When Hominins Conquered Highlands—an Acheulean Site at 3000 m a.s.l. on Mount Dendi/Ethiopia
High-altitude mountain habitats are regarded as unfavorable for human occupation (e.g., Aldenderfer 2014) and basic findings of high altitude human physiology research impressively illustrate this: e.g., prevalence of high-altitude hypoxia, increased UV radiation, increased loss of water, and higher basal metabolic rates (Beal 2001; Berghold and Schaffert 2009). Therefore, it seems reasonable that hominins would only be pushed into such conditions by decreasing land resources in the lowlands following rapid population increase or ecological changes (Basell 2008; Foerster et al. 2015). In fact, the scarce archeological evidence for a pre-Holocene occupation of high mountain ranges hypothesized a late colonization of these environments. Archeological investigations on Mount Dendi (3270 m a.s.l.) located on the Ethiopian Plateau question this assumption.
Dendi is a large silicic volcanic complex constituted by both trachytic and rhyolitic lava flows and domes. The rhyolites and trachytes are pale gray and have an aphanitic or microporphyritic texture with Sanidine phenocryst. Obsidian lava flows on top of these deposits have been an important raw material source for the production of stone artifacts. The lava flows are overlain by younger pyroclastic deposits dominated by pyroclastic surge lapilli tuffs and minor ignimbrite and volcanic ashes. These products mainly constitute the floor of the caldera except the volcanic ash which is found everywhere in the caldera (Zinaye 2014).
A notable feature is the presence of two connected lakes formed within the central depression (Fig. 1) surrounded by the caldera rim rising up to 440 m above lake surface. A large number of springs in the steep slopes of the caldera drain into the lakes, with the exception of some tributaries of the Huluka River, which joins the Blue Nile (Abay) Basin. During high water levels, the lakes flow out to this basin. The current abundance of water is recharged primarily during the main rainy season from June to September (annual mean precipitation: ~ 1400 mm) (Degefu et al. 2014; Adimassu et al. 2014).
Archeological prospections concomitant to coring activities on the crater lakes by a CRC 806 research group in 2012 (Wagner et al. 2018), led to the first discovery of Stone Age sites. Further archeological surveys in 2013 and 2015 resulted in the recording of a total of 71 archeological sites from all Stone Age periods. A test-excavation in a small rockshelter (DEN12-A01) yielded four Holocene Later Stone Age layers with a tool spectrum dominated by backed microliths (Schepers et al. in prep.). The Middle Stone Age is mainly represented by single finds of obsidian points that might have been lost during hunting trips. Of outstanding importance are ten Early Stone Age (ESA) sites with large obsidian bifaces that demonstrate the presence of hominins at high altitudes already during the Middle Pleistocene. This short report will focus on one of these sites, DEN-12-A02 that is located at an altitude of 3000 m a.s.l. and yielded finds in stratigraphic context. The altitude has been the same at time of the deposition, given that the impact of tectonic uplift was insignificant during the last 500 ka in the southeastern Ethiopian Plateau (Pik et al. 2003; Xue et al. 2018).
Stratigraphy and Micromorphology
The hand axe was found 2 m below the actual surface in the uppermost level of the terrace embankments that was further excavated. The profile DEN15-A02 prepared at this location exposed dark reddish colored clayey slope deposits, more than 2 m thick (Fig. 2b) interspersed with heavily weathered pyroclastic rock fragments. Features of sediment redeposition such as sharp boundaries between soil horizons, bedding, laminations, or rotational features were not apparent. Pronounced pedogenesis resulted in a sequence of A/Bwt/Btg/Bt horizons, with all horizons containing at least 37% clay. The lowermost Bt with 54% clay shows a particularly strong pedogenic alteration. This is reflected by several generations of illuvial clay coatings, frequent manganese oxide and iron hydroxide nodules and coatings as well as by a strongly developed soil structure. Stagnic color pattern in the Btg horizon (46% clay) indicates reduction/oxidation processes caused by seasonal water saturation. Micromorphological evidence and granulometric composition suggest that an eluvial horizon is lacking, probably due to erosion of the upper part of the profile. The lower part of the sequence, however, does not show any characteristic feature of reworking along the slope, hence, an in situ preservation of the lower part of the sediment sequence is suspected. Considering the different generations of illuvial clay coatings and intercalation by manganese coatings (Fig. 2d), it is highly likely that formation of the lower Bt horizon started a long time ago and was polygenetic, i.e., it continued during several glacial/interglacial cycles. Radiocarbon dating of organic samples has a maximum age limit of around 50,000 years, while a much higher age of several 100,000 years is expected for the find horizon. As luminescence dating of vulcanogenic minerals has proven unreliable (e.g., Tsukamoto et al. 2007), we did not carry out these radiometric methods. In addition, palaeomagnetic measurements yielded no results.
