We propose that vertebrate teeth evolutionarily arose where outpocketings of the endoderm-lined pharynx (pouches), contacted the ectoderm-lined skin to form gill slits. Using zebrafish, we will test the correlation between tooth and gill slit formation, reveal the extent of ectoderm and endoderm in pouches and gill slits, and test the requirement for endoderm and ectoderm in tooth formation.