biologylair:

The genome of Monosiga brevicollis, the choanoflagellates photographed above, was recently sequenced.

Choanoflagellates,single-celled, aquatic, eukaryotic microbes are currently biological giants in unlocking the key to the evolutionary transition from unicellular to multicellular organisms, particularly in animals.

Sponges, or animals of the phylum Porifera , serve as the most primitive extant group of animals. (You can see this on this  previously posted phylogenetic tree of the history of life). Interestingly enough, aquatic sponges possess flagellated feeding structures  called choanocytes, which are nearly morphologically identical to the single-celled organisms we call choanoflagellates.

Photo Courtesy: National Science Foundation