Andreas Vesalius
Dissections of cadavers (dead human bodies) was forbidden for many centuries, and this limited previous work on anatomy. Before Vesalius, most information about the human body came from Galen.  By dissecting human bodies, Vesalius showed that many of the things Galen wrote about were wrong. Galen was primarily misinformed about the human heart, liver and lower jaw bone. It was found that Galen's work was based on Barbary apes, but he also dissected pigs, which led to even more inaccurate observations.