Final answer:
The notion of a lost race of Mound builders emerged in the 19th century as Europeans refused to believe that native peoples could have constructed the large earthen structures they found in the Mississippi Valley.
Step-by-step explanation:
The idea of a lost race of Mound builders in North America was a concept that surfaced during a time when Europeans encountered large earthen structures in the Mississippi Valley and refused to believe that the native peoples had constructed them. This notion, which was fraught with skepticism about the capabilities of the indigenous societies, emerged chiefly in the 19th century during the period of the early archaeological thought and speculation in North America. During this time, there were arguments among scholars and laypeople alike about whether the sophisticated mounds, such as those at the Cahokia site, could have been the work of the ancestors of the Native American peoples or a different, unknown civilization that had since vanished.