All This Has Happened Before. . .

Lonche Man dental x-rays

Okay, I'll take a stab at this.  This is an x-ray of Stone Age dental work. This is knowledge that was forgotten and then remembered.  Here's the link to the Oxford University Press' Very Short Inroductions Series.

Advanced analytical methods, based on radioactivity and radiation, have recently revealed that therapeutic dental filling was in use during the Stone Age. As part of the team that performed the study, I worked with experts in radiocarbon dating, synchrotron radiation imaging, dentistry, palaeo-anthropology, and archaeology. Our discovery was based on the identification of an extraneous substance on the surface of a canine from a Neolithic human mandible.