How to interpret past human mobility patterns

celtic-europe-national-geographic

New paper (behind paywall), Interpreting Past Human Mobility Patterns: A Model, by Reiter and Frei Eur J Archaeol (2019).

Interesting excerpts (modified for clarity; emphasis mine):

Present investigations of mobility can be divided into two main groups: 1) individual mobility, and 2) group mobility.

Research approach

(…) it is arguable that, ‘the reality of a mobile existence is far more complex than the ordering principles used to describe it’ (Wendrich & Barnard, 2008: 15). It seems that the most accurate means of modelling mobility is through a thorough examination of a variety of phenomena in combination with archaeological context. Notable examples of these defining criteria include:

  1. Mobility (length of time, season);
  2. Number of journeys;
  3. Segment of the population which moved (as defined by gender, age, health, occupation, or social position);
  4. General socio-political organization;
  5. Logistics and available modes of transport.

means-identifying-individual-vs-group-mobility

In an ideal world, these five categories should be investigated via multiple samples from multiple individuals from a site, region, and culture group who represent the full gamut of ages, sexes, and social levels. Unfortunately, the fragmentary nature of the archaeological record rarely includes material suitable for covering all parameters.

A mobility model

Thirty years ago, David Anthony criticized archaeologists for their approach to migration: ‘instead of developing the needed tools, archaeologists have avoided the subject’ (Anthony, 1990: 895).

Although there are (and always will be) holes in the record, we propose a mobility model composed of four over-arching mobility patterns which we have named as follows:

NOTE. Cases explored in the paper are within brackets.

  1. Non-migratory [no mobility: The Case of Singen (Germany)];
  2. Point-to-point migratory [The Case of the Skrydstrup Woman (Denmark)];
  3. Back-and-forth [The case of Haraldskaer Woman (Denmark)];
  4. Repeated mobility, subdivided into
    • Cyclical mobility [The cases of Nieder-Mörlen (Germany) and Ötzi (Italy)]
    • Non-cyclical mobility [The cases of the medieval Silk Road, Roman York, Viking Age Trelleborg, and La Tène Bohemia]

human-mobility-model

All told, the mobility patterns identified in the present model cleave to three overarching kinds of mobility: non-mobility, single mobility/migration, and multiple movements. The causes of non-mobility and different types of mobility can be manifold.

Non-mobility may include lack of sufficient funds or surplus, social obligations, health status, age, and social standing (serf, slave, landed gentry).

Single, unidirectional movements may have been caused by marriage alliances; family movements; social, political, or economic instability; violence (enslavement, kidnapping); or health issues.

By contrast, individuals who show evidence of multiple movement were likely to have been moving because mobility formed part of their employment, beliefs (ritual), or lifestyle. Although a warrior or soldier, herder, trader, or traveller within an extensive kinship network may present very different mobility patterns, they are all unified by the fact that their chosen occupation or social group(s) exhibit some form of mobility mandate.

The causes of back-and-forth mobility are difficult to define as different reasons could spur a single to-and-from journey to a specific place of cultural, religious, or personal importance.

Repeated mobility, be it cyclical or more irregular (non-cyclical), can also be closely related to social status. For example, a peddler, small-scale trader, or migrant worker’s identity and integration (or nonintegration) into the society (or societies) with which they have contact can be defined by their transitory lifestyles. (…) both the profession and its mobile nature removed metalworkers from ‘normal’ society; in many cases, they formed a separate social category (Neipert, 2006). This could also be the case with warriors. Although contact with migratory workers or specialists was necessary for temporary collaboration, prolonged contact might involve severe social change (Neaher, 1979; Bollig, 1987).

Related

Viking Age town shows higher genetic diversity than Neolithic and Bronze Age

sigtuna-vikings

Open access Genomic and Strontium Isotope Variation Reveal Immigration Patterns in a Viking Age Town, by Krzewińska et al., Current Biology (2018).

