The holozoan protists play a crucial role in understanding the evolutionary steps leading to the emergence of multicellular animals from single-celled ancestors.

[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.

[13] Based on phylogenetic and phylogenomic analyses, the cladogram of Holozoa is shown below:[17][18][6][2] Cristidiscoidea Fungi Dermocystida Ichthyophonida Corallochytrium Syssomonas Tunicaraptor Filasterea Choanoflagellata Metazoa Uncertainty remains around the relationship of the two most basal groups, Ichthyosporea and Pluriformea.

[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.

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.

[3] Proposed Ediacaran fossil "embryos" of early metazoans, discovered in the Doushantuo Formation, have been reinterpreted as non-animal protists within Holozoa.

[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.

[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).

[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.