Who first began to doubt Galen's ideas?

The first significant challenge to Galen's ideas came during the Renaissance, particularly through the work of anatomists like Andreas Vesalius. Vesalius conducted detailed dissections and observations that contradicted some of Galen's anatomical descriptions, which were primarily based on animal dissections rather than human studies. His landmark work, "De humani corporis fabrica," published in 1543, marked a turning point in the understanding of human anatomy and led to a broader skepticism towards Galenic doctrines in the medical community.