Gallifreyans were outwardly indistinguishable from humans. Although they looked identical to humans and many humanoids, they had a variety of biological differences that set them apart. The Doctor told Madame Vastra that the Time Lords "became what they did" through prolonged exposure to the Time Vortex. According to the Eleventh Doctor, this took billions of years for the people of Gallifrey and that "[i]t didn't just happen." After being subjected from alterations by Madame Kovarian and her associates, River Song, a human who was conceived inside the Doctor's TARDIS, also developed at least some of these traits before she was born. (TV: A Good Man Goes to War)