Interesting excerpts (emphasis mine, some references deleted for clarity):

The town of Sigtuna in eastern central Sweden was one of the pioneer urban hubs in the vast and complex communicative network of the Viking world. The town that is thought to have been royally founded was planned and organized as a formal administrative center and was an important focal point for the establishment of Christianity [19]. The material culture in Sigtuna indicates that the town had intense international contacts and hosted several cemeteries with a Christian character. Some of them may have been used by kin-based groups or by people sharing the same sociocultural background. In order to explore the character and magnitude of mobility and migration in a late Viking Age town, we generated and analyzed genomic (n = 23) and strontium isotope (n = 31) data from individuals excavated in Sigtuna.

y-dna-vikings

The mitochondrial genomes were sequenced at 1.5× to 367× coverage. Most of the individuals were assigned to haplogroups commonly found in current-day Europeans, such as H, J, and U [14, 26, 27]. All of these haplotypes are present in Scandinavia today.

The Y chromosome haplogroups were assigned in seven males. The Y haplogroups include I1a, I2a, N1a, G2a, and R1b. Two identified lineages (I2a and N1a) have not been found in modern-day Sweden or Norway [28, 29]. Haplogroups I and N are associated with eastern and central Europe, as well as Finno-Ugric groups [30]. Interestingly, I2a was previously identified in a middle Neolithic Swedish hunter-gatherer dating to ca. 3,000 years BCE [31].

In Sigtuna, the genetic diversity in the late Viking Age was greater than the genetic diversity in late Neolithic and Bronze Age cultures (Unetice and Yamnaya as examples) and modern East Asians; it was on par with Roman soldiers in England but lower than in modern-day European groups (GBR and FIN; Figure 2B). Within the town, the group excavated at church 1 has somewhat greater diversity than that at cemetery 1. Interestingly, the diversity at church 1 is nearly as high as that observed in Roman soldiers in England, which is remarkable, since the latter was considered to be an exceptionally heterogeneous group in contemporary Europe [39].

pca-vikings
A PCA plot visualising all 23 individuals from Sigtuna used in ancient DNA analyses (m – males, f – females).

Different sex-related mobility patterns for Sigtuna inhabitants have been suggested based on material culture, especially ceramics. Building on design and clay analyses, some female potters in Sigtuna are thought to have grown up in Novgorod in Rus’ [40]. Moreover, historical sources mention female mobility in connection to marriage, especially among the elite from Rus’ and West Slavonic regions [41, 42]. Male mobility is also known from historical sources, often in connection to clergymen moving to the town [43].

Interestingly, we found a number of individuals from Sigtuna to be genetically similar to the modern-day human variation of eastern Europeans, and most harbor close genetic affinities to Lithuanians (Figure 2A). The strontium isotope ratios in 28 adult individuals with assigned biological sex and strontium values obtained from teeth (23 M1 and five M2) show that 70% of the females and 44% of the males from Sigtuna were non-locals (STAR Methods). The difference in migrant ratios between females and male mobility patterns was not statistically significant (Fisher’s exact test, p = 0.254 for 28 individuals and p = 0.376 for 16 individuals). Hence, no evidence of a sex-specific mobility pattern was found.

(…) As these social groups are not mirrored by our genetic or strontium data, this suggests that the inclusion in them was not based on kinship. Therefore, it appears as if socio-cultural factors, not biological bonds, governed where people were interred (i.e., the choice of cemetery).

diversity-yamna
Average pairwise genetic diversity measured in complete Sigtuna, St. Gertrud (church 1) and cemetery 1 (the Nunnan block) compared to both ancient and modern populations ranked by time period (Yamnaya, Unetice, and GBR-Roman, Roman Age individuals from Great Britain; GBR-AS, Anglo-Saxon individuals from Great Britain; GBR-IA, Iron Age individuals from Great Britain; JPT-Modern, presentday Japanese from Tokyo; FIN-Modern, present-day Finnish; GBR-Modern, present-day British; GIHModern, present-day Gujarati Indian from Houston, Texas). Error bars show ±2 SEs.

Interesting from this paper is the higher genetic (especially Y-DNA) diversity found in more recent periods (see e.g. here) compared to Neolithic and Bronze Age cultures, which is probably the reason behind some obviously wrong interpretations, e.g. regarding links between Yamna and Corded Ware populations.

