Tracing ancient human migration

Introduction

The theme of the 51st International Symposium of the Society for the Study of Human Biology (SSHB 2009) was human dispersals, during which noted speakers presented the state of the art on the dynamics of out of Africa migration and subsequent dispersals and worldwide diasporas of our species over time.

Human migration is an important cause of change in the genetic and demographic structure of human populations. In a unique synthesis of our understanding of human biology in general, research into human dispersal draws together past and present patterns of population movements and their influence on the genetic, epidemiological and demographic make up of populations.

The scientific tools for reinvestigating human dispersals are innovative archaeological approaches, particularly molecular techniques. With the rapid growth of biomolecular archaeology, researchers can now retrieve molecular information from ancient biological specimens, thus gaining new insights into the biological, demographic and cultural processes underlying human dispersal. Driven by rapid methodological advances in biotechnology and drawing upon ever-expanding modern molecular genetic databases, biomolecular archaeology has begun to unravel the complex mechanisms of prehistoric and historic dispersals of human populations, as well as dietary adaptations and the timing and context of the domestication of animals and plants. Novel approaches being developed in the study of human dispersals are informed by a broad range of complementary state-of-the-art analytical methods for examining ancient materials, including human and animal bones and teeth, as the primary resource; advanced archaeological strategies; innovative phylogeographic approaches to the genetic data of extant populations, and shifts in human diet.

The Symposium started its proceedings with one of the main issues in anthropology: The primary dispersal of our species out of Africa. New archaeological evidence from the Arabian peninsula and the Indian subcontinent that has not been part of the standard human evolutionary story, new fieldwork findings as well as a reappraisal of research on the southern route along the Indian Ocean edge were presented. All data demonstrate that the pattern of human migrations was more fascinating and intricate than previously thought.

Another major topic is the study of the Neolithic transition, one line of biomolecular research applied to the reconstruction of human dispersal, building on the pioneer work of Ammerman and Cavalli-Sforza (1984), which has opened up exciting prospects for assessing the socio-economic impact of agriculture and pastoralism and for determining the sociobiological processes of domestication. Working in this area speakers presented, among other things, the results of their investigation of the dietary status, genetic composition and mobility of inhabitants of Europe between 9000 and 7000 years ago, as well as an analysis of the domestic animals they relied upon and which may have moved with them. This period coincided with the arrival of farming in the area, either as a series of processes or as a rapid event, which irreversibly transformed the whole of Europe and had far-reaching effects on human demography, health, social organization and ideology.

The transition from hunting–gathering to agriculture and pastoralism was an ecological change that profoundly affected our evolutionary history and constitutes a milestone event in human history. The earliest evidence for domestic animals and plants has been found in the Levant and Anatolia dating to around 10000 years ago, whence farming practices subsequently spread north-westwards into the Balkans, where the earliest farming sites date to ca 9000 BP. From there, two broad routes involving Early Neolithic cultural groups are traditionally cited to explain the transmission of farming: A Northern route (the Danubian culture) and a Southern route through the Mediterranean (the Cardial impressed ware culture). However, many archaeologists are wary of tracking the spread of farming based solely on cultural observations, and the transmission of farming was probably much more complicated than such simple maps would imply. The application of well-developed biomolecular methods to human and animal skeletal remains from key prehistoric sites has started to resolve long-standing questions concerning the origins and dispersals of early farmers, the impact the arrival of agriculture had on pre-existing indigenous groups in terms of demography, genetic change and subsistence, and the extent to which their legacy is still observable today.

Generations of researchers have taken various approaches to studying the origins of agriculture; new settlement patterns, technologies and material culture have been cited to herald the first arrival of farming. So far, all have fallen short of explaining the reasons for why critical change came about in food production in Europe: Was it the result of revolution or evolution; did farming totally replace hunting–gathering; did it trigger a large-scale movement and replacement of people and ideas; and were these processes homogeneous through space and time?

A recent re-evaluation of radiocarbon dates for Cardial-impressed sites (Zilhão 2001) has suggested that farming spread through the Western Mediterranean incredibly quickly by maritime pioneer colonization: The implication being that the Southern route of expansion differed considerably from the Neolithization of other parts of Europe. Thanks to rapid advances in the application of biomolecular methods to archaeology, this specific problem can now be investigated within a wider context using currently available state-of-the-art methods as presented by the invited speakers.

DNA and ancient DNA (aDNA) analyses

The study of DNA and particularly of aDNA has tended to be used to tackle questions regarding evolution and migrations of our species and to study the dispersal of crops and domestic livestock (Brown 1999; Troy et al. 2001). The primary focus of aDNA research has been on mitochondrial DNA (mtDNA), since mammalian cells contain several thousand copies of the mt genome, which greatly enhances its survival in ancient tissues. Furthermore, because of its fast evolution and maternal mode of inheritance, mtDNA is an ideal candidate for genealogical study. Extensive work on characterizing mtDNA variation in modern geographically dispersed populations has produced a well-resolved world phylogeny (Torroni et al. 2006). On a finer scale, phylogeographic analysis of modern mtDNA has been employed to determine the degree of genetic replacement associated with the migration of farmers from the Near East into Europe (Richards et al. 2003; Currat and Excoffier 2005; Tarsi et al. 2006).

