アイテム詳細

要旨

INTRODUCTION
Almost half the world’s population speaks a language of the Indo-European language family. It remains unclear, however, where this family’s common ancestral language (Proto-Indo-European) was initially spoken and when and why it spread through Eurasia. The “Steppe” hypothesis posits an expansion out of the Pontic-Caspian Steppe, no earlier than 6500 years before present (yr B.P.), and mostly with horse-based pastoralism from ~5000 yr B.P. An alternative “Anatolian” or “farming” hypothesis posits that Indo-European dispersed with agriculture out of parts of the Fertile Crescent, beginning as early as ~9500 to 8500 yr B.P. Ancient DNA (aDNA) is now bringing valuable new perspectives, but these remain only indirect interpretations of language prehistory. In this study, we tested between the time-depth predictions of the Anatolian and Steppe hypotheses, directly from language data. We report a new framework for the chronology and divergence sequence of Indo-European, using Bayesian phylogenetic methods applied to an extensive new dataset of core vocabulary across 161 Indo-European languages.

RATIONALE
Previous phylolinguistic analyses have produced conflicting results. We diagnosed and resolved the causes of this discrepancy, two in particular. First, the datasets used had limited language sampling and widespread coding inconsistency. Second, some analyses enforced the assumption that modern spoken languages derive directly from ancient written languages rather than from parallel spoken varieties. Together, these methodological problems distorted branch-length estimates and date inferences. We present a new dataset of cognacy (shared word origins) across Indo-European. This dataset eliminates past inconsistencies and provides a fuller and more balanced language sample, including 52 nonmodern languages for a denser set of time-calibration points. We applied ancestry-enabled Bayesian phylogenetic analysis to test rather than enforce direct ancestry assumptions.

RESULTS
Few ancient written languages are returned as direct ancestors of modern clades. We find a median root age for Indo-European of ~8120 yr B.P. (95% highest posterior density: 6740 to 9610 yr B.P.). Our chronology is robust across a range of alternative phylogenetic models and sensitivity analyses that vary data subsets and other parameters. Indo-European had already diverged rapidly into multiple major branches by ~7000 yr B.P., without a coherent non-Anatolian core. Indo-Iranic has no close relationship with Balto-Slavic, weakening the case for it having spread via the steppe.

CONCLUSION
Our results are not entirely consistent with either the Steppe hypothesis or the farming hypothesis. Recent aDNA evidence suggests that the Anatolian branch cannot be sourced to the steppe but rather to south of the Caucasus. For other branches, potential candidate expansion(s) out of the Yamnaya culture are detectable in aDNA, but some had only limited genetic impact. Our results reveal that these expansions from ~5000 yr B.P. onward also came too late for the language chronology of Indo-European divergence. They are consistent, however, with an ultimate homeland south of the Caucasus and a subsequent branch northward onto the steppe, as a secondary homeland for some branches of Indo-European entering Europe with the later Corded Ware–associated expansions. Language phylogenetics and aDNA thus combine to suggest that the resolution to the 200-year-old Indo-European enigma lies in a hybrid of the farming and Steppe hypotheses.