The sample 84001, a “first-generation short-distance migrant” of haplogroup N1c-L392 (N1a in the new nomenclature) brings yet more proof of how:

  • Admixture changes completely within a certain number of generations. In this case, the N1c-L392 sample clusters within the genetic variation of modern Norwegians, near to the Skane Iron Age sample, and not with its eastern origin (likely many generations before).
  • This haplogroup appeared quite late in Fennoscandia but still managed to integrate and expand into different ethnolinguistic groups; in this case, this individual was probably a Viking of Nordic language, given its genetic admixture and its non-local (but neighbouring Scandinavian) strontium values.

Related

Improving environmental conditions favoured higher local population density, which favoured domestication

agricultural-origins

New paper (behind paywall) Hindcasting global population densities reveals forces enabling the origin of agriculture, by Kavanagh et al., Nature Human Behaviour (2018)

Abstract (emphasis mine):

The development and spread of agriculture changed fundamental characteristics of human societies1,2,3. However, the degree to which environmental and social conditions enabled the origins of agriculture remains contested4,5,6. We test three hypothesized links between the environment, population density and the origins of plant and animal domestication, a prerequisite for agriculture: (1) domestication arose as environmental conditions improved and population densities increased7 (surplus hypothesis); (2) populations needed domestication to overcome deteriorating environmental conditions (necessity hypothesis)8,9; (3) factors promoting domestication were distinct in each location10 (regional uniqueness hypothesis). We overcome previous data limitations with a statistical model, in which environmental, geographic and cultural variables capture 77% of the variation in population density among 220 foraging societies worldwide. We use this model to hindcast potential population densities across the globe from 21,000 to 4,000 years before present. Despite the timing of domestication varying by thousands of years, we show that improving environmental conditions favoured higher local population densities during periods when domestication arose in every known agricultural origin centre. Our results uncover a common, global factor that facilitated one of humanity’s most significant innovations and demonstrate that modelling ancestral demographic changes can illuminate major events deep in human history.

cultural-variables-population-densities
Path diagram for piecewise-SEM exploring the effects of environmental and cultural variables on population densities of foraging societies. Measured variables are represented by the large boxes and R2 GLMM values (see Methods) are provided for response variables. n = 220. Red arrows depict negative relationships among variables, black arrows positive relationships, and dashed grey arrows depict non-significant paths (P ≥ 0.05). Standardized coefficients are presented for all paths (small boxes) and arrow widths are scaled to reflect the magnitude of path coefficients.

Interesting excerpts:

(…) our results are consistent with the surplus hypothesis, which suggests that improving environmental conditions and the potential for increased population density may have facilitated the domestication of plants and animals in agricultural origin centres4,7 (Fig. 3). Several factors may explain the links between environmental conditions, potential population density and the origin of domestication. For one, rates of innovation may scale positively with the number of potential innovators13,14. In turn, the likelihood of domestication innovations may have increased in environments that could support increasingly higher densities of foraging people.

In addition, foraging societies may have become more sedentary to take advantage of locally abundant resources, some of which were later domesticated35. Our results indicate that residential mobility scales negatively with population density in foraging societies (Fig. 1). Therefore, increasingly sedentary lifestyles may have contributed further to increases in population density and the potential for innovation. Increases in the productivity of wild progenitors of important domesticates may have also facilitated growing population densities and the viability of cultivation for food production15,16.

population-density-foragers
Predictions of potential population density for foragers. a–c, Predicted population densities at 4,000 (a), 10,000 (b) and 21,000 (c) YBP. Blue hues depict potential population densities below the median population density of observed foraging societies, and red hues depict potential population densities above the median. The second red hue and above are greater than the mean population density of observed foraging societies. Note the increase in area, through time, with potential population densities greater than the mean of observed foraging societies (number of 0.5° × 0.5° cells: 21,000 YBP = 3,027; 4,000 YBP = 4,673). For example, a notable increase in the number of red cells in the Sudanic savannah and Ganges of East India (Northeast India) between panels c and a.

It is also possible that improving environmental conditions may have resulted in a situation where necessity drove the origins of domestication. For example, population densities may have increased in foraging societies that occupied productive, coastal areas, causing an outflow of groups into regions with less ideal conditions where the cultivation of plants and animals was required to secure adequate food resources6,17,18. Our results cannot support, or refute, the possible influence the outflow of people from hospitable locations to less ideal environments may have played. A detailed understanding of the movements of ancient populations is required for more rigorous testing of the role that forced habitation of marginal environments may have played in the origins of domestication at particular sites.

See also: