1. Introduction
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1. Introduction India harbours a tremendous cultural, social, linguistic and genetic diversity. In India, almost all the major religions are represented. People of India speak languages belonging to four language families - Austro-Asiatic, Dravidian, Tibeto-Burman and Indo-European. Socially, population of India can be divided into Tribals and Non- tribals. Non-tribal population mostly comprises of various communities from Hindu religious fold, hierarchically arranged in relation to each other through varna and caste system. Besides, it also includes various religious communities such as Muslims, Christians, Sikhs, Buddha, Jain etc (Reddy et al., 2010). Tribal populations constitute about 8% of the total population. There are approximately 400 tribal groups in India. Genetically, India is quite heterogeneous, exhibiting diversity, which is more than any other comparable region of the world (Majumder, 1998). Origins of such diverse populations of India remain obscure. However, four potential sources are suggested: First corresponds to early Palaeolithic occupation of India, second to migration of farmers possibly speaking proto-Dravidian languages from western Iran during Neolithic, third to the arrival of Indo-European speaking pastoral nomads roughly 3500 years ago and the last of the Sino-Tibetan speakers in the North-Eastern India (Vidyarthi, 1983; Cavalli-Sforza et al., 1994; Gadgil et al., 1997; Cordaux and Stoneking, 2003). A composite picture has emerged out of these studies: that the Indian subcontinent was peopled rapidly by the first wave of modern humans out of Africa and probably much earlier than Europe Thangaraj et al. (2005) The majority of the Indian mtDNA belongs to Indian specific haplogroups, which show a deep coalescence age and have diversified in-situ without influence from outside regions (Kivisild et al. 1999, 2003; Metspalu et al. 2004) The mtDNA haplogroups are shared between populations and remarkably, do not cluster according to language families or social groups (castes, 1 tribes and religion etc.) (Cordaux et al. 2003; Majumder 2001; Roychoudhury et al. 2000, 2001). Indian Y chromosome haplogroups too show a deep coalescence age implying that the pre-Holocene and Holocene migrations have primarily shaped the Y chromosome boundaries (Sengupta et al. 2006). Thus, in essence, these findings emphasize the continuity of their gene pool, contrary to the traditional arguments and models, where the genetic structure and heterogeneity of Indian populations were often traditionally attributed to multiple migrations (Vidyarthi, 1983; Cavalli-Sforza et al., 1994; Gadgil et al., 1997). Studies using genome wide SNPs (Reich et al., 2009; Moorjani et al., 2013; Basu et al., 2016) have shown a much more complex history- despite the antiquity and continuity of the early gene pool of Indians, there has been multiple admixture events, and Indian populations exhibit at least 4 ancestry sources. This admixture event may have happened ~ 1,900 to 4,200 years ago, followed by extensive adoption of practice of endogamy. Admixture events with ‘Iranian Agriculturists’ and ‘Steppe pastoralists’ have also been indicated by ancient DNA (Narasimhan et al., 2018). Thus, the peopling in India has a much more complicated history, which has also been reshaped by cultural and demographic events. Such migrations cannot be studied in isolation, but within the framework of evolution of modern humans. It is now widely accepted that modern humans evolved in Africa relatively recently and spread to other parts of the world through many dispersal events (Foley, 1998). However, the routes by which modern humans spread to the other parts of the world remain poorly understood. Indian subcontinent, in this regard, is a crucial geographic area as it is located at the crossroads of Africa, Eurasia and Pacific, (Cavalli-Sforza et al., 1994; Cann, 2001) and it has also been suggested that India served as a corridor for the dispersal of the modern humans that started from Africa 100,000 years ago (Cann, 2001). It has also been suggested that colonisation of the 2 Indian subcontinent may have happened ~60000 years ago, through southern route (Mellars et al., 2013), and a demographic expansion of populations may have taken place in the subcontinent (Atkinson et al., 2008). Thus, it seems that, lying at the crossroads; India has been peopled by various human groups carrying diversity of genes and culture at different times. How was India peopled? When? What routes did different people take? Where did the people move? - such issues remain at the centre of anthropological enquiry today. Migration of people is not merely a matter of demographic change, but it also implies migration of genes, ideas, customs, languages and technology. Apart from the changes in gene pool, it also involves cultural contacts producing variety of responses and thus bringing about socio-cultural changes, in both the cultures, the migrant and the autochthones. For example, formation of Varna based hierarchical society and caste system is attributed to arrival of Indo-European language speakers. Among many other socio-cultural changes, introduction of agricultural technology and various crops and linguistic changes are important. This scenario raises a few questions; assuming that such contacts would involve introduction and exchange of genes, could the traces of migrations and past cultural contacts be discerned using molecular techniques? What were those interactions? Whether language change and introduction of agriculture were accompanied by changes in gene pool or it was only a process of cultural diffusion? Whether these contacts wiped out the original populations or there was genetic admixture of varying levels? Interdisciplinary perspective on Indian Subcontinent Indian subcontinent was peopled rapidly by the first wave of modern humans out of Africa (Thangaraj et al., 2005). Majority of the Indian mtDNA belong to Indian specific haplogroups, which show a deep coalescence age and have diversified in-situ 3 (Kivisild et al., 1999a, 2003; Metspalu et al., 2004). mtDNA haplogroups are shared between populations and do not cluster according to language families or social groups (castes, tribes, religion etc) (Roychoudhury et al., 2000, 2001; Majumder, 2001b; Cordaux et al., 2003). Indian Y chromosome haplogroups too show a deep coalescence age implying that the pre-Holocene and Holocene migrations have shaped the Y chromosome boundaries (Sengupta et al., 2006). Thus, these findings emphasize the continuity of their gene pool, contrary to the traditional arguments and models, where the genetic structure and heterogeneity of Indian populations was often attributed to the several migrations (Vidyarthi, 1983; Cavalli-Sforza et al., 1994; Gadgil et al., 1997). Recent examination of skeletal record from the subcontinent has also emphasised such continuity of populations (Walimbe, 2007). Today, India has 50000 - 60000 endogamous groups (Gadgil and Malhotra, 1983). It follows that these endogamous groups are formed primarily due to process of fission with strong founder effects (Nakatsuka et al., 2017), possibly after large scale admixture migrating populations (Reich et al., 2009; Basu et al., 2016). It is suggested that fissioning of groups and lack of fusion with other groups of same occupation and social rank, maintained through strict endogamy, created several endogamous groups (Karve, 1961; Karve and Malhotra, 1968; Malhotra and Vasulu, 1993). MtDNA sequences are shown to be useful in detecting past demographic events (Harpending et al., 1998; Harpending and Rogers, 2000) apart from their use in deciphering the population affinities. Application of such techniques have demonstrated different demographic histories of several tribal populations from India (Cordaux et al., 2003). Tribal populations of southern India showed reduced diversity and no signal of population expansion, in contrast, North Indian populations showed signatures of Population expansions (Cordaux et al., 2003). 4 Even different demographic histories were discerned for two castes inhabiting the same geographic area (Mountain et al., 1995a). Taking advantage of the global mtDNA phylogenetic tree (Torroni et al., 2006), studies using phylogeographic approach have produced the detailed timeline of evolution of various mtDNA haplogroups native to South Asia (Palanichamy et al., 2004; Sun et al., 2006; Chandrasekar et al., 2009) and haplogroups that may have entered later (Palanichamy et al., 2015b; Silva et al., 2017; Sylvester et al., 2019) Thus, today, advances in molecular genetics and population genetic analyses not only allow us to study the population affinities but also to study the demographic changes that might have happened in the past. It is possible to date the demographic events and mutational events. Such dating is vital for contextualising genetic data within archaeological evidences or hypotheses. For example, is adoption of agriculture associated with population fissioning? What were demographic consequences; population expansions or bottlenecks? Are there signatures of Admixture/populations replacement/language replacement? Within this broader framework, this study specifically aims to understand the genetic affinities and demographic history of tribal communities of Maharashtra, which have not been studied so far using molecular markers. Genetic implications of Migrations: Some Scenarios The change from Hunting-Gathering lifestyle to Agriculture led to an explosive increase in population