A rapid coastal migration along the "southern route" from Africa
into southern Asia, some time before 50 kyBP has been strongly
suggested by studies on present day world populations (especially those
based on mitochondrial DNA) [1,11,18-21,28].
The founder analysis of the mtDNAs in this study suggests 50.0 ± 1.5
kyBP for such arrival which is well within the lower bound range of
earlier estimates and perhaps more consistent with the earliest and
most pronounced population expansion in southern Asia around 52 kyBP
suggested in [28].
Magnitude of this southern Asian growth phase suggests that over half
of the global human population lived in Indian subcontinent between ~45
to 20 kyBP and population size peaked at over 60% around 38kyBP [28].
These population expansion estimates are largely in agreement with high
mtDNA diversity and star like non-overlapping pattern of numerous
lineages of macrohaplogroup M reported previously [4,6,9,7].
Though the stage upto here is clear, the contribution and role of
this sizably large earliest settlers' component in the contemporary
Indian populations, coupled with later migrations during the last 10
thousand years (ky) from west and east of the subcontinent has been an
issue of controversy. Cordaux et al. [22]
based on the non-overlapping pattern of mtDNA phylogeny between India
and east Eurasia has supported the argument of Cavalli-Sforza et al. [29]
that in India the genetic traces of early migrations along the southern
route were erased by the subsequent migrations, which shaped the
present-day mtDNA gene pool of India. However presence of numerous
autochthonous lineages in India emerging directly from the root of the
founder macrohaplogroups M, N and R [4,6,7,9] during the estimated population growth period in southern Asia (~45 to 20 kyBP)[28] indicates the presence of large component of earliest settlers in the contemporary Indian populations.
In the quest of finding the carriers of the genetic legacy of the
earliest settlers among the contemporary Indian populations, some
previous studies on mtDNA variation by calculating nucleotide diversity
and expansion time (as per methods of Slatkin et al. [30])
for different linguistic groups of India, distinguished Austro-Asiatic
speaking tribes as the oldest and the carriers of the said legacy [31,32]. Basu et al. [14]
also supported the view by reporting that the frequency of the ancient
haplogroup M2 among the Austro-Asiatic tribal populations is as high as
19% and they lack the younger haplogroup M4. However Metspalu et al. [11],
so as this study, rejects such claims as linguistic groups of India do
not cluster into distinct branches of the Indian mtDNA tree, [[6,10,13],
this study] calculating the beginning of expansion for those groupings
is problematic, whereas lack of coding region information in Basu et
al. [14]
have lead to an over estimation of M2 frequency. Moreover our results
indicate that M2 frequency variation among the studied tribes can be
better explained by recent population expansion/demographic events than
as a function of deep rooting ancestry. The nucleotide diversity though
appears better parameter, is also predisposed to influence of past
demographic events. The phylogenetic inferences based on such
parameters should be strictly viewed in reference to the demographic
events, particularly for India.
Our analysis of mtDNA variation in populations of India indicate
that the Dravidian tribes extending from southern to central India and
tribes of eastern India irrespective of the linguistic affiliation
shows equally deep rooted M2 ancestry ~37 ky (Figure 2), comparable nucleotide diversity (Table 2) and similar past demographic history (Figure 3).
However Indo-European tribes of western and central India except
Kathodi/Katkari and Andh tribes harbour only M2a1 branch representing a
time depth of ~8 ky. Kathodi/Katkari and Andh tribe encompasses other
braches of M2a, but lacks M2b. All these Indo-European tribes shows
appreciable frequency of M2 (Table 1), but they are low on nucleotide diversity (Table 2).
Thus it would be highly speculative to tag any one or a group of
populations based on linguistics or geography as the representatives of
earliest settlers, rather it indicate to the fact that earliest
settlers' component is more pronounced in the areas extending from
southern to eastern India, and shows decline towards north and
northwest India, a cline similar to that of M in general [[5,11],
this study]. However a decline of earliest settlers' component across
tribe to higher caste gradient may also be accepted in the respective
regions as indicated in [13].
