Human occupation of the Afroalpine Bale Mountains at the onset of the African Humid Period

The reasons for the intermittent human use of harsh Afroalpine environments in prehistory remain unclear. High-resolution glacial and archaeological chronologies from Ethiopia’s Bale Mountains now offer insights into landscape change and human adaptations at high altitudes.

Objectives
This study investigates the behavioral signatures of human occupation in Africa’s largest alpine environment around 15,000 years ago, focusing on local site use and integration into regional networks amid deglaciation and the abrupt onset of African Humid Period wet conditions.

Methods
This research integrates surface exposure dating of moraine boulders and radiocarbon dating of archaeological rock shelter deposits with detailed analyses of lithic materials from three stratified sites in the Bale Mountains. We use multivariate statistical analyses of electron microprobe data to determine the geochemical provenance of obsidian artifacts. Lithic technological analysis is based on systematic recording of artifact attributes to reconstruct key stages of production. Functional analyses include use-wear and residue studies conducted using stereomicroscopy, reflected light microscopy, and scanning electron microscopy (SEM–EDX).

Results
This study provides a detailed reconstruction of the final deglaciation phase in the Bale Mountains and identifies distinct patterns of lithic acquisition, production, and use across three contemporaneous sites. Dimtu, located on the formerly glaciated plateau and representing the highest known stratified archaeological site in Africa, is distinguished by a focus on the production of rare but specific pointed flakes. Simbero exhibits standardized backed tool production and evidence of hafting, while the Webi Gestro assemblage includes bladelets and notched tools; wear on unretouched bladelets indicates their use in transverse and longitudinal motions for processing activities and possibly as projectile elements. Geochemical results reveal obsidian exchange between high altitudes and lowlands, suggesting extensive social networks reinforced by technological and behavioral parallels.

Conclusions
Human strategies at high altitudes closely mirror contemporaneous lowland behavior, revealing synchronous patterns across ecological zones. Similar patterns during other periods point to broader systemic dynamics. Conventional refugium-based explanations fail to fully capture these patterns, highlighting the need to examine diachronic shifts in the scale, connectivity, and intensity of prehistoric networks across ecozones.