United States researchers investigating how single-celled organisms evolved to become multicellular stumbled across a strange phenomenon during their experiments: Single-celled eukaryotes called choanoflagellates, which are the closest living relatives to animals, begin to sexually reproduce in response to a protein produced by bacteria. Why this happens in natural settings is still unclear, though they speculate that it could help the choanoflagellates easily mate with others from the same species. The finding is presented August 31 in the journal Cell.