Title: Genetic Evidence for the Demic Diffusion of Agriculture to India
Abstract: The process by which agriculture diffused from the Fertile Crescent within the past 10,000 years has been widely discussed, but as yet there is no consensus: Was it mostly a demic diffusion (with massive spread of people) or rather a cultural diffusion (without substantial migration of people)? The demic diffusion model ([ 1 ][1]) predicts a substantial genetic input from migrating agriculturalists, whereas the cultural diffusion model ([ 2 ][2]) predicts no major changes at the genetic level. Hence, a way to test these competing hypotheses would be to compare genetic variation in traditional agriculturalists, traditional hunter-gatherers, and recent agriculturalists (i.e., former hunter-gatherers who recently shifted to agriculture). Under the demic diffusion model, recent agriculturalists are expected to show closer genetic affinities to traditional agriculturalists than do hunter-gatherers; whereas under the cultural diffusion model, recent agriculturalists should resemble hunter-gatherers genetically.
Genetic studies of Europeans have led to conflicting conclusions, partly because the genetic composition of pre-agricultural European populations is unknown. However, there still exist in India both nonagricultural groups and groups that recently adopted agriculture ([ 3 ][3], [ 4 ][4]), allowing a formal test of the demic versus cultural diffusion models of the spread of agriculture. Caste groups can be considered as traditional agriculturalists in this context, because they introduced key innovations (such as iron technology) that facilitated the expansion of agriculture toward south India ([ 4 ][4]). There are also tribal groups in south India who are generally considered to be the aboriginal inhabitants of the region and who have traditionally been hunter-gatherers ([ 3 ][3], [ 4 ][4]). Some of them still survive through hunting and gathering or unskilled labor, whereas others have shifted to an agriculturalist subsistence strategy within the past 3000 years ([ 4 ][4]).
Genetic affinities were deduced from the frequencies of 14 Y-chromosome haplogroups analyzed in 583 males, including 71 tribal south Indian hunter-gatherers, 60 tribal south Indian recent agriculturalists, and 283 south Indian and 169 north Indian traditional agriculturalists ([ 5 ][5]). Pairwise genetic (Fst) distances separating the different categories of individuals indicate that south Indian recent agriculturalists are significantly more closely related to traditional agriculturalists than are traditional hunter-gatherers ([Fig. 1A][6]) ( t test on jackknifed Fst values, P < 0.01). This conclusion is supported by a multidimensional scaling analysis that simultaneously compared all four categories; it also reveals that recent south Indian agriculturalists are overall genetically more similar to traditional agriculturalists than to traditional hunter-gatherers ([Fig. 1B][6]).
![ Fig. 1. ][7] Fig. 1.
Genetic affinities between southern traditional hunter-gatherers (Trad. HG south), southern recent agriculturalists (Recent AG south), southern traditional agriculturalists (Trad. AG south), and northern traditional agriculturalists (Trad. AG north). ( A ) Fst distances were deduced from Y chromosome (below diagonal) and mtDNA (above diagonal) data. ( B and C ) Multidimensional scaling plots were based on (B) Y chromosome and (C) mtDNA Fst distances. SV, stress value.
Genetic affinities were also deduced from sequences of the mitochondrial DNA (mtDNA) control region analyzed in 632 individuals, including 229 tribal south Indian hunter-gatherers, 201 tribal south Indian recent agriculturalists, and 140 south Indian and 62 north Indian traditional agriculturalists ([ 5 ][5]). Pairwise Fst distances separating the different categories of individuals indicate that south Indian recent agriculturalists are significantly more closely related to traditional agriculturalists than are traditional hunter-gatherers ([Fig. 1A][6]) ( t test on jackknifed Fst values, P < 0.001). This conclusion is also supported by a multidimensional scaling analysis ([Fig. 1C][6]); the latter also shows that recent south Indian agriculturalists are overall genetically more similar to traditional agriculturalists than to traditional hunter-gatherers. Thus, mtDNA variation shows trends that markedly parallel those deduced from Y-chromosome variation. This is also reflected in the fact that mtDNA and Y-chromosome genetic distances among traditional and recent agriculturalists and traditional hunter-gatherers are highly and significantly correlated (Mantel test, r2 = 0.805, P < 0.05).
The patterns of Y-chromosome and mtDNA variation in south India indicate that groups that have changed recently from a hunting-gathering subsistence strategy to agriculture have concomitantly received genes from traditional agriculturalists. This demonstrates the validity of the demic diffusion model for the spread of agriculture to India. It also underscores the importance of cultural processes such as the diffusion of agriculture in shaping human genetic variation.
Supporting Online Material
[www.sciencemag.org/cgi/content/full/304/5674/1125/DC1][8]
Materials and Methods
References and Notes
1. [↵][9] A. J. Ammerman, L. L. Cavalli-Sforza, Neolithic Transition and the Genetics of Populations in Europe (Princeton Univ. Press, Princeton, NJ, 1984).
2. [↵][10] A. Whittle, Europe in the Neolithic (Cambridge Univ. Press, Cambridge, 1996).
3. [↵][11] K. S. Singh, People of India: India's Communities (Oxford Univ. Press, Delhi, 1998).
4. [↵][12] V. N. Misra, J. Biosci. 26, 491 (2001).
[OpenUrl][13][PubMed][14][Web of Science][15]
5. [↵][16] Materials and methods are available as supporting material on Science Online.
6. We thank P. P. Majumder and M. V. Usha Rani for assistance. Supported by the Max Planck Society (Germany) and the Indian Department of Biotechnology.
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[8]: http://www.sciencemag.org/cgi/content/full/304/5674/1125/DC1
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Publication Year: 2004
Publication Date: 2004-05-20
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 38
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