Researchers seeking the evolutionary roots of the animal kingdom have discovered a bacterium, Vibrio fischeri, that acts as an aphrodisiac by releasing an enzyme that sends the choanoflagellate, Salpinogoeca rosetta, one of the closest living relatives of animals, into a full mating frenzy. Choanoflagellates are eukaryotes. Their cells have a membrane-bound nucleus containing their genetic material, and they live free as single cells and in multicellular colonies.