The evolution of animals from a unicellular ancestor involved many innovations. Choanoflagellates, unicellular and colonial protozoa closely related to Metazoa, provide a potential window into early animal evolution. We have found that choanoflagellates express representatives of a surprising number of cell signaling and adhesion protein families that have not previously been isolated from nonmetazoans, including cadherins, C-type lectins, several tyrosine kinases, and tyrosine kinase signaling pathway components. Choanoflagellates have a complex and dynamic tyrosine phosphoprotein profile, and cell proliferation is selectively affected by tyrosine kinase inhibitors. The expression in choanoflagellates of proteins involved in cell interactions in Metazoa demonstrates that these proteins evolved before the origin of animals and were later co-opted for development.