The best remembered sequence occurs in the twelfth episode, when Attenborough encounters a group of mountain gorillas in Dian Fossey's sanctuary in Rwanda. The primates had become used to humans through years of being studied by researchers. Attenborough originally intended merely to get close enough to narrate a piece about the apes' use of the opposable thumb, but as he advanced on all fours toward the area where they were feeding, he suddenly found himself face to face with an adult female. Discarding his scripted speech, he turned to camera and delivered a whispered ad lib: