WASHINGTON (Nov. 28, 2012)—A study by the Genographic Project has given new insight into how demographic factors have shaped genetic diversity in Indian populations. Among the most surprising findings was that genetic differences between tribal and caste groups in Tamil Nadu seem to pre-date the arrival of the Indo-Europeans in the region by approximately 2,000 years.
Published today in the journal PLOS ONE, the study was led by principal investigator Ramasamy Pitchappan of the Genographic Project’s Indian Regional Center at Chettinad Academy of Research & Education in Chennai.
Contemporary Indian populations exhibit great cultural, morphological and linguistic diversity. The study sought to answer the contentious question of whether India’s contemporary genetic patterns are a result of long-term occupation, perhaps dating to just after humans left Africa about 60,000 years ago, or if they have been substantially impacted by more recent migration into the region.
“Our conclusions provide a new framework to better understand the relative impacts of demographic events and other cultural, social and economic factors that might have influenced modern genetic diversity in India,” Pitchappan, the senior author, said.
Indian populations can be broadly divided into “tribal” and “non-tribal.” Tribal groups constitute 8 percent of the Indian population and are characterized by traditional modes of subsistence such as hunting and gathering. In contrast, the majority of the non-tribal populations are classified as castes under the Hindu Varna (color caste) system, which groups the population based primarily on occupation. The system embodies strict marital rules preventing marriage among different castes.
The study applied a novel analytical strategy to unravel the population structure and genetic history of the southernmost state of India, Tamil Nadu, which is known for its rigid caste system. One of the aims of the study was to explore whether genetic differences observed among Tamil Nadu populations could be attributed to the establishment of the Hindu Varna system approximately 2,000 years ago by Indo-Europeans from northern India. The genetic data were also interpreted in reference to the paleoclimatic, archaeological and historical evidence from this region.
A total of 1,680 men from 12 tribal and 19 non-tribal (caste) Tamil Nadu populations were analyzed for markers on the paternally inherited Y-chromosome. Overall, the populations were characterized by Y-chromosome lineages (81 percent) that likely originated within India. The results suggest a minimal genetic influence in Tamil Nadu from the main western Eurasian migrations reported in the last 10,000 years, including the spread of agricultural populations from the Fertile Crescent during the Neolithic period. Although non-tribal groups exhibited a slightly higher proportion of non-Indian paternal lineages than tribal populations, the common paternal lineages shared among them are likely drawn from the same ancestral genetic pool that emerged in India during the late Pleistocene and early Holocene (10,000-30,000 years ago).
The genetic data also revealed that genetic differentiation among populations in Tamil Nadu began as early as 6,000 years ago, with no significant genetic admixture among them for at least the last 3,000 years. These results indicate a minimal genetic impact from the Indo-European migrations into the region over the past 2,000 years. These results are consistent with the earliest historical records of the region that document a highly structured society prior to the establishment of the Hindu Varna system. Rather, the timing of the Y-chromosomal differentiation among Tamil Nadu populations seems to fit better with the emergence of agricultural technology in South India and the resulting demographic shifts during the Neolithic period.
Genographic Project manager and one of the lead authors of the study David Soria-Hernanz explains that “the rigorous sampling and analytical approach used in the study allowed us to dissect the confounding relationships among multiple socio-cultural factors in Tamil Nadu, allowing us to further explore and test in detail the relationships between social structure and genetics.”
Project director and National Geographic Explorer-in-Residence Spencer Wells noted, “This study is a wonderful example of how human culture, and particularly the shift to an agricultural mode of subsistence during the Neolithic period, has had a profound impact on modern patterns of genetic diversity.”
Background: The Genographic Project seeks to chart new knowledge about the migratory history of the human species and answer age-old questions surrounding the genetic diversity of humanity. The project is a nonprofit, multi-year, global research initiative. At the core of the project is a global consortium following an ethical and scientific framework and responsible for sample collection and analysis in their respective regions. Members of the public can participate in the Genographic Project by purchasing a Genographic Participation Kit — Geno 2.0 — from the Genographic website (www.genographic.com), where they can also choose to donate their genetic results to the research effort. A portion of the proceeds of the kits help further research and support a Legacy Fund for indigenous and traditional peoples’ community-led language revitalization and cultural projects.
NOTES: To view the PLOS ONE article, visit: http://dx.plos.org/10.1371/journal.pone.0050269.
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