The answer indeed has to do with similarities between genes coding for proteins involved in adherence and attachment in choanoflagellates and animals. Let me explin it to you a little further. Choanoflagellates are a group of protists. There is Morphological Evidence that can explain how close these protists are as closing living relatives to animals. Not only is morphological evidence the ones that help to conclude that but also molcular evidence. The choanoflagellates are a group of of free-living unicellular and colonial flagellate eukaryotes.