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Scientific Understanding of Consciousness |
Language Evolution from Africa
Science 15 April 2011: Vol. 332 no. 6027 pp. 346-349 Phonemic Diversity Supports a Serial Founder Effect Model of Language Expansion from Africa Quentin D. Atkinson1,2 1Department of Psychology, University of Auckland, Private Bag 92019, Auckland, New Zealand. 2Institute of Cognitive and Evolutionary Anthropology, University of Oxford, 64 Banbury Road, Oxford OX2 6PN, UK. (paraphrase) Human genetic and phenotypic diversity declines with distance from Africa, as predicted by a serial founder effect in which successive population bottlenecks during range expansion progressively reduce diversity, underpinning support for an African origin of modern humans. Recent work suggests that a similar founder effect may operate on human culture and language. Here I show that the number of phonemes used in a global sample of 504 languages is also clinal and fits a serial founder–effect model of expansion from an inferred origin in Africa. This result, which is not explained by more recent demographic history, local language diversity, or statistical non-independence within language families, points to parallel mechanisms shaping genetic and linguistic diversity and supports an African origin of modern human languages. The number of phonemes—perceptually distinct units of sound that differentiate words—in a language is positively correlated with the size of its speaker population in such a way that small populations have fewer phonemes. Languages continually gain and lose phonemes because of stochastic processes. If phoneme distinctions are more likely to be lost in small founder populations, then a succession of founder events during range expansion should progressively reduce phonemic diversity with increasing distance from the point of origin, paralleling the serial founder effect observed in population genetics. A founder effect has already been used to explain patterns of variation in other cultural replicators, including human material culture and birdsong. A range of possible mechanisms predicts similar dynamics governing the evolution of phonemes and language generally. This raises the possibility that the serial founder–effect model used to trace our genetic origins to a recent expansion from Africa could also be applied to global phonemic diversity to investigate the origin and expansion of modern human languages. Here I examine geographic variation in phoneme inventory size using data on vowel, consonant, and tone inventories taken from 504 languages in the World Atlas of Language Structures (WALS), together with information on language location, taxonomic affiliation, and speaker demography Consistent with previous work, speaker population size is a significant predictor of phonemic diversity (Pearson’s correlation r = 0.385, df = 503, P < 0.001), with smaller population size predicting smaller overall phoneme inventories. The same relationship holds for vowel (r = 0.378, df = 503, P < 0.001) and tone (r = 0.230, df = 503, P < 0.001) inventories separately, with a weaker, though still significant, effect of population size on consonant diversity (r = 0.131, df = 503, P = 0.003). To account for any non-independence within language families, the analysis was repeated, first using mean values at the language family level and then using a hierarchical linear regression framework to model nested dependencies in variation at the family, subfamily, and genus levels. These analyses confirm that, consistent with a founder effect model, smaller population size predicts reduced phoneme inventory size both between families (family-level analysis r = 0.468, df = 49, P < 0.001) and within families, controlling for taxonomic affiliation {hierarchical linear model: fixed-effect coefficient (β) = 0.0338 to 0.0985 [95% highest posterior density (HPD)], P = 0.009}. The single major cline in phonemic diversity is consistent with a linguistic founder effect operating under conditions of rapid expansion from a most likely origin in Africa. This supports a picture of language spread that is congruent with similar analyses of human genetic and phenotypic diversity. Phonemic diversity appears to be highly stable within major language families, indicating that, despite the many sociolinguistic processes at work, robust statistical patterns in global variation can persist for many millennia and could plausibly reflect a time scale on the order of the African exodus. Outside Africa, the highest levels of phonemic diversity are found in language families thought to be autochthonous to Southeast Asia. This also fits with genetic evidence, indicating that Southeast Asia experienced particularly pronounced population growth immediately after the African exodus, meaning that languages in this region should have been least affected by population bottlenecks and would have had the most time to recover diversity. Although distance from Africa explains much less of the variation in phonemic diversity (19%) than in neutral genetic markers (80 to 85%), the effect is comparable to that obtained from analysis of human mitochondrial DNA (18%) or phenotypic data (14 to 28%). To the extent that language can be taken as an example of cultural evolution more generally, these findings support the proposal that a cultural founder effect operated during our colonization of the globe, potentially limiting the size and cultural complexity of societies at the vanguard of the human expansion. An origin of modern languages predating the African exodus 50,000 to 70,000 years ago puts complex language alongside the earliest archaeological evidence of symbolic culture in Africa 80,000 to 160,000 years ago. Truly modern language, akin to languages spoken today, may thus have been the key cultural innovation that allowed the emergence of these and other hallmarks of behavioral modernity and ultimately led to our colonization of the globe. (end of paraphrase)
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