The now-extinct harp seal population that inhabited the Baltic Sea from the Mesolithic to the Iron Age is an enigma. It occurred outside the species' contemporary Arctic range, likely deviated from typical harp seal migratory behaviour, and experienced body size reductions and dramatic population fluctuations leading up to its extinction. Here we use ancient DNA analyses to shed more light on the evolutionary history of the Baltic Sea harp seal population, including its origin, timing of colonisation, diversity and factors contributing to its demise. We generated 49 ancient Baltic and eight ancient Arctic harp seal mitogenomes, which we analysed together with 53 contemporary Arctic harp seal mitogenomes. We detected limited phylogeographic resolution among ancient and contemporary populations, which we interpret as a late Pleistocene range expansion from a common refugial population with subsequent gene flow. Ancient Baltic harp seals were significantly genetically differentiated from contemporary harp seal populations and retained their own genetic composition throughout time. The genetic diversity of Baltic harp seals decreased over time, yet was comparable to that of contemporary populations. This suggests that Baltic harp seals formed a distinct breeding population, which may occasionally have received immigrants from the Arctic but was itself confined in the Baltic Sea until the end. We hypothesise that loss of genetic diversity and the ultimate extinction of the Baltic harp seal population was a consequence of population fluctuations caused by climatic change, reduced salinity and biological productivity, and periodic intense human harvest.

Mesolithic harpoons are structurally complex weapon points and significant sources of archaeological information. Nonetheless, separating different types of information inherent in harpoon point shapes (e.g., aspects relating to mechanical performance, personal or group craft variation and chronology) using descriptive approaches is difficult. In this study, we employed an exploratory geometric morphometric approach to the analysis of 28 Mesolithic harpoon points, and 144 undated harpoon points from the circum-Baltic Sea area in order to retrieve both functional and spatiotemporal information. By analyzing harpoon structure statistically through (i.) a shape proxy (harpoon silhouette) and (ii.) harpoon sub-structures (barbs), we have been able to reveal information related to both variation in shape convention and functional constraints. Barb shape results revealed statistically significant chronological and geographical groupings with spatiotemporal barb-shape trajectories made visible and objective evaluation of how barb-shape conventions impacted functional variation. In addition, harpoon silhouette shape distributions were shown to have potential as sources for robust artifact classifications in relation to functional constraints and raw-material engagement. These results suggest that morphometric approaches similar to the ones we have employed offer promising ways of addressing specific archaeological questions in the context of harpoon point shapes and, more generally, other complex weapon point forms.

The grey seal (Halichoerus grypus) has been part of the Baltic Sea fauna for more than 9000 years and has ever since been subjected to extensive human hunting, particularly during the early phases of its presence in the Baltic Sea, but also in the early 20th century. In order to study their temporal genetic structure and to investigate whether there has been a genetically continuous grey seal population in the Baltic, we generated mitochondrial control region data from skeletal remains from ancient grey seals from the archaeological sites Stora Förvar (Sweden) and Neustadt (Germany) and compared these with modern grey seal data. We found that the majority of the Mesolithic grey seals represent haplotypes that is not found in contemporary grey seals, indicating that the Baltic Sea population went extinct, likely due to human overexploitation and environmental change. We hypothesize that grey seals recolonised the Baltic Sea from the North Sea. during the Bronze Age or Iron Age, and that the contemporary Baltic grey seal population is direct descendants of this recolonisation. Our study highlights the power of biomolecular archaeology to understand the factors that shape contemporary marine diversity. 

In recent years, non-human ancient DNA studies have begun to focus on larger sample sizes and whole genomes, offering the potential to reveal exciting and hitherto unknown answers to ongoing biological and archaeological questions. However, one major limitation to the feasibility of such studies is the substantial financial and time investments still required during sample screening, due to uncertainty regarding successful sample selection. This study investigates the effect of a wide range of sample properties including latitude, sample age, skeletal element, collagen preservation, and context on endogenous content and DNA damage profiles for 317 ancient and historic pinnipedsamples collected from across the North Atlantic. Using generalised linear and mixed-effectmodels, we found that a range of factors affected DNA preservation within each of the species under consideration. The most important findings were that endogenous content varied significantly according to context, the type of skeletal element, the collagen content and collection year. There also appears to be an effect of the sample’s geographic origin, with samples from the Arcticgenerally showing higher endogenous content and lower damage rates. Both latitude and sample age were found to have significant relationships with damage levels, but only for walrus samples. Sex, ontogenetic age and extraction material preparation were not found to have any significantrelationship with DNA preservation. Overall, the skeletal element and sample context were found to be the most influential factors and should therefore be considered when selecting samples for large-scale ancient genome studies.

The harp seal (Pagophilus groenlandicus), today a subarctic species with breeding populations in the White Sea, around the Jan Mayen Islands and Newfoundland, was a common pinniped species in the Baltic Sea during the mid- and late Holocene. It is puzzling how an ice dependent species could breed in the Baltic Sea during the Holocene Thermal Maximum (HTM), and it remains unclear for how long harp seals bred in the Baltic Sea and when the population became extirpated. We combined radiocarbon dating of harp seal bones with zooarchaeological, palaeoenvironmental and stable isotope data to reconstruct the harp seal occurrence in the Baltic Sea. Our study revealed two phases of harp seal presence and verifies that the first colonization and establishment of a local breeding population occurred within the HTM. We suggest that periods with very warm summers but cold winters allowed harp seals to breed on the ice. Human pressure, salinity fluctuations with consequent changes in prey availability and competition for food resources, mainly cod, resulted in physiological stress that ultimately led to a population decline and local extirpation during the first phase. The population reappeared after a long hiatus. Final extinction of the Baltic Sea harp seal coincided with the Medieval Warm Period. Our data provide insights for the first time on the combined effects of past climatic and environmental change and human pressure on seal populations and can contribute with new knowledge on ongoing discussions concerning the impacts of such effects on current arctic seal populations.

Exploitation of aquatic resources, especially fishing, was a fundamental part of human subsistence during the Late Mesolithic Ertebolle culture (5400-4000/3900 cal BC) in Southern Scandinavia. In this pilot study we examine three cod otoliths from two Late Mesolithic locations in eastern Denmark to see whether local environmental conditions are reflected in the strontium ratios of the fish, to source the fish to either the Atlantic or the Baltic Sea and finally, to explore how fishing was conducted in relation to the settlements. We used laser ablation multi-collector inductively coupled plasma mass spectrometry for sequential sampling of the otoliths for strontium isotope analysis. All three otoliths yielded Sr-87/Sr-86 values that fall within the range of Baltic Sea water, thus indicating that cod caught by Mesolithic fishers in the Danish straits belonged to the Baltic Sea stock. Our results suggest that cod were not caught in waters immediately adjacent to the archaeological sites but rather came from the Kattegat. We could not detect any substantial change in habitat between juvenile and mature stages of the fishs lives. Our study shows the potential of isotopic analysis to address issues regarding the individual ecological history of fish and human fishing strategies.