Previous research suggests that the complex symbolic, technological, and socioeconomic behaviors that typify had roots in the middle Pleistocene<200,000 years ago, but data bearing on human behavioral origins are limited. We present a series of excavated Middle Stone Age sites from the Olorgesailie basin, southern Kenya, dating from ≥295,000 to ~320,000 years ago by argon-40/argon-39 and uranium-series methods. Hominins at these sites made prepared cores and points, exploited iron-rich rocks to obtain red pigment, and procured stone tool materials from ≥25- to 50-kilometer distances. Associated fauna suggests a broad resource strategy that included large and small prey. These practices imply notable changes in how individuals and groups related to the landscape and to one another and provide documentation relevant to human social and cognitive evolution.

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One of the most prominent but least understood demographic phenomena in the precontact Southwest is the disappearance of the Hohokam from the valleys of southern Arizona. Despite extensive research, no widely accepted explanation has been offered. We argue that the failure to identify a satisfactory cause is due to excessive focus on catastrophic phenomena and terminal occupations, and a lack of attention to gradual demographic processes. Based on a combination of macro-regional population studies and local research in the lower San Pedro River valley, we present an explanation for gradual population decline precipitated by social and economic coalescence beginning in the late A.D. 1200s. In the southern Southwest an influx of immigrants from the north led to a shift from a dispersed, extensive settlement/subsistence strategy to increased conflict, aggregation, and economic intensification. This shift resulted in diminished health and transformation from population growth to decline. Over approximately 150 years gradual population decline resulted in small remnant groups unable to maintain viable communities. Small, terminal populations were ultimately unable to continue identifiable Hohokam cultural traditions and consequently disappeared from the archaeological record of southern Arizona, either through migration or a shift in lifestyle that rendered them archaeologically invisible.