How did one young anatomist show that Galen had been describing animals, not people?

A young anatomist named Andreas Vesalius did it by checking Galen, point by point, against actual human bodies, and by publishing what he found in a book too good to ignore. In Bologna he discovered that all of Galen's research had been restricted to animals, because the tradition of Rome did not allow the dissection of human bodies. Galen had dissected Barbary macaques, which he considered structurally closest to man, and his research produced many errors owing to the limited anatomical material available to him.

Vesalius did not have that problem. Unlike Galen, he was able to procure a steady supply of human cadavers for dissection, after a judge at the Padua criminal court agreed to supply him with the cadavers of executed criminals. What he found, one bone and organ at a time, did not match the textbook. Galen thought the lower jaw was made of two bones; Vesalius showed it was one. Galen had said the breastbone had seven parts, as in an ape; in humans, Vesalius found three.

In 1543, he published his findings in a seven-book work, On the Fabric of the Human Body, known as the Fabrica. Its detailed plates and the likelihood that the artists were present at the dissections made it an instant classic, and the Fabrica emphasized the priority of dissection. Vesalius was only 28 years old when the first editions were published.

Think of it as a map. For fourteen centuries doctors had been using a map drawn from the wrong territory, and Vesalius redrew it from the real one. His was not the first work based on dissection, but its quality made it an instant classic. What changed is what students could trust: the body in front of them, rather than the authority on the page.