I'm not super sure. If I recall correctly, we've known for a while that something was going on, because surface hearing alone couldn't account for all of the water evaporating from oceans, but we couldn't tell what. In defense of humanity here, the concept of photons interacting with something as comparably massive as molecules is kinda wild. We were caught way off guard when the photoelectric effect was announced, and that's photons interacting with whole atoms instead of just elementary particles. The idea of the photomolecular effect is thus even wilder.