Archeological surveys of the Devils Postpile region indicate that humans began crossing the Sierra Crest west of Mammoth Lakes and traveling through the Devils Postpile area at least 7,500 years ago. Although the area may have been more habitable during warmer climatic periods, the Postpile region typically saw less human activity than areas to the east and west because of its difﬁcult access, high elevation, heavy snowpack, volcanic activity, and territorial conﬂicts.
A San Francisco State College survey located obsidian sites dating from 2,500 to 5,000 years ago along the banks of the Middle Fork San Joaquin within the monument. A 1993 study conﬁrmed that the source of the obsidian was the Casa Diablo geothermal area in Long Valley, one of the most signiﬁcant obsidian sources in the region and an important meeting place for trans-Sierra trade. The limited quantities of obsidian and lack of evidence of long-term habitation suggests that the Middle Fork Valley was probably used seasonally from the west and east sides of the Sierra as part of a trade route to and from Casa Diablo, and occasionally for hunting and collection of natural resources.
The numerous bedrock mortars found along the western edge of the Long Valley suggest that eastside populations likely acquired acorns via trans-Sierra trade. An archeological survey also noted basalt chips interspersed with obsidian in the monument. These chips appear to be from local sources and may have constituted an important trade item.
Surveys throughout California have shown that the trans-Sierra obsidian trade dropped off markedly beginning around 2,500 years ago, due to unknown circumstances but possibly due to a volcanic blast that covered much of the Devils Postpile region in pumice about 3,350 years ago would likely have halted human use of the area for some time.
Before the arrival of Euro-Americans, the population of the Sierra Nevada may have reached 90,000 to 100,000, but the Middle Fork Valley probably received only seasonal use because of its high elevation and heavy snowpack. Archeologists have suggested that a prolonged drought, possibly less than 1,000 years ago, may have compelled people living in the Great Basin to migrate west to the base of the Sierra and over the crest. According to some tribal histories, however, contemporary American Indians are descendants of the original inhabitants of the area.