Post by Admin on Jan 7, 2022 21:18:41 GMT
Fig. 5.
Admixture Graph Model.
The largest deviation between empirical and theoretical f-statistics is |Z|=2.9, indicating a good fit considering the large number of f-statistics analyzed. Admixture events are shown as dotted lines labeled by proportions, with the minor ancestry in gray. The present-day groups are shown in orange ovals, the ancient ones in blue, and unsampled groups in white. (The ovals and admixture events are positioned according to guesses about their relative dates to help in visualization, although the dates are in no way meant to be exact.) In this graph we do not attempt to model the contribution of WSHG and Anatolian farmer-related ancestry, and thus cannot model Central_Steppe_EMBA, the proximal source of Steppe ancestry in South Asia (instead we model the Steppe ancestry in South Asia through the more distally related Yamnaya). However, the admixture graph does highlight several key findings of the study, including the deep separation of the AASI from other Eurasian lineages, and the fact that some Austroasiatic-speaking groups in South Asia (e.g. Juang) harbor ancestry from a South Asian group with a higher ratio of AASI-related to Iranian farmer-related ancestry than any groups on the Modern Indian Cline, thus revealing that groups with substantial Iranian farmer-related ancestry were not ubiquitous in peninsular South Asia in the 3rd millennium BCE when Austroasiatic languages likely spread across the subcontinent.
To shed light on the formation of the statistically reconstructed ANI, we return to the Swat Valley time transect that formed the Steppe Cline after 2000 BCE. The Modern Indian Cline intersects the Steppe Cline at a position close to the position of the Kalash, the group in northwest South Asia with the highest ANI ancestry proportion (56) (Fig. 4). The DATES-based estimate of admixture in the Kalash is 110 ± 12 generations (56), suggesting a post-IVC date of formation of the ANI paralleling the post-IVC date of formation of the ASI. Further evidence for a post-IVC integration of Steppe ancestry into South Asia comes from ancient individuals on the Steppe Cline (along which the ANI could theoretically have formed) whose admixture date for Steppe ancestry is also post-IVC. Specifically, we estimate the date of admixture into the Late Bronze Age and Iron Age individuals from the Swat District of northernmost South Asia to be on average 26 generations before the date that they lived, corresponding to a 95% confidence interval of ~1900–1500 BCE. This time scale for the arrival of Steppe ancestry in the region is consistent with our observation of 6 outlier individuals in Turan who lived between ~2000–1500 BCE and who carry this ancestry in mixed form (Fig. 2), and with our finding that the R1a Y chromosome associated with Central_Steppe_MLBA ancestry in South Asia is also present in the Swat District Late Bronze and Iron Age individuals (2 copies).
Taken together, these results show neither of the two primary source populations of the Modern Indian Cline, the ANI and ASI, was fully formed before the turn of the 2nd millennium BCE.
Steppe Ancestry in South Asia is Primarily from Males and Disproportionately High in Brahmins
In the Late Bronze Age and Iron Age individuals of the Swat Valley, we detect a significantly lower proportion of Steppe admixture on the Y chromosome (only 5% of the 44 Y chromosomes of the R1a-Z93 subtype that occurs at 100% frequency in the Central_Steppe_MLBA males) compared to 20% on the autosomes (Z = −3.9 for a deficiency from males under the simplifying assumption that all the Y chromosomes are unrelated to each other since admixture and thus statistically independent), documenting how Steppe ancestry was incorporated into these groups largely through females (Fig. 4). However, sex bias varied in different parts of South Asia, as in present-day South Asians we observe a reverse pattern of excess Central_Steppe_MLBA-related ancestry on the Y chromosome compared to the autosomes (Z = 2.7 for an excess from males) (13, 57) (Fig. 4). Thus, the introduction of lineages from Steppe pastoralists into the ancestors of present-day South Asians was mediated mostly by males. This bias is similar in direction to what has been documented for the introduction of Steppe ancestry into Iberia in far western Europe, although the bias is less extreme than reported in that case (58).
Our analysis of Steppe ancestry also identified 6 groups with a highly significantly elevated ratio of Central_Steppe_MLBA-to-Indus_Periphery_West-related ancestry compared to the expectation for the model at the Z < −4.5 level. The strongest two signals were in Brahmin_Tiwari (Z = −7.9) and Bhumihar_Bihar (Z = −7.0). More generally, there is a notable enrichment in groups that consider themselves to be of traditionally priestly status: 5 of the 6 groups with Z < −4.5 were Brahmins or Bhumihars even though they comprise only 7–11% of the 140 groups analyzed (p<10−12 by a χ2 test assuming all the groups evolved independently). We caution that this is not a formal test as there is an unknown degree of shared ancestry among groups since they formed by mixture, and because our decisions about which groups to include in the analysis was not made in a blinded way; for example, we excluded four “Catholic Brahmin” groups with strong evidence of substantial shared ancestry in the last millennium (10) which makes them not statistically independent (Table S5, Fig. 4 (13)). Nevertheless, the fact that traditional custodians of liturgy in Sanskrit (Brahmins) tend to have more Steppe ancestry than is predicted by a simple ASI-ANI mixture model provides an independent line of evidence, beyond the distinctive ancestry profile shared between South Asia and Bronze Eastern Europe mirroring the shared features of Balto-Slavic and Indo-Iranian languages (59), for a Steppe origin for South Asia’s Indo-European languages prior to ~2000 BCE.
