Mitogenomes show discontinuity in Gotland’s LN – EBA transition

New paper (behind paywall) The stone cist conundrum: A multidisciplinary approach to investigate Late Neolithic/Early Bronze Age population demography on the island of Gotland, by Fraser et al. J. Archaeol. Sci. (2018) 20:324-337.

Interesting excerpts (emphasis mine):

Unfortunately, due to poor preservation, mitochondrial haplotype calls were only obtained from the EBA individuals in this study. However, some interesting findings were observed. We find two adult local individuals with unique haplogroup lineages [H1a, H1e], and two juvenile individuals with haplogroup lineages [H2a and T1a] previously found exclusively in the CWC individuals analyzed here, all four dated to BA I showing that new lineages had already been established on Gotland at this time period. Another unique haplogroup lineage [K1b] is found in the child dated to BA III at Suderkvie, indicating continued migration to the island. Additionally, the two nonlocal individuals [LN II; ans010 and BA I; hgb010], were adding to the already existing haplogroup lineages [U5b and T2b, respectively] previously found in the PWC individuals analyzed here. The T2b lineage has also been found in the TRB individuals from the dolmen (Table S4).

Our ancestral contribution modeling for the maternal lineages showed that, among the models tested, the only models with a good fit were 55/45 CWC/TRB contribution (Fig. 6), or a 3-way mixture of 55% CWC, 40% TRB, and 5% PWC (Fig. S15). All other models had poor support, including the models with 100% contribution from either group present on Gotland in the preceding period. The PWC group had the weakest fit of all models which is quite surprising as the PWC was the only preceding group that was established on the island in the beginning of the LN period. Although separated temporally by more than 500 years, the individuals from the Hägur burial seems to have been well established in the Eksta area which previously was inhabited by PWC groups. However, the new haplogroup lineages [H1a and H2a] in the Hägur burial suggest some migration to Gotland in a period succeeding the PWC. Archaeologically, it is difficult to reconcile a 55/45 CWC/TRB contribution on Gotland as the temporal range of the TRB culture ends around c. 2700 cal BCE, and presently there is little archaeological evidence of assimilation of TRB and BAC/CWC on Gotland during the latter part of the MN period (e.g. Andersson, 2016). The apparent decline of human activity on the island post TRB, and also later postPWC is intriguing. As we do not see cultural assimilation of TRB and PWC on Gotland one can only speculate as to why TRB disapears from the archaeological record.

The introduction of new female lineages and the mtDNA haplogroup variation within these stone cist burials, together with an increase of nonlocal individuals, and a dietary shift, indicates that a demographic event has happened. The LN period shows traces of activity all over the island compared to the MN period with ten TRB, and eighteen PWC sites (e.g. Bägerfeldt, 1992; Luthander, 1988; Wallin, 2010; Österholm, 1989). This pattern seems to continue into the EBA as seen by the well-established local groups identified by their Sr-signals, as well as the monumental burials.

Fig. 1. Map indicating distribution of TRB-North group megalithic tombs (Blomqvist, 1989; Midgley, 2008; Sjögren, 2003; Tilley, 1999) and PWC areas (Larsson, 2009) modified from (Malmström et al., 2009). Swedish megalithic TRB burial sites included in the analyses: 1. Gökhem passage grave, Falköping, Västergötland, 2. Alvastra dolmen, Östergötland, 3. Mysinge passage grave, Resmo, Öland, 4. Ansarve dolmen, Tofta, Gotland, and 5. the Ostorf TRB burial ground, Mecklenburg-Vorpommern, Germany. From another recent paper by Fraser et al. (2017)

From the conclusions:

We find a shift in population demography compared to the preceding cultural developments on the island recorded from the Neolithic TRB, and sub-Neolithic PWC groups. We find that these burials were used by local groups that were well established in the regions where the burials were situated. These individuals also displayed a different dietary pattern than that noted for the preceding TRB and PWC groups on the island. We also detect sporadic reuse of the MN TRB dolmen in the LN period by nonlocal individuals, who also shows deviating dietary patterns to the LN/EBA individuals in the stone cist burials.

We see an increase of new mitochondrial lineages in the EBA individuals, of which some also were noted in the CWC reference dataset used in this study. Our modeling for maternal ancestry suggests a 3-way model of 55% CWC, 40% TRB, and 5% PWC. Given the broad absence of archaeological evidence for the typical BAC/CWC burials, as well as no archaeological evidence of assimilation of TRB and PWC during the latter part of the MN period on Gotland, it seems probable that the major process of admixture did not occur on the island. Instead, the data indicates an admixture process that occurred elsewhere and prior to migrating to Gotland. Thus, our results suggest later migration to the island during the LN period by people with a new economy, as well as new burial customs. A likely scenario, taking all these factors into account, is a sizable migration of people, with a ~50/50 (maternal) ancestry in TRB and CWC associated groups, possibly admixing with much smaller local groups of PWC associated individuals on the island.

A. Gotland cultural timelines (Apel et al., 2018; Fraser et al., 2018). B: Approximate dates for the Scandinavian Early Neolithic to Early Bronze Age time
divisions; EN: onset of TRB, MN A: onset of PWC, MN B: onset of BAC, LN-EBA time division from Vankilde (1996).

A quite interesting study that supports the predicted greater mobility during the Nordic Early Bronze Age, compared to earlier periods.

Obviously, the use of previous CWC and TRB mtDNA samples (dated necessarily before 2300 BC) to assert that the picture found during the EBA (ca. 1700-1100 BC) is due to the admixture of both cultures is not tenable.

It was more likely a mixture of descendant populations in Scandinavia, after the arrival of the Bell Beaker elites (ca. 2400 BC) and their admixture with the local population, bringing maritime integration and unifying trends to Scandinavia.