In 1957 he began taking pictures with an endoscope, an instrument that can see inside a body cavity, but when Lennart Nilsson presented the rewards of his work to LIFE’s editors several years later, they demanded that witnesses confirm that they were seeing what they thought they were seeing. Finally convinced, they published a cover story in 1965 that went on for 16 pages, and it created a sensation. Then, and over the intervening years, Nilsson’s painstakingly made pictures informed how humanity feels about . . . well, humanity. They also were appropriated for purposes that Nilsson never intended. Nearly as soon as the 1965 portfolio appeared in LIFE, images from it were enlarged by right-to-life activists and pasted to placards.