Neolithic Europe - Genetics

Genetics

Archaeologists agree that the technologies associated with agriculture originated in the Levant/Near East and then spread into Europe. However, debate exists whether this resulted from an active migratory process from the Near East, or merely due to cultural contact between Europeans and Near Easterners. Currently, three models summarize the proposed pattern of spread:

1. Replacement model: posits that there was a significant migration of farmers from the Fertile Crescent into Europe. Given their technological advantages, they would have displaced or absorbed the less numerous hunter-gathering populace. Thus, modern Europeans are primarily descended from these Neolithic farmers.

2. Cultural diffusion: in contrast, this model supposes that agriculture reached Europe by way of a flow of ideas and trade between the Mesolithic European population and Anatolian farmers. There was no net increase in migration during this process, and therefore, modern Europeans are descended from the "original" Palaeolithic hunter-gatherers.

3. Pioneer model: recognises that models 1) and 2) above may represent false dichotomies. This model postulates that there was an initial, small scale migration of farmers from the Near East to certain regions of Europe. They might have enjoyed localized demographic expansions due to social advantages. The subsequent spread of farming technologies throughout the rest of Europe was then carried out by Mesolithic Europeans who acquired new skill through trade and cultural interaction.

Genetic studies have been utilised in the study of pre-historic population movements. On the whole, scientists agree that there is evidence for a migration during the Neolithic. However, they cannot agree on the extent of this movement. The conclusions of studies appear to be 'operator dependent'. That is, results vary depending on what underlying mutation rates are assumed, and conclusions are drawn from how the authors 'envisage' their results fit with known archaeological and historic processes. Consequently, such studies must be interpreted with caution.

Perhaps the first scholar to posit a large-scale Neolithic migration, based on genetic evidence, was Luigi Luca Cavalli-Sforza. By applying principal component analysis to data from "classical genetic markers" (protein polymorphisms from ABO blood groups, HLA loci, immunoglobulins, etc.), Cavalli-Sforza discovered interesting clues about the genetic makeup of Europeans. Although being very genetically homogeneous, several patterns did exist. The most important one was a north-western to south-eastern cline with a Near Eastern focus. Accounting for 28% of the overall genetic diversity in the European samples in his study, he attributed the cline to the spread of agriculture from the Middle East c. 10,000 to 6,000 years ago.

Cavalli-Sforza's explanation of demic diffusions stipulated that the clines were due to the population expansion of neolithic farmers into a scarcely populated, hunter-gathering Europe, with little initial admixture between agriculturalists and foragers. The predicted route for this spread would have been from Anatolia to central Europe via the Balkans. However, given that the time depths of such patterns are not known, "associating them with particular demographic events is usually speculative". Apart from a demic Neolithic migration, the clines may also be compatible with other demographic scenarios (Barbujani and Bartorelle 2001), such as the initial Palaeolithic expansion, the Mesolithic (post-glacial) re-expansions., or later (historic) colonizations.

Studies using direct DNA evidence have produced varying results. A notable proponent of Cavalli-Sforza's demic diffusion scenario is Chikhi. In his 1998 study, utilising polymorphic loci from seven hypervariable autosomal DNA loci, an autocorrelation analysis produced a clinal pattern closely matching that in Cavalli-Sforza’s study. He calculated that the separation times were no older than 10,000 years. "The simplest interpretation of these results is that the current nuclear gene pool largely reflects the westward and northward expansion of a Neolithic group".

Although the above studies propounded a 'significant' Neolithic genetic contribution, they did not quantify the exact magnitude of the genetic contribution. Dupanloup performed an admixture analysis based on several autosomal loci, mtDNA and NRY haplogroup frequencies. The study was based on the assumption that Basques were modern representatives of Palaeolithic hunter-gatherers’ gene pool, and Near Eastern peoples were a proxy population for Neolithic farmers. Subsequently, they used admixture analysis to estimate the likely components of the contemporary European gene pool contributed by the two parental populations whose members hybridized at a certain moment in the past. The study suggested that the greatest Near Eastern admixture occurs in the Balkans (~80%) and Southern Italy (~60%), whilst it is least in peoples of the British Isles (estimating only a 20% contribution). The authors concluded that the Neolithic shift to agriculture entailed major population dispersal from the Near East.

