5. Galen
Galen was one of the most significant physicians of the ancient world. He did anatomical work on other primates. It led to him realizing that there are anatomical similarities in humans despite whatever maladies they may present with. He recognized that these similarities in human anatomy and biology meant that there were specific illnesses that people could be diagnosed with. These diseases were attributed to the body not functioning correctly. Galen was the medical authority throughout the Middle Ages. It means that his teachings influenced Western medicine for at least 1400 years.