Coevolution of languages and genes on the island of Sumba, eastern Indonesia

Abstract

Numerous studies indicate strong associations between languages and genes among human populations at the global scale, but all broader scale genetic and linguistic patterns must arise from processes originating at the community level. We examine linguistic and genetic variation in a contact zone on the eastern Indonesian island of Sumba, where Neolithic Austronesian farming communities settled and began interacting with aboriginal foraging societies approximately 3,500 years ago. Phylogenetic reconstruction based on a 200-word Swadesh list sampled from 29 localities supports the hypothesis that Sumbanese languages derive from a single ancestral Austronesian language. However, the proportion of cognates (words with a common origin) traceable to Proto-Austronesian (PAn) varies among language subgroups distributed across the island. Interestingly, a positive correlation was found between the percentage of Y chromosome lineages that derive from Austronesian (as opposed to aboriginal) ancestors and the retention of PAn cognates. We also find a striking correlation between the percentage of PAn cognates and geographic distance from the site where many Sumbanese believe their ancestors arrived on the island. These language-gene-geography correlations, unprecedented at such a fine scale, imply that historical patterns of social interaction between expanding farmers and resident hunter-gatherers largely explain community-level language evolution on Sumba. We propose a model to explain linguistic and demographic coevolution at fine spatial and temporal scales.

Fig. 1.

Models for the evolution of languages and genes at two scales. Each circle represents a population of languages or genes, with parent populations shown on top and descendant populations shown below downward-facing arrows. Open circles, invading farming populations; filled circles, resident aboriginal populations. (A) Replacement models at larger geographic and temporal scales include two favored models in the literature: language replacement (i.e., the languages of an incoming population replace those of resident groups without gene flow) (10) and Diamond and Bellwood's (7) basic hypothesis (i.e., linguistic and genetic replacement by an incoming group with subsequent coevolution of descendant languages and genes). (B) An alternative model with codominant effects at smaller geographic and temporal scales involves both genetic admixture (e.g., demic diffusion) and the incursion of words that do not trace to PAn (horizontal arrows) in each descendant population after arrival of a founding Austronesian population (circle at center and top). A greater number of noncognates enters the population in the western part of Sumba where there are lower frequencies of Austronesian Y chromosome lineages (larger filled circles and thicker horizontal arrows) relative to the central part of Sumba.

Fig. 2.

Phylogenetic and geographic distributions of languages and Y chromosomes on Sumba. (Upper) Phylogenetic tree of Sumban language groups (A–E) (see Materials and Methods). (Lower) Map of Sumba showing geographic distribution of language groups (A–E) and Y chromosome haplogroups (C, K, M, and O). Pie charts represent frequencies of four Y chromosome haplogroups at eight locations sampled for both DNA and languages. Haplogroup O (green) is unevenly distributed, with lower frequencies in the western portion of the island. Small black dots indicate 20 additional language samples for which paired DNA samples were not available.

Fig. 3.

Scatterplot of the percentage of PAn cognates versus the percentage of Austronesian Y chromosomes (haplogroup O) found at each location.

Fig. 4.

Map showing approximate geographic locations of 29 language samples (black dots). Each of the three locations show a strongly significant correlation between their geographic distance from all other locations, and the percentage of PAn cognates retained are listed on the map (1, Wunga: r, −0.503; P, 0.006; 2, Rambangaru: r, −0.015; P, 0.005; 3, Kanatang: r, −0.507; P, 0.006).