Phylogenetic tree of the Holozoa showing the position of animals (Metazoa), choanoflagellates (Choanoflagellata), filastereans (Filasterea) and ichthyosporeans (Ichthyosporea) within this clade (left). The choanoflagellates include Choanoeca flexa (top right), which has flagella that point inwards when the organism is in bright light. In the dark, the cells contract in a coordinated manner that causes the flagella to point outwards, a movement reminiscent of the contractions that cause tissues to curve during animal development (figure adapted from Brunet et al., 2019). The ichthyosporean Sphaeroforma arctica (bottom right) also exhibits actomyosin contractility, invaginating its membrane to generate multiple cells out of a polarized epithelial layer. This movement is comparable to processes that occur during embryonic development in flies (Dudin et al., 2019).