How did animal experimentation become so established to begin with?
There have always existed abundant human bodies, tissue, and blood to augment our knowledge base. But when western Catholicism prevailed, papal decree forbade autopsy. In the second century AD, a Roman physician named Galen performed endless animal experiments and generated over 500 treatises on animal physiology.
Galen's false hypotheses - declaring that animals possess the same physiology as humans - helped dim the light throughout the Dark Ages, but the Renaissance offered a slight reprieve. When competitive intellectual inquiry overwhelmed Church injunctions, autopsies revealed animal-based inaccuracies and shed light on the true nature of human disease.
In the 1600s - 1800s, when so little was known about physiology, one could learn basic things from animals, because all mammals have things in common at the gross level: they all have hearts, lungs and livers, for example. Today, our studies are at the molecular level precisely where the differences between species are greatest. In the mid 19th century, Claude Bernard took up animal experimentation. His tremendous zeal and proliferation of data created a market for animal experimentation.
"From a history of misconceptions..."
In the 1930s, a disaster over ethylene glycol poisoning established animal testing as routine in drug development. The disaster of thalidomide, a drug for morning sickness that led to over 10,000 babies with birth defects, spurred governments to mandate animal testing as a supposed guarantee of drug safety. Never mind that animal tests had failed to predict the thalidomide tragedy itself.