Direct U-series dating of the Apidima C human remains (Chapter 2)

Abstract

The site of Apidima, in southern Greece, is one of the most important Paleolithic sites in Greece and southeast Europe. One of the caves belonging to this cave complex, Cave A, has yielded human fossil crania Apidima 1 and 2, showing the presence of an early Homo sapiens population followed by a Neanderthal one in the Middle Pleistocene. Less known are the human remains reportedly recovered from Cave C at Apidima. These include a number of isolated elements, but also a partial skeleton interpreted as a female burial, Apidima 3, proposed by Pitsios (e.g., Pitsios 1999) to be associated with Aurignacian lithics and to date to ca. 30 ka. In light of the rarity of the Upper Paleolithic in Greece, and the general scarcity of human remains associated with the Aurignacian, the remains from Apidima Cave C are potentially very significant in elucidating the arrival of the early Upper Paleolithic populations in Europe. Here we undertake direct Uranium-series dating of three human samples from Cave C, including the burial, to help clarify their chronology. Results suggest a minimum age of terminal Pleistocene for all three samples.

Introduction

Apidima is a cave complex situated on the coast of the Mani Peninsula, southern Greece, consisting of five caves (A-E) formed in the Upper Cretaceous–Late Eocene limestone of the coastal cliffs of the inner Mani (Fig. 1). The caves are situated very near the current sea level, Cave A being the lowermost at ca. 4 m above sea level (asl) and Caves C and D the highest (at ca. 19 m and 24 m asl, respectively). The caves were investigated by a team from the Museum of Anthropology of the Medical School of the National and Kapodistrian University of Athens between 1978 and 1985, and several important discoveries were made. These included two human fossil crania of Middle Pleistocene age from Cave A (Pitsios 1985, 1995, 1999; Harvati and Delson 1999; Harvati 2000; Harvati et al. 2009, 2011, 2019), considered among the most important paleoanthropological finds from southeast Europe. Their recent re-investigation showed the presence of an early modern human population, followed by a Neanderthal one, at the site in the Middle Pleistocene, and provided evidence of an early Homo sapiens dispersal out of Africa that was both earlier and geographically more widespread than previously thought (Harvati et al. 2019). However, a number of less known, but potentially very important human remains have also been recovered from Cave C. These include a burial hypothesized to be of early Upper Paleolithic age, a find that, if confirmed, would be unique in Greece (Pitsios 1985, 1995, 1999; Mompheratou and Pitsios 1995; Ligoni and Papagrigorakis 1995; Harvati et al. 2009; Tourloukis and Harvati 2018) and in Europe (d’Errico and Vanhaeren 2015).

Human remains reported from Cave C include a partial skeleton, as well as isolated dental remains and skeletal elements likely representing additional individuals (e.g., Mompheratou and Pitsios 1995; personal observation). The skeleton (LAO 1/S3, or Apidima 3) is represented by much of the postcranium, a mandibular fragment preserving the left molar series and possibly isolated teeth. It has been interpreted as a burial of a young woman. Sex was attributed on the basis of the pelvic morphology (Pitsios 1999), whereas age was estimated from dental attrition (Ligoni and Papagrigorakis 1995; Pitsios 1999). More than 40 (41 reported by Pitsios 1985, 43 by Pitsios 1999) pierced shells of Nassa neritea (Karali 1995) were reportedly recovered around the upper part of the skeleton (Pitsios 1999) and were considered to represent personal ornaments associated with the burial. A few lithic artifacts reportedly found together with this skeleton were tentatively assigned to the Aurignacian (Darlas 1995). Pitsios (1999) proposed a date of ca. 30 ka for this burial on the basis of his own stratigraphic observations, the tentative attribution of the lithics to the Aurignacian by Darlas (1995) and on ESR dates from cave sediments by Liritzis and Maniatis (1995).

