont ancestor around 1070 million years ago (Mya).[16] The choanoflagellates, animals and filastereans group together as the clade Filozoa. Within Filozoa, the choanoflagellates and animals group together as the clade Choanozoa.[13] Based on phylogenetic and phylogenomic analyses, the cladogram of Holozoa is shown below:[17][18][6][2]




1110 Mya












960 Mya
1070 Mya
1250 Mya

Uncertainty remains around the relationship of the two most basal groups, Ichthyosporea and Pluriformea.[4] They may be sister to each other, forming the putative clade Teretosporea.[19] Alternatively, Ichthyosporea may be the earliest-branching of the two, while Pluriformea is sister to the Filozoa clade comprising filastereans, choanoflagellates and animals. This second outcome is more strongly supported after the discovery of Syssomonas.[2][6]

The position of Tunicaraptor, the newest holozoan member, is still unresolved. Three different phylogenetic positions of Tunicaraptor have been obtained from analyses: as the sister group to Filasterea, as sister to Filozoa, or as the most basal group of all Holozoa.[2][20]

Environmental DNA surveys of oceans have revealed new diverse lineages of Holozoa. Most of them nest within known groups, mainly Ichthyosporea and Choanoflagellata. However, one environmental clade does not nest within any known group and is a potential new holozoan lineage. It has been tentatively named MASHOL (for 'marine small Holozoa').[21]

Unicellular ancestry of animals[edit]

Unicellular holozoans do not go through animal embryonic development, but they display developmental processes using similar molecules. An actomyosin network controls the cellularization of both an ichthyosporean coenocyte (A) and a fruit fly blastoderm (B). Similarly, actomyosin contraction allows both the shaping of choanoflagellate colonies (C) and the gastrulation of animal embryos.

The quest to elucidate the evolutionary origins of animals from a unicellular ancestor requires an examination of the transition to multicellularity. In the absence of a fossil record documenting this evolution, insights into the unicellular ancestor of animals are obtained from the analysis of shared genes and genetic pathways between animals and their closest living unicellular relatives. The genetic content of these single-celled holozoans has revealed a significant discovery: many genetic characteristics previously thought as unique to animals can also be found in these unicellular relatives. This suggests that the origin of multicellular animals did not happen solely because of the appearance of new genes (i.e. innovation), but because of pre-existing genes that were adapted or utilized in new ways (i.e. co-option).[7][6] For example:

Additionally, many biological processes seen in animals are already present in their unicellular relatives, such as sexual reproduction and gametogenesis in the choanoflagellate Salpingoeca rosetta and several types of multicellular differentiation.[7]

Fossil record[edit]

A fossilized sample of Bicellum brasieri, a billion-year-old potential holozoan.

A billion-year-old freshwater microscopic fossil named Bicellum brasieri is possibly the earliest known holozoan. It 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]

Proposed Ediacaran fossil "embryos" of early metazoans, discovered in the Doushantuo Formation, have been reinterpreted as non-animal protists within Holozoa. According to some authors, although they present possible embryonic cleavage, they lack metazoan synapomorphies such as tissue differentiation and nearby juveniles or adults. Instead, its development is comparable to the germination stage of non-animal holozoans. They possibly represent an evolutionary grade in which palintomic cleavage (i.e. rapid cell divisions without cytoplasmic growth in between, a characteristic of animal embryonic cleavage)[23] was the method of dispersal and propagation.[24]



Prior to 2002, a relationship between Choanoflagellata, Ichthyosporea and the animal-fungi divergence was considered on the basis of morphology and ultrastructure. Early phylogenetic analyses gave contradicting results, because the amount of available DNA sequences was insufficient to yield unambiguous results. The taxonomic uncertainty was such that, for example, some Ichthyosporea were traditionally treated as trichomycete fungi.[1]

Holozoa was first recognized as a clade in 2002 through a phylogenomic analysis by Franz Bernd Lang, Charles J. O'Kelly and other collaborators, as part of a paper published in the journal Current Biology. The study used complete mitochondrial genomes of a choanoflagellate (Monosiga brevicollis) and an ichthyosporean (Amoebidium parasiticum) to firmly resolve the position of Ichthyosporea as the sister group to Choanoflagellata+Metazoa. This clade was named Holozoa (from Ancient Greek ὅλος (holos) 'whole', and ζῷον (zoion) 'animal'), meaning 'whole animal', referencing the wider animal ancestry that it contains.[1]

Holozoa has since been supported as a robust clade by every posterior analysis,[20] even after the discovery of more taxa nested within it (namely Filasterea since 2008,[13] and the pluriformean species Corallochytrium and Syssomonas since 2014[25] and 2017[6] respectively). As of 2019, the clade is accepted by the International Society of Protistologists, which revises the classification of eukaryotes.[4]


In classifications that use traditional taxonomic ranks (e.g. kingdom, phylum, class), all holozoan protists are classified as subphylum Choanofila (phylum Choanozoa,[a] kingdom Protozoa) while the animals are classified as a separate kingdom Metazoa or Animalia.[26] This classification excludes animals, even though they descend from the same common ancestor as choanofilan protists, making it a paraphyletic group rather than a true clade. Modern cladistic approaches to eukaryotic classification prioritise monophyletic groupings over traditional ranks, which are increasingly perceived as redundant and superfluous. Because Holozoa is a clade, its use is preferred over the paraphyletic taxon Choanofila.[4]


  1. ^ a b The term "Choanozoa" has been used since 1991 by Cavalier-Smith as a paraphyletic phylum of opisthokont protists,[27] 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 favor of a renamed Choanozoa to fit the clade Metazoa+Choanoflagellata.[4]


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