Chronology of Ksar Akil (Lebanon) and implications for the colonization of Europe by anatomically modern humans

Abstract

The Out-of-Africa model holds that anatomically modern humans (AMH) evolved and dispersed from Africa into Asia, and later Europe. Palaeoanthropological evidence from the Near East assumes great importance, but AMH remains from the region are extremely scarce. 'Egbert', a now-lost AMH fossil from the key site of Ksar Akil (Lebanon) and 'Ethelruda', a recently re-discovered fragmentary maxilla from the same site, are two rare examples where human fossils are directly linked with early Upper Palaeolithic archaeological assemblages. Here we radiocarbon date the contexts from which Egbert and Ethelruda were recovered, as well as the levels above and below the findspots. In the absence of well-preserved organic materials, we primarily used marine shell beads, often regarded as indicative of behavioural modernity. Bayesian modelling allows for the construction of a chronostratigraphic framework for Ksar Akil, which supports several conclusions. The model-generated age estimates place Egbert between 40.8-39.2 ka cal BP (68.2% prob.) and Ethelruda between 42.4-41.7 ka cal BP (68.2% prob.). This indicates that Egbert is of an age comparable to that of the oldest directly-dated European AMH (Peştera cu Oase). Ethelruda is older, but on current estimates not older than the modern human teeth from Cavallo in Italy. The dating of the so-called "transitional" or Initial Upper Palaeolithic layers of the site may indicate that the passage from the Middle to Upper Palaeolithic at Ksar Akil, and possibly in the wider northern Levant, occurred later than previously estimated, casting some doubts on the assumed singular role of the region as a locus for human dispersals into Europe. Finally, tentative interpretations of the fossil's taxonomy, combined with the chronometric dating of Ethelruda's context, provides evidence that the transitional/IUP industries of Europe and the Levant, or at least some of them, may be the result of early modern human migration(s).

Figure 1. Stratigraphic and photographic documentation of the Ksar Akil excavations.

(a) Stratigraphic sequence as established by the early excavations of Boston College. The section drawing of the 23 m-deep stratigraphy illustrates both the archaeological levels (in Latin numerals) and the broad techno-typologically distinct phases (Mousterian, Initial Upper Palaeolithic, etc.) that these levels have been ascribed to; (b) The discovery of Egbert (Ksar Akil 1) in 1938. Close up of the skull in situ. Image copyright Pitt Rivers Museum, University of Oxford (accession number: 1998.294.820); (c) Inferior view of the partial right maxilla of Ethelruda (Ksar Akil 2), modified after .

Figure 2. Comparison of the start boundaries for each archaeological phase, produced by the two Bayesian models for Ksar Akil (shown in Text S1 (SI Section III), Figs. S4–5 ).

The boundaries, undated events, reflect the most likely age for the beginning of each of the major technocomplexes. Models are shown in different colours. The only area of significant discrepancy is the start boundary of the late Upper Palaeolithic phase (Phase 3), starting with layer XIII directly following Stone Complex 2. The start boundary for the Mousterian is very tentative, since there are no determinations from the lowermost part of the Mousterian phase. The boundaries and PDFs are compared to the NGRIP δ¹⁸O record and the Greenland Interstadials are numbered, as are the two relevant Heinrich Events 3 and 4.

Figure 3. Comparison of the modelled ages (Probability Distribution Function; PDF) obtained for Egbert and Ethelruda with age estimates of AMH from other Palaeolithic sites between 50,000–30,000 years ago.

The PDFs for the Ksar Akil fossils (Model 1 in green, Model 2 in black), as derived from the Bayesian modelling (Text S1 (SI Section III), Figs. S4–5), are plotted against the currently available determinations for AMH from Europe and Africa , . The likelihoods for the directly-dated specimens are shown in dark grey, whereas the PDFs for those dated indirectly, in light blue. Egbert is contemporaneous with the oldest directly-dated European modern human (Peştera cu Oase, [54]) and falls within the earlier part of the ranges for both Nazlet Khater and Hofmeyer, the African AMH; these dates, however, are very imprecise. The Peştera cu Oase date is a mean of two determinations, one ultrafiltered and one not. The age estimate for Ethelruda is broadly similar to that for the AMH maxilla from Kent's Cavern, but not older than the AMH teeth from Cavallo in Italy. The radiocarbon determinations were calibrated with the INTCAL09/ Marine09 curve and the modelling was performed using OxCal v.4.1.7 .