The Upper Paleolithic is very rare in Greece and is known from only a handful of sites (e.g., Harvati et al. 2009; Harvati 2016; Tourloukis and Harvati 2018). Furthermore, human remains associated with the Aurignacian are very scarce throughout Europe, usually consisting of isolated specimens (most frequently teeth) rather than burials, even though a total absence of Aurignacian burials is not conclusive (Riel-Salvatore and Gravel-Miguel 2013; d’Errico and Vanhaerean 2015). Elaborate inhumations with ornaments, such as beads manufactured from shells, are overall scarce and usually more common in the middle and later parts of the Upper Paleolithic (Riel-Salvatore and Gravel-Miguel 2013). A possible early Upper Paleolithic chronology for Apidima C and the human remains found there is therefore of great interest. However, the age estimate proposed by Pitsios (1999) is largely conjectural. Pitsios (1999) does not specify how his stratigraphic observations can indicate a temporal range. The attribution of the lithics to the Aurignacian is tentative (Darlas 1995) and their association with the skeleton cannot be ascertained from the information published; with the exception of one specimen, a blade, the exact provenance of the lithics is either not specified or reported as probably unrelated to the context of the burial (Mompheratou and Pitsios 1995: 37; but see also Darlas 1995: 59). Finally, while Liritzis and Maniatis (1995) produced two ESR dates of 20–30 ka and 25–45 ka for two travertine samples, these samples were taken from the opening of Cave D and B, respectively, and therefore have no bearing on either Cave C or the burial uncovered there (see also Harvati et al. 2009).

Here we conduct direct dating of the human remains from Apidima C, including the burial as well as two isolated teeth, using U-series dating in order to resolve this question. This effort was undertaken as part of the new research program of the Museum of Anthropology of the Medical School, National and Kapodistrian University of Athens, Greece, in collaboration with the Paleoanthropology group at the University of Tübingen, Germany, and the University of Bergen, Norway.

Materials & Methods

Samples were selected from the Museum of Anthropology’s collections of human remains excavated at Apidima C in the 1980s (Mompherratou and Pitsios 1995). LAO 1 S5 (Fig. 2A) is an isolated upper molar with extensive crown attrition (advanced stage 2, erosion across the entire dentine layer). It was found in the same context as the second specimen, LAO 1 S6 (Fig. 2B), a likely isolated premolar. Its extreme degree of attrition (stage 3, exposed pulp cavity; see Burns 2015) makes its exact anatomical allocation difficult. Two further specimens were selected from the bones associated with the burial of the female skeleton, Apidima 3: A fragment of the sternum (LAO 1 S3_18; Fig. 2C) and a fragment of a pelvic iliac bone (LAO 1 S3_12; Fig. 2D). Permission for sampling was obtained from the Ministry of Culture and Sports, Athens (ΥΠΠΟΑ/ΓΔΑΠΚ/ΔΣΑΝΜ/ΤΕΕ/Φ77/299995/215105/2663/281). All specimens were 3-d scanned before sampling using a handheld structured-light scanner with a maximum scanning accuracy of 50 microns, and high resolution casts were obtained of the two dental remains so as to create a complete record of their anatomy before the sampling procedure was undertaken. Of the four specimens, the iliac fragment (LAO 1 S3_12) did not preserve an appropriate cross-section for analysis and was therefore not used. The remaining samples were assigned the following laboratory reference numbers: LAO 1 S5 (isolated molar): 3776, LAO 1 S6 (isolated premolar): 3777, LAO 1 S3_12 (sternum fragment from female burial): 3778 (see Fig. 3A).

U-Series Analysis

U-series dating is based on the different chemical behavior of uranium (U) and thorium (Th). While uranium is water solvable, thorium is not. As a result, minerals precipitated from water contain uranium isotopes (specifically ²³⁸U, ²³⁴U and ²³⁵U). In a closed system, ²³⁴U decays to ²³⁰Th. The activity ratio of the two isotopes can be used to determine a U-series age. The ²³⁰Th/²³⁴U activity ratio starts with zero and grows over time about 600,000 years) into equilibrium when the ²³⁰Th/²³⁴U ratio is indistinguishable from unity. However, bones and teeth are not closed systems; they accumulate their uranium while they are buried in the ground. The actual U-uptake history can be highly complex (Grün et al. 2014), but generally leads to age calculations that underestimate the burial age of the specimen. It is possible to address the problem of the unknown U-uptake history with a variety of diffusion models (e.g., through the diffusion-adsorption model described by Pike et al. 2002, or the diffusion-adsorption-decay model of Sambridge et al. 2012). However, all these models are based on continuous diffusion processes and cannot recognize longer initial phases with no or little U-diffusion. This problem can be addressed in teeth by combining U-series and ESR methods (Grün et al. 1988). In the context of this study, ESR analysis was not feasible because of time constraints. To reiterate, the U-series ages reported here are apparent closed system age estimates, which most likely underestimate the burial ages of the specimens.

The U-series analyses were carried out using laser ablation, inductively coupled plasma multi-collector mass spectrometry (LA-ICP-MCMS), which minimizes sample destruction of valuable human fossils (e.g., Groucutt et al. 2018). The analyses followed the procedures that were detailed by Grün et al. (2014). Two different analytical strategies were applied: analyzing spots (each for 60 s) along transects (3776B and 3778) and drilling holes with the laser in stationary position for 20 minutes (3776A and 3777), the latter procedure was applied to minimize sample damage (Benson et al. 2013). Sample 3776 was very fragile and a fragment of one of the roots split off. As a result, this individual tooth was analyzed by drilling four holes into the main part and two transects across the root fragment.

All isotope ratios in this paper are activity ratios with 2-σ errors. Ages were calculated with the Isoplot (Ludwig 2012).

Results

The results of the individual spot analyses are shown in Table 1 and those of the holes in Table 2. The data in Table 2 were binned for 10 cycles (corresponding to approximately 10s ablation). As can be seen from Table 2, the analyses of the first three holes of sample 3776A1 to A3 are associated with large errors due to the low U-concentrations (< 2.3 ppm at the surface). The other holes (3776A4 and 3777-1 to 3777-4) had higher U-concentrations at the surface but the ablation efficiency rapidly decreased with measurement length so that only the data of the first 160 to 200 s were used for age calculations. Samples 3776 and 3777 have extremely high elemental U/Th ratios, indicating that there was no interference from detrital Th. The U/Th ratios for sample 3778 are somewhat higher, particularly for 3778B. All individual LA spots and holes return finite age results, indicating that the teeth have apparently not experienced uranium leaching. This could, however, be only confirmed by combining U-series with ESR data (Grün et al. 1988).

The ²³⁰Th/²³⁸U and ²³⁴U/²³⁸U are shown in Figure 3B. There seems to be an overall trend of slightly increasing ²³⁴U/²³⁸U ratios with increasing ²³⁰Th/²³⁸U ratios, but the large errors (due to low U-concentrations and young ages) prevent any meaningful interpretations. For sample 3776, the results of the transects and holes are compatible. The biggest difference is observed for sample 3778 where the two transects yielded distinctively different ²³⁰Th/²³⁸U results, and subsequently apparent ages. Sample 3778B has also distinctively higher U and Th concentrations, which may indicate some incorporation of detrital U and Th into the sample. However, corrections for detrital Th lead only to slightly younger results (by 0.2 ka). The age differences observed between samples 3778A and B could be simply due to some delayed U-uptake or some more complex processes that we cannot really address with the two transects. The distribution of the U-series ages does not indicate that U-leaching has occurred.

Because of the large associated errors, the apparent U-series ages of the three samples are overall statistically indistinguishable.

Please refer to the tables in the downloadable PDF.

Discussion & Conclusions

Our age results imply that all samples have experienced a U-uptake event that corresponds to the Pleistocene/Holocene transition at 11.7 ka b2k (before the year 2000; Walker et al. 2018). Considering that the samples were most likely an open system for some time, particularly in view of the young apparent ages, it can be reasonably envisaged that the U-uptake took place during the terminal Pleistocene. However, it is important to consider that our dates represent minimum age constraints for the samples, and therefore do not exclude an earlier Upper Paleolithic age for the human remains and burial. Nevertheless, in light of these findings, the association of the Apidima 3 skeleton with the Aurignacian lithics, as well as the attribution of the lithics to the Aurignacian, should be re-evaluated. A better understanding of the chronology of the human samples would be gained by ESR analyses on samples 3776 and 3777 (e.g., Brumm et al. 2016). Unfortunately, radiocarbon dating of these samples (e.g., Higham at al. 2014) was not possible due to poor collagen preservation (Higham pers. comm.). However, it might be possible to apply that method to the pierced shell remains associated with Apidima 3 (Douka 2017). Finally, the question of association of the human remains with the material cultural remains recovered at the site can only be resolved through renewed fieldwork and excavation aiming to resolve the stratigraphy, depositional context and site formation processes at Apidima C. In summary, our results confirm a Pleistocene age for the Apidima C human remains, but further research is necessary to assess their association with the early Upper Paleolithic assemblages described for this site.

Acknowledgements

The authors would like to thank Yuexing Feng, University of Queensland, for his invaluable help with the Laser Ablation ICP-MS measurements and Nick Thompson for his help with photographing the specimens used in our analysis. M. Duval’s research is funded by the Australian Research Council Future Fellowship FT150100215 and the Spanish Ramón y Cajal Fellowship RYC2018-025221-I. This research was supported by the European Research Council ERC CoG CROSSROADS (724703). We are grateful to the Greek Ministry of Culture and Sports for their support and to the anonymous reviewer who greatly helped improve this manuscript.

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