Results derived from analysis of the non-recombining portion of the Y- chromosomes (NRY) produced, at least initially, similar gradients to the classic demic diffusion hypothesis. Two significant studies were Semino 2000 and Rosser 2000, which identified haplogroups J2 and E1b1b (formerly E3b) as the putative genetic signatures of migrating Neolithic farmers from Anatolia, and therefore represent the Y-chromosomal components of a Neolithic demic diffusion. This association was strengthened when King and Underhill (2002) found that there was a significant correlation between the distribution of Hg J2 and Neolithic painted pottery in European and Mediterranean sites. However, studies of the ancient Y-DNA from the earlier Neolithic cave burials of Cardium pottery culture men shows they were mainly haplogroup G2a. These 'Neolithic lineages' accounted for 22% of the total European Y chromosome gene pool, and were predominantly found in Mediterranean regions of Europe (Greece, Italy, southeastern Bulgaria, southeastern Iberia).

However later Y-DNA based studies, exploiting an increased understanding of the phylogenetic relationships, performing micro-regional haplogroup frequency analysis, reveal a more complicated demographic history. The studies suggest that "the large-scale clinal patterns of Hg E and Hg J reflect a mosaic of numerous small-scale, more regional population movements, replacements, and subsequent expansions overlying previous ranges". Rather than a single, large-scale 'wave of advance' from the Near East, the apparent Hg J2 cline is produced by distinct populations movements emanating from different part of the Aegean and Near East, over a period stretching from the Neolithic to the Classical Period. Similarly, haplogroup E1b1b was also thought to have been introduced into the Balkans by Near Eastern agriculturalists. However, Cruciani et al. (2007) recently discovered that the large majority of haplogroup E1b1b lineages in Europe are represented by the sub-clade E1b1b1a2- V13, which is rare outside Europe. Cruciani, Battaglia and King all predict that V13 expanded from the Balkans. However, there has been no consensus as to exact timing of this expansion (King and Battalia favour a neolithic expansion, possibly coinciding with the adoption of farming by indigenous Balkaners, whilst Cruciani favours a Bronze Age expansion), nor as to where V13 actually arose (but point to somewhere in the southern Balkans or Anatolia) Overall, Y-chromosome data seems to support the "Pioneer model", whereby heterogeneous groups of Neolithic farmers colonized selected areas of southern Europe via a primarily maritime route. Subsequent expansion of agriculture was facilitated by the adoption of its methods by indigenous Europeans, a process especially prominent in the Balkans.

The data from mtDNA is also interesting. European mtDNA haplogroup frequencies show little, if any, geographic patterning, a result attributed to different molecular properties of mtDNA, as well as different migratory practices between females and males (Semino 2000). The vast majority of mtDNA lineages (60–70%) have been dated to have either emerged in the Mesolithic or Palaeolithic., whereas only 20% of mitochondrial lineages are "Neolithic". However, this conclusion has been questioned. Any undetected heterogeneity in the founder population would result in an overestimation in the age of the current population's molecular age. If this is true, then Europe could have been populated far more recently, e.g. during the Neolithic, by a more diverse founding population (Barbujani et al. 1998, from Richards 2000). As Chikhi states: "We argue that many mitochondrial lineages whose origin has been traced back to the Palaeolithic period probably reached Europe at a later time". However, Richards et al. (2000) maintain these findings even when founding population heterogeneity is considered. In one such study, Wolfgang Haak extracted ancient mtDNA from what they present as early European farmers from the Linear Pottery Culture in central Europe. The bodies contained a 25% frequency of mtDNA N1a, a haplogroup which they assumed to be linked to the Neolithic. Today the frequency of this haplogroup is a mere 0.2%. Haak presented this as supportive evidence for a Palaeolithic European ancestry.

Formerly there had been much debate about whether the westerly spread of agriculture from the Near East was driven by farmers actually migrating, or by the transfer of ideas and technologies to indigenous hunter-gatherers. However, in a very recent study in 2010, researchers have studied the genetic diversity of modern populations to throw light on the processes involved in these ancient events. The new study, funded by the Wellcome Trust, examines the diversity of the Y chromosome. Mark Jobling, who led the research, said: "We focused on the commonest Y-chromosome lineage in Europe, carried by about 110 million men, it follows a gradient from south-east to north-west, reaching almost 100% frequency in Ireland. We looked at how the lineage is distributed, how diverse it is in different parts of Europe, and how old it is." The results suggested that the lineage R1b1b2 (R-M269), like E1b1b or J lineages, spread together with farming from the Near East. Prior archaeological and metrological studies had arrived at similar conclusions in support of the migrationist model.

Dr Patricia Balaresque, first author of the study, added: "In total, this means that more than 80% of European Y chromosomes descend from incoming farmers. In contrast, most maternal genetic lineages seem to descend from hunter-gatherers. To us, this suggests a reproductive advantage for farming males over indigenous hunter-gatherer males during the switch from hunting and gathering, to farming".

A study of Neolithic skeletons in the Great Hungarian Plain found a high frequency of eastern Asian maternal (mtDNA) haplogroups.