Animals and Their Unicellular Ancestors


Animals belong to the Opisthokonta, one of the major divisions of the eukaryotic Tree of Life. This supergroup also includes other well‐known groups such as fungi and choanoflagellates, in addition to some newly discovered unicellular taxa as the Ichthyosporea or the Filasterea. To unveil the origin of animal multicellularity, it is vital to understand the evolution of their single‐celled relatives, as they might hold key genetic clues that might help us understand how the unicellular ancestors of animals became animals. Our current knowledge of unicellular animal relatives, their specific phylogenetic relationships and the role they might play in future research is being improved, thanks to molecular data.

Key Concepts:

  • Animals are members of the opisthokonts.

  • The Opisthokonta clade includes animals, fungi and several unicellular lineages.

  • Choanoflagellates are the closest animal unicellular relatives.

  • Ichthyosporea and Filasterea lineages are most closely related to animals than to fungi.

  • The opisthokonts include several multicellular types.

  • The opisthokont is a key evolutionary window to understand the unicellular to multicellular transition.

Keywords: Opisthokonta; multicellularity; phylogeny; evolution; Metazoa; Fungi; choanoflagellates; Ichthyosporea; Filasterea

Figure 1.

Phylogeny of the Opisthokonta. Summary of the phylogenetic relationships of the opisthokonts, based on recent molecular phylogenies. Amoebozoans are sister group to Opisthokonta, together forming the Unikonta. Shaded boxes remark widely accepted supergroups. See text for further discussion.

Figure 2.

Pictures representatives of different Opisthokonta. (a) Picture of the frog Phyllomedusa vaillantii. Courtesy of Carlos Palacín and Alejandro de Mendoza. (b) Picture of the fungi Amanita muscaria. (c) Scanning microscopy picture of the filasterean Ministeria vibrans. (d) Scanning microscopy picture of the ichthyosporean Creolimax fragrantissima in colonial stage. Courtesy of Arnau Sebé‐Pedrós.



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Further Reading

Ruiz‐Trillo I, Lane CE, Archibald JM and Roger AJ (2006) Insights into the evolutionary origin and genome architecture of the unicellular opisthokonts Capsaspora owczarzaki and Sphaeroforma arctica. Journal of Eukaryotic Microbiology 53: 1–6.

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Paps, Jordi, and Ruiz‐Trillo, Iñaki(Oct 2010) Animals and Their Unicellular Ancestors. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0022853]