The origin of animals is one of the most intensely studied evolutionary events, and our
understanding of this transition was greatly advanced by analyses of unicellular relatives of
animals, which have shown many “animal-specific” genes actually arose in protistan ancestors
long before the emergence of animals [1-3]. These genes have complex distributions and the
protists have diverse lifestyles, so understanding their evolutionary significance requires both a
robust phylogeny of animal relatives, and a detailed understanding of their biology [4, 5]. But
discoveries of new animal-related lineages are rare and historically biased to bacteriovores and
parasites. Here, we characterize the morphology and transcriptome content of a new animalrelated
lineage, predatory flagellate Tunicaraptor unikontum. Tunicaraptor is an extremely small
(3-5 μm) and morphologically simple cell superficially resembling some fungal zoospores, but it
survives by preying on other eukaryotes possibly using a dedicated but transient “mouth”, which
is unique for unicellular opisthokonts. The Tunicaraptor transcriptome encodes a full
complement of flagellar genes, and the flagella-associated calcium channel which is only
common to predatory animal relatives and missing in microbial parasites and grazers.
Tunicaraptor also encodes several major classes of animal cell adhesion molecules, as well as
transcription factors and homologs of proteins involved in neurodevelopment that have not been
found in other animal-related lineages. Phylogenomics including Tunicaraptor challenges the
existing framework used to reconstruct the evolution of “animal specific” genes, and emphasizes
that the diversity of animal-related lineages may be better understood only once the smaller,
more inconspicuous animal-related lineages are better studied.