Wednesday, June 13, 2012

Prehistory Via The Genetics of Goats and Sheep

Recent studies attempt to fathom the process of goat and sheep domestication respectively using the uniparental genetics of the modern populations of these animals as a tool to achieve this end.

Goat Herding Sailors

The abstract of the goat paper, Pereira (2009), includes the following:

Our analyses indicate a remarkably high diversity of maternal and paternal lineages in a sample of indigenous goats from the northwestern fringe of the African continent. Median-joining networks and a multidimensional scaling of ours and almost 2000 published mtDNA sequences revealed a considerable genetic affinity between goat populations from the Maghreb (Northwest Africa) and the Near East.

It has been previously shown that goats have a weak phylogeographic structure compatible with high levels of gene flow, as demonstrated by the worldwide dispersal of the predominant mtDNA haplogroup A. In contrast, our results revealed a strong correlation between genetic and geographical distances in 20 populations from different regions of the world.

The distribution of Y chromosome haplotypes in Maghrebi goats indicates a common origin for goat patrilines in both Mediterranean coastal regions. Taken together, these results suggest that the colonization and subsequent dispersal of domestic goats in Northern Africa was influenced by the maritime diffusion throughout the Mediterranean Sea and its coastal regions of pastoralist societies whose economy included goat herding. Finally, we also detected traces of gene flow between goat populations from the Maghreb and the Iberian Peninsula corroborating evidence of past cultural and commercial contacts across the Strait of Gibraltar.

The similar conclusion of the abstract of Zeder (2008), on "Domestication and early agriculture in the Mediterranean Basin: Origins, diffusion, and impact," also deserves to be restated together with the goat paper:

The past decade has witnessed a quantum leap in our understanding of the origins, diffusion, and impact of early agriculture in the Mediterranean Basin. In large measure these advances are attributable to new methods for documenting domestication in plants and animals.

The initial steps toward plant and animal domestication in the Eastern Mediterranean can now be pushed back to the 12th millennium cal B.P. Evidence for herd management and crop cultivation appears at least 1,000 years earlier than the morphological changes traditionally used to document domestication. Different species seem to have been domesticated in different parts of the Fertile Crescent, with genetic analyses detecting multiple domestic lineages for each species.

Recent evidence suggests that the expansion of domesticates and agricultural economies across the Mediterranean was accomplished by several waves of seafaring colonists who established coastal farming enclaves around the Mediterranean Basin.

This process also involved the adoption of domesticates and domestic technologies by indigenous populations and the local domestication of some endemic species. Human environmental impacts are seen in the complete replacement of endemic island faunas by imported mainland fauna and in today's anthropogenic, but threatened, Mediterranean landscapes where sustainable agricultural practices have helped maintain high biodiversity since the Neolithic.

Genocidal Sheep Farmers

The abstract of the sheep paper, Chessa (2009), which uses a more sophisticated methodology involving genetic traces of ancient retroviruses argues (in part):
The domestication of livestock represented a crucial step in human history. By using endogenous retroviruses as genetic markers, we found that sheep differentiated on the basis of their “retrotype” and morphological traits dispersed across Eurasia and Africa via separate migratory episodes. Relicts of the first migrations include the Mouflon, as well as breeds previously recognized as “primitive” on the basis of their morphology, such as the Orkney, Soay, and the Nordic short-tailed sheep now confined to the periphery of northwest Europe. A later migratory episode, involving sheep with improved production traits, shaped the great majority of present-day breeds.
In short, sheep have experienced a major episode of population replacement sometime after the early Neolithic, leaving only a few relict populations at the European fringe. The result is likely to be less controversial than evidence of human population replacement, of course, because sheep are not people and we expect, or at least do not fault, herders, for consciously practicing sheep eugenics.

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