Da Vinci dissected cadavers to study the human body's inner workings. This was illegal at that time, so the scientist had to work in dark, secret places. (For the most part his subjects were bodies of people who had been hanged.) These drawings contributed greatly to modern science. Historians believe the Egyptian priest Imhotep may have actually made similar anatomical notations thousands of years earlier, but his notes are lost, buried with him in his unknown tomb. So, for all intents and purposes, da Vinci is the first great anatomist. Of course he's also been accused of being a homosexual for a few reasons, one being that he made several drawings of female sexual organs but none of male sexual organs.
Because of all the pains he went to to study the human body first-hand, da Vinci's were considered the most realistic human depictions to date.