The time depth of M2 lineage and diversity indices in Indo-European
speakers of western region extending up to central India posits the
expansion of earliest settlers' component into these areas during the
post Last Glacial Maximum(LGM) growth (~12 to 7 kyBP) of population
(Figure 3) or perhaps little later (Figure 3-pannel
'F'). However this requires further investigation. It is only during
this rapid growth; regional and population specific branching patterns
appear on the more or less homogenous M2 phylogeny. The possible
explanation would be the earliest settlers of India prior to this rapid
population growth had lived in an extended enclave and there had been
continuous gene flow across population boundaries. The second but more
plausible reason of such homogeneity could be that earliest settlers by
virtue of large population size during ~45 to 20 kyBP [28] and Indian ecological setting which, favoured tendency to isolate and subjugate [33] might have been differentiated into populations distributed far apart as suggested in recent studies [4,9].
But during the LGM and late glacial aridity, climate across India and
south Asia generally seems to have been much more arid than present.
Geomorphological indicators from the landmass of India suggest dune
mobility in the northwest [34], and greatly reduced river flow in north central India during the span of time that covered the full glacial [35].
Offshore indicators of salinity (due to runoff from the land) suggest
that LGM aridity was substantially greater than at present. Indicators
of upwelling intensity in the Indian Ocean suggest that the summer
monsoon was much weaker than present at the LGM, but reaching its
weakest at around 15,800 – 12,500 C14 years ago, that is 17,800-13,800 calibrated or 'real' years ago [26].
During this period of cold and more arid conditions rainforest
retreated and was replaced by dry grasslands. However, some monsoon
forests and woodlands in southern India and scrub, open woodland in
eastern India probably existed in presently moist forest climates. This
appears to be harsh conditions for an hunting gathering based
subsistence, thus to fight the adverse, probably shrinking populations
might have come close to each other in a more habitable area allowing a
free gene flow between populations, whereas ancestral population of the
Kathodi/Katkari M2a2 lineage appears to have remained isolated during
this period. In the post LGM growth period, though population spread
over wide geographical regions. Maternal gene flow is evident in the
geographical neighbors suggesting fluidic population specific
boundaries until recently at least among the tribes.
The next important event on the Indian scene is the beginnings of agriculture and use of pottery [36-41].
Cultivation of plants/agriculture diffused from the Fertile Crescent
within the past 10,000 years. The steady advance beyond this stage
seems however to have been primarily driven by the crop-animal complex
derived from the mid-east, reaching the tip of southern India around 3
to 2 kyBP [42,43].
The diffusion of pottery traditions, which arise in response to the
need to store and cook grains, shows evidence of the influences from
northwest and northeast, with the western influence predominating over
much of the country. Thus the Black and Red ware reflects western,
while the Corded ware Chinese influence [44-46].
Two other technological innovations, known to have originated outside
of India, the domestication of horse, around 6 kyBP on the shores of
Black Sea in present Ukraine, and the use of iron around 5 kyBP in
Anatolia in present day Turkey, appears in the Indian archeological
records (around 2 kyBP) soon after the agriculture [42].
The recent study investigating the cultural or demic diffusion model of
agriculture in India supported the demic diffusion model which predicts
a substantial genetic input from migrating agriculturalists [47].
The advent of agriculture and perhaps migrating agriculturists brought
about dramatic changes in the economy, technology and demography of
human societies. Human habitat in the hunting-gathering stage was
essentially on hilly, rocky and forested regions, which had ample wild
plant and animal food resources. Agriculture led to the emergence of
villages and towns and perhaps brought with it the division of society
into occupational groups. [43].
Crop cultivation resulted in the loss of the traditional habitat of
hunter-gatherers by deforestation, fragmenting and marginalizing
numerous such populations, many of whom were assimilated into
agriculturally based subsistence economies [48],
thereby catalyzing some sort of regional similarities across tribe
caste continuum. Our results on reconstructed past population
demography indicating decline of earliest settlers' population (female
population here) during this period in almost all the geographical
regions except western (Figure 3)
is consistent with the above proposition and suggests that demic
diffusion of these technologies were rapid, perhaps involving large
migrating populations with these technologies.
The highest frequency of east Eurasian- specific mtDNA haplogroups [11,22] and absence of M2 an earliest settlers component (Table 1)
among Tibeto-Burman speaking tribes of northeastern states of India
suggests that, despite the more recent migrations to India, these
populations remained relatively isolated, explaining the close
correlation between genetic and linguistic results [49,50].
This contrasts with the situation observed in other regions of India,
where linguistic structure shows very little concordance with the
genetic structures.
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