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.

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. 

This thesis aims to study and describe the ancient populations of grey and harp seals in the Baltic Sea, and to present new methodological approaches for general use in ancient DNA studies.

The dissertation is comprised of five studies: a review of the use of paleogenetics in studying ancient human-marine mammal interactions; a method paper investigating patterns of DNA preservation in ancient pinniped samples; a method paper presenting a genetic sex identification method for ancient pinnipeds; a population genomic study of the Baltic grey seal; and a population genomic study of the now extinct Baltic harp seal.

Guidelines for ancient DNA sample selections were deduced from broad-scale statistical modelling of factors influencing DNA preservation in pinniped bones, the most significant of which included type of bone element, collagen content, and whether the bone derive from a cave context. Modern ringed seal samples with known sex were used to test an alternative pinniped sex identification method using the annotated dog genome as a reference for quantification of the relative representation of X chromosome reads. Reliable sex identification was shown to require a minimum of 5,000 total reads mapped to the reference genome. A total of 69 mitochondrial control regions were generated for Baltic grey seals, which revealed that the Mesolithic data largely represent extinct haplotypes, the main of which continued until the Early Neolithic. A population replacement prior to the early Bronze Age introduced mitochondrial variation resembling that of modern Baltic greys seals. The level of genetic differentiation between the Baltic harp seal population and the three contemporary breeding populations, suggests that the White Sea population is the most likely ancestor of the Baltic harp seal breeding population. An increase in genetic diversity, following a hiatus with no Baltic harp seals, combined with the measures of genetic differentiation from this period, further suggests that a second colonization likely occurred from the White Sea during the early Bronze Age.

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 aurochs (Bos primigenius) was once widespread in Europe, Asia and North Africa. The aurochs was both the ancestor of domestic cattle, and co-existed alongside them for millennia post domestication, before going extinct in 1627. Several studies have suggested that admixture occurred between wild aurochs populations and domestic cattle. To contribute towards our understanding of this process, we generated near complete mitochondrial genomes (between 15063 and 16338 nucleotides) from material derived from the horn of the last aurochs bull (died in 1620) as well as five medieval period Scandinavian drinking horns that have been attributed to aurochs based on their size. Phylogenetic analysis on the data shows that three drinking horns carry European aurochs haplotype P, while two of the drinking horns and the horn of the last aurochs bull carry modern domestic taurine cattle T haplotypes. Our results therefore demonstrate that drinking horns may represent a unique source of material with which to study aurochs genetics, and that the last European aurochs likely underwent a degree of admixture with domestic cattle. We anticipate that future analysis of the nuclear DNA content of such horns will be able to shed further light into the specifics of these admixture events.

The harp seal is a cold adapted seal species, which requires a suitable formation of pack ice during winter and spring to secure successfully breeding. Today harp seals live in sub-Arctic regions and the North Atlantic, but during the middle Holocene Period, a breeding population existed in the Baltic Sea. In order to investigate the genetic affinity and faith of the now extinct Baltic harp seal population we analysed contemporary and ancient mitogenomes from across the species contemporary and ancient range. Ancient mitochondrial genomes were generated for a total of 49 Baltic harp seals ranging from Late Mesolithic to the Iron Age, together with five Neolithic samples from the White Sea and two Neolithic samples from the Northwest Atlantic Ocean. The ancient data was compared to published modern harp seal data assigned to the present breeding populations around Newfoundland, the Greenland Sea and the White Sea. Surprisingly only limited phylogenetic resolution was found among the ancient and modern localities. The statistical measures for genetic differentiation, however, identified significant levels of population genetic differentiation between the Baltic harp seal population and all modern populations, which suggest an independent breeding population in the Baltic Sea. The low level of genetic differentiation to the White Sea population indicate a shared ancestry between the Baltic and White Sea. The generated Skyline plot suggest second wave of colonization after a hiatus in the Baltic Sea. Interestingly, the genetic diversity in the Baltic harp seal population was significantly higher than in any of the modern populations. However, a drastic decrease in genetic diversity is observed from the Bronze Age to the Iron Age, which might be linked to effects of high hunting pressure and climatic changes towards the final extirpation of harp seals in the Baltic.