Here's why:
* Morphological Similarities: Choanoflagellates share striking similarities with the collar cells (choanocytes) found in sponges, the simplest animals. Both have a single flagellum surrounded by a collar of microvilli, which are used to filter food from the water.
* Genetic Evidence: Molecular analysis, particularly of ribosomal RNA genes, strongly supports the close evolutionary relationship between choanoflagellates and animals. They share a unique set of genes not found in other protists.
* Potential Ancestor: It's widely believed that a choanoflagellate-like ancestor gave rise to animals.
While other protists, such as the amoebozoans, have been suggested to be closely related to animals in the past, the evidence overwhelmingly points to choanoflagellates as the closest living relatives.