Final answer:
Choanoflagellates are animal-like protists in the Opisthokonta supergroup, closely related to sponges, providing insight into early animal evolution.
Step-by-step explanation:
The supergroup Opisthokonta includes choanoflagellates, which are animal-like protists that are closely related to sponges. Choanoflagellates are characterized by their unique single posterior flagellum that pushes the cell forward, as opposed to the anterior flagella seen in other protists that pull cells along. These protists are known for their similarity to the choanocyte cells in sponges, particularly in their collar structure, which is used for feeding. This resemblance suggests that both choanoflagellates and sponges may have evolved from a common ancestor, highlighting choanoflagellates as a crucial link in understanding the early evolution of animals.
The close relationship between choanoflagellates and animals is evident from the structure and function of their cells. Choanoflagellates possess a collar of microvilli surrounding their flagellum, which is used to trap and ingest bacteria. This mechanism is remarkably similar to the feeding cells of sponges, which further supports the hypothesis that animals, including sponges (Porifera), arose from an ancestral protist resembling colonial choanoflagellates. As such, these simple yet intriguing protists provide valuable insight into the early stages of animal multicellularity and the evolution of Metazoa.