A young German scientist named Bridgette Tenenbaum discovered a species of sea slug that could secrete pure stem cells, later called ADAM. This substance could be used to enhance one's body; improving physical or mental capabilities, curing diseases, and healing injuries. ADAM was used to develop a series of genetic modifications called plasmids, that could give humans special powers (which used a serum called EVE) and additional innate abilities.
Dr. Tenenbaum created the Little Sisters as a way to recover ADAM from Rapture's dead. The Little Sisters were young girls implanted with the sea slug within the lining of their stomachs, and mentally conditioned to recognize the dead as "angels". Upon seeing an angel, they were further conditioned to use a hypodermic syringe to extract the used ADAM, take it into themselves, and convert the material within their body back into a usable form. To protect the Sisters from others, each was assigned a Big Daddy, genetically modified humans who were placed into large and powerful diving suits that followed the Sisters around and attacked anyone that attempted to harm the Sisters or themselves.