However, as compared with mapping earlier Paleolithic migrations and later post-Neolithic migrations, estimating the contribution of genes from early farmers based on modern data alone is extremely challenging; inevitably, a wide range of values has been reported. What aDNA analysis of Early Neolithic and late European hunter–gatherer samples from Central Europe has clearly demonstrated is that high frequencies of mtDNA haplotypes are found only at low frequencies in modern Europeans (Haak et al. 2005; Bramanti et al. 2009). But the large genetic differences between hunter–gatherers, early farmers and present day populations cannot be explained by population continuity alone. This would imply that the first farmers, at least from this part of Europe, were not the descendants of local hunter–gatherers, but instead migrated into Central Europe at the onset of the Neolithic, neither did they make a sizeable contribution to the modern gene pool, which is surprising given the success of farming itself. This study challenges the idea of mass migrations of farmers during the Neolithic and is more consistent with the movement of ideas without genetic displacement. On this point, Haak and Bramanti's studies are limited both geographically and in terms of the number of samples analysed. To date, few aDNA studies have been conducted on prehistoric human remains from Southern Europe (Tarsi et al. 2006) and none on the first agricultural populations in this region, coinciding with the Impressed ware cultures of the hypothetical Mediterranean route of agricultural expansion. The hypothesized rapid maritime expansion through this region may imply some genetic replacement or initial seeding of genetically distinct populations. Evidence for this hypothesis can only be obtained from direct DNA analysis of early agricultural populations.

Animal DNA

The presence of domestic animals in the Neolithic is useful to determine their origins and whether they moved with the early farmers or were domesticated independently. Sheep and goats were imported from their centre of domestication in the Near East, as there were no related wild species in Southern Europe during the Neolithic. Whether South European cattle were separately domesticated from European wild cattle (Bos primigenius) or were derived from Middle Eastern stock is a controversial issue. Some authors have reported a clear distinction between mtDNA sequences from Neolithic domestic and wild cattle in Central Europe – data that also fit with the analysis of British samples. However, aDNA analysis of samples from Southern Europe has detected domestic taurine haplotypes in wild auroch samples, raising the possibility that cattle may have been independently domesticated in this region. If true, this suggests continuous exploitation of wild cattle in the early Neolithic, rather than a total reliance on imported domestic species. Notably, the latter is more likely to be expected if rapid colonization had occurred (Loftus et al. 1994; Beja-Pereira et al. 2006; Bollongino et al. 2006; Achilli et al. 2008). Analysis of a-mtDNA from wild aurochs and domestic taurine cattle from Southern Europe and comparison with previously published sequences may help to clarify this issue.

Stable isotope analysis of bone collagen

Another approach to the study of the Neolithic transition is by stable isotope analysis. Since collagen preserved in ancient tissues is synthesized mainly from dietary protein, the carbon (¹³C) and nitrogen (¹⁵N) stable isotope ratios in extracted collagen reflect the ratios in foods that were eaten during an individual's lifetime or, more accurately, the residence time of collagen prior to death (10 years in human bone). C and N stable isotope ratios vary in different foodstuffs: ¹³C values can distinguish marine from terrestrial consumers (Richards and Hedges 1999) and C3 plants from C4 plants consumers. As ¹⁵N values indicate the trophic level of the food consumed, they can be used to distinguish carnivores from herbivores in terrestrial ecosystems and between the more complex trophic levels in a marine ecosystem. Using sulphur (³⁴S), another isotope, consumers of marine, freshwater and terrestrial resources can be distinguished from one another.

Application of stable isotope analysis to ancient human bones from Atlantic European coastal regions has revealed a rapid shift in diets with the arrival of agriculture and a marked decrease in the exploitation of marine resources (Richards et al. 2003; Milner et al. 2004). This finding gives credence to the idea that farming had fairly quickly replaced fishing and gathering. Application of the technique to human remains obtained from late forager and early agricultural sites on or near the Mediterranean coast will enable researchers to verify whether a similar change in diet occurred when the first domestic animal and plants appeared in this region.

Strontium isotope analysis of tooth enamel

Strontium (Sr) isotope analysis of human and animal tooth enamel has shed light on population mobility. Strontium isotopes in human tooth enamel reflect the local geology of the area where individuals grew up, since the teeth form during early childhood. It follows then that if the tooth Sr isotopic signal does not match the local geological isotopic signal where an individual was buried, the individual probably shifted residence during their lifetime.

Strontium isotope analysis applied to human remains from early farming sites in Central Europe has revealed a surprisingly high degree of residential mobility among these populations (Bentley et al. 2004). Because agriculture is often associated with increased sedentism, this finding may imply that some early farmers buried at these sites were the first settlers who actually migrated to their place of interment during their lifetime. This technique could prove a valuable tool for uncovering new evidence if applied to human and animal teeth from late forager and early agricultural sites on or near the Mediterranean coastline: A distinctive marine Sr isotope signature in human remains would indicate that individuals spent their first years of life on or near the coast, as expected if migrations and mobility followed predominantly maritime routes. Similarly, information from animals will show whether they were moved around the landscape or restricted to coastal zones and whether they were imported or brought to the site, as would be expected following maritime colonization of early farming populations.

In conclusion from the various communications it was stressed that, in order to achieve a deeper understanding of the Neolithic transition crossways of Europe, and across Southern Europe in particular, more sites need to be investigated and interpreted within the wider context of European and Mediterranean archaeology. Integration of research at the European level would set the basis for a much stronger analysis and interpretation of regional variability in the Mediterranean.

Declaration of interest: The author reports no conflicts of interest. The author alone is responsible for the content and writing of the paper.