Investigating microscopic and molecular organic matter to explore the role of climate in neanderthal social and cultural evolution at a regional scale

Resumen

The role of climate change is a recurrent theme in debates concerning the replacement of Neanderthals by anatomically modern humans across Europe and western Asia. The Iberian Peninsula is widely regarded as a key geographic location for understanding this process, owing to the apparent late survival of Neanderthal populations along its southern and western fringes. Despite this, relatively few long-term terrestrial palaeoclimate records are available against which hypotheses concerning climatic and ecological factors can be tested. Global climatic datasets from analyses of ice-cores obtained from the polar regions have provided a framework for understanding broad fluctuations in climate over geological timescales. At a higher scale of resolution, analysis of speleothems and palynological studies of marine cores off the Atlantic and Mediterranean coasts provide our most complete records of Pleistocene conditions throughout Iberia. Nevertheless, the extent to which these records accurately reflect regional and local terrestrial conditions in the areas inhabited by Neanderthal groups remains unclear. The persistence of locally favourable conditions in certain parts of the peninsula during periods of wider global climatic downturn cannot be discounted, and relatively few site-specific palaeoecological studies from human occupation contexts have been carried out. This presents a challenge for understanding long-term human-environment dynamics and the processes which drove the Middle to Upper Paleolithic transition. Compound-specific isotope analyses of sedimentary organic compounds, particularly hydrocarbons derived from fossil leaf waxes (n-alkanes), have recently emerged as a powerful palaeoclimate proxy for reconstructing palaeohydrological (hydrogen isotope) and palaeoecological (carbon isotope) conditions at a high spatial and temporal resolution. This technique has been successfully applied in a wide range of terrestrial environments, however, has rarely been utilised in archaeological contexts. Here we combine compound-specific carbon and hydrogen isotope analysis of sedimentary n-alkanes with archaeological soil micromorphology, a well-established geoarchaeological method, to better understand the sedimentary context of the target organic compounds and any potential biases which might arise due to anthropogenic activity or post-depositional processes. We investigate palaeoenvironmental conditions at the Middle Palaeolithic sites of Abric del Pastor (MIS 5/4) and El Salt (MIS 3) in Alcoy, Eastern Iberia, in combination with traditional palaeoenvironmental proxies employed in archaeological research, including charcoal, microvertebrates, macro-faunal remains and pollen. In addition, an experimental study has been carried out to assess the degree of thermal alteration on hydrogen isotope signatures of different plant organs of Celtis australis, a common anthropogenic fuel source at the study sites. Our results reveal the strengths of integrated microscopic and molecular approaches to address archaeological questions, highlight new avenues for future research, and add new levels of detail to our understanding of the regional environmental variability that framed the disappearance of Neanderthal populations in Eastern Iberia.