I've been enjoying reading through this thread this morning!
Some contributions from my field:
Eugene Dubois discovering the remains of an H. erectus individual in Java, monumentally confirming that there were other hominin species aside from our own, and lending enormous weight to the proposal of a human evolutionary lineage. Raymond Dart's discovery of the Taung child, the first hominin to be placed in the Australopithecine genus, likewise. It re-animated interest in Africa, which ultimately proved the right place to look for our earliest ancestors, and greatly expanded the conversation on what, physically, we were looking for.
Steno's laws: the basic guidelines of how the stratigraphic landscape forms. These have never been replaced; they remain the rules of thumb for budding geologists, archaelogists, paleontologists, and anyone else who digs in the dirt to find knowledge.
The discovery of a three-fold structure of phoneme->morpheme->sentence that underlies all human language and made the systematic comparison of languages plausible. Especially fascinating in that it was independently discovered by several cultures at different times, thus implicitly confirming that the pattern was real rather than imposed once linguists were able to share notes.
Franz Boas on the expedition to Baffin Island, realizing that cultural superiority was largely contextual and that cultural bias therefore must be controlled for if meaningful cultural observations are to take place.
It was already mentioned in this thread, but I wanted to point out something specific about the discovery of DNA; along with its many applications to natural biology and medicine, it also freed up the social sciences to explore beyond the limitations of racial pseudoscience by finally putting the nail in that particular coffin and thus bridging the ideological gap that had formed between biologists/medical professionals and social scientists. We've greatly profited from the subsequent exchange of information between those fields, and the not-coincidental development of the sociomedicine and medical anthropology subfields over the twenty years that followed have saved millions of lives.
The discovery that the metallic properties of blood allowed for the physical mapping of activity within otherwise hidden anatomical structures - ie., the realization that led to the development of the Magnetic Resonance Imager (MRI). The impacts on medicine are too extensive to list, and when this logic was applied to the study of the brain, neuroscience was able to take an enormous leap forward. We can barely have a conversation about social/cultural topics these days without some discussion of neuroscience, and Raymond Damadian's device is largely to be credited for this, elevating neuroscience from a mostly speculative science to a truly experimental one.
