Neolithic expansions: how the European foragers were assimilated

From the paper:

Recently, mitochondrial DNA (mtDNA) from skeletal remains of European early farmers and late hunter-gatherers has been retrieved [7]–[13]. The frequency of mtDNA haplogroups, defined by substitutions shared by related mtDNA types (Phylotree.org-mtDNA tree build 12), in early farmers across Europe [7], [10]–[13] was found to be overall similar to those in modern Europeans (Figure 1, Figure S4, Figure S5), while pre-Neolithic hunter-gatherers appear to be quite distinct (Figure 1). In particular, 83% (19 out of 23) of hunter-gatherers analyzed to date carry mtDNAs belonging to haplogroup U [9], [10], [14] and none of the hunter-gatherers fall in haplogroup H. In contrast, haplogroup U has been found in only 13 of 105 (around 12%) individuals from early farming cultures of Europe and it occurs in less than 21% of modern Europeans, while haplogroup H comprises between 25% and 37% of mtDNAs retrieved from early farming cultures (Figure S4) and is in about 30% of contemporary Europeans (Figure 1). The mtDNA data thus suggest that the pre-Neolithic populations in Europe were largely replaced by in-coming Neolithic farming groups, with a maximum mtDNA contribution of around 20% from pre-Neolithic hunter-gatherers [8]–[10].

Age estimated by looking at nucleotide difference between genomes descended from a common ancestor. See left: haplogroup H is in green and shows low nucleotide differences; U is in brown, and it is clear that the former are much more recently related to each other than the latter. This is consistent with recent founders for H and older founders for U.

The following paragraph explains exactly what the mtDNA evidence says happened:

The high frequency of H-type mtDNAs in European Neolithic populations and its complete absence in pre-Neolithic hunter-gatherers suggests that H-type mtDNAs arrived with early farmers in Europe. The population size increase observed between 9,000 and 5,000 YBP likely represents the population expansion that accompanied the Neolithic revolution. In contrast, U-type mtDNAs show an increase in population size around 15,000 to 10,000 YBP, which coincides with the end of the last glacial maximum in Europe and a northwards expansion of hunter-gatherer populations. The data suggests that this population remained rather constant after 10,000 YBP until the onset of the Neolithic revolution. However, the H-type mtDNA population size seems to experience an exponential increase around 7,000 YBP, suggesting that both populations are not yet fused. After 4,000 YBP, no archaeological remains of hunter-gatherers were found in central Europe [22]. From approximately that time on, both H- and U-type mtDNAs expand in a similar way. This may reflect fusion of the two populations where these mtDNAs were prevalent.

So, in short: U experienced after the end of the last Ice Age, as European hunters found new room to expand in space, and economic opportunities to expand in population size. By 10,000 yBP they had maximized their population size. Subsequently, incoming Neolithic H-bearers started expanding as they arrived in Europe and started growing in numbers and filling up the landscape. At some point, (c. 4ky BP) all the hunter-gatherers had been assimilated, and the composite gene pool consisted of the descendants of mainly the Neolithic farmers and a minority of assimilated hunter-gatherers. Since H and U were no longer tied to different subsistence strategies, they underwent expansion in similar fashion.

PLoS ONE 7(3): e32473. doi:10.1371/journal.pone.0032473

Complete Mitochondrial Genomes Reveal Neolithic Expansion into Europe

Qiaomei Fu, Pavao Rudan, Svante Pääbo, Johannes Krause

The Neolithic transition from hunting and gathering to farming and cattle breeding marks one of the most drastic cultural changes in European prehistory. Short stretches of ancient mitochondrial DNA (mtDNA) from skeletons of pre-Neolithic hunter-gatherers as well as early Neolithic farmers support the demic diffusion model where a migration of early farmers from the Near East and a replacement of pre-Neolithic hunter-gatherers are largely responsible for cultural innovation and changes in subsistence strategies during the Neolithic revolution in Europe. In order to test if a signal of population expansion is still present in modern European mitochondrial DNA, we analyzed a comprehensive dataset of 1,151 complete mtDNAs from present-day Europeans. Relying upon ancient DNA data from previous investigations, we identified mtDNA haplogroups that are typical for early farmers and hunter-gatherers, namely H and U respectively. Bayesian skyline coalescence estimates were then used on subsets of complete mtDNAs from modern populations to look for signals of past population expansions. Our analyses revealed a population expansion between 15,000 and 10,000 years before present (YBP) in mtDNAs typical for hunters and gatherers, with a decline between 10,000 and 5,000 YBP. These corresponded to an analogous population increase approximately 9,000 YBP for mtDNAs typical of early farmers. The observed changes over time suggest that the spread of agriculture in Europe involved the expansion of farming populations into Europe followed by the eventual assimilation of resident hunter-gatherers. Our data show that contemporary mtDNA datasets can be used to study ancient population history if only limited ancient genetic data is available.

Link