The analysis of all archeological finds from Mount Dendi is ongoing and a large contextual study, which will present detailed data from all sites, is planned for a comprehensive research article. Despite the preliminary stage of this work, we wanted to share a few observations about the DEN-12-A02 assemblage.
~ Altitude m a.s.l.
134 Lithics including at least 18 Acheulean bifaces, dating to the end of OIS 7
92% bones Ursus spelaeus
More than 70 sites ranging from the Oldowan to the Late Stone Age. Besides few Early Acheulean sites, especially a later phase of the African Acheulean is well-represented (Gombore II, dated to ~ 0.8 myr ago, Garba I, dated to ~ 0.5 myr ago)
Chavaillon and Piperno 2004
Eight localities dated between 1.45 and 0.7 myr ago with more than 50 hand axes/large cutting tools (LCT)
de la Torre 2011
The site DEN12-A02 shows that hominins might have settled in high-altitude regions already before the advance of Homo sapiens sapiens. This would question the concept of modern humans’ superiority in a wide range of domains, such as subsistence strategies and hunting equipment in comparison to extinct hominins (for a critical discussion of this concept see: Villa and Roebroeks 2014). In any case, for millennia modern Homo sapiens seemed to have no reasons to colonize high mountains in Europe.
Why hominins conquered the tropical high-altitude habitat already at this early time is still an open question. Was the area a common habitat or was it only used as a refugium during times of environmental stress? Is the case of Dendi, an exception from the general settlement scheme or is the absence of sites the result of a research deficit in high-altitude regions? It is striking that with only one exception, all Acheulean sites in high-altitude environments are located in Ethiopia. Under certain environmental conditions, high mountains in tropical zones might have been favorable ecological niches despite the general stress factor for hominins in high altitudes. Fresh water availability may have played a key role. Modern circulation patterns suggest the Ethiopian Highlands are likely to have received more rainfall than surrounding regions of northern Africa and the Horn during prehistoric times. The region receives moisture from both the Atlantic and Indian Ocean system (Umer et al. 2004) and as a major topographic feature, the highlands capture high orographic rainfall. In the case of Mount Dendi, the crater lakes, if they already existed, might have been important water reservoirs during arid periods.
The ongoing analysis of lacustrine and terrestrial sediment cores from the crater lakes and the caldera deposits hopefully will elucidate the climate history and environmental conditions of the area and might help to answer these questions.
We thank the local residents of Mt. Dendi for their support and participation in field research. The Ethiopian Authority for Research and Conservation of Cultural Heritage (ARCCH) and regional and zonal cultural bureaus facilitated fieldwork and museum studies. We thank all participants of the fieldwork, which had to stand the sometimes-harsh environmental conditions. Special thanks to Asfawossen Asrat Kassaye and Tamrat Endale for their support during the fieldwork and fruitful discussion, Wei Chu for his helpful comments, Ingrid Koch for the lithic drawings, and Lutz Hermsdorf-Knauth for help with the figures. We are grateful to the University of Cologne for administrative support, especially Dr. W. Schuck from the office of the CRC 806. Finally, we are grateful to the two anonymous reviewers for their very helpful comments and suggestions that significantly improved the manuscript.
Funding was provided by the Deutsche Forschungsgemeinschaft (DFG) in frame of the Collaborative Research Center 806 at the University of Cologne.
Compliance with Ethical Standards
Conflict of Interest
On behalf of all authors, the corresponding author states that there are no conflicts of interest.
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