Date Published: July 17, 2019
Publisher: Public Library of Science
Author(s): Marcus J. Hamilton, Robert S. Walker, Yang Xu.
The expansion of the human species out of Africa in the Pleistocene, and the subsequent development of agriculture in the Holocene, resulted in waves of linguistic diversification and replacement across the planet. Analogous to the growth of populations or the speciation of biological organisms, languages diversify over time to form phylogenies of language families. However, the dynamics of this diversification process are unclear. Bayesian methods applied to lexical and phonetic data have created dated linguistic phylogenies for 18 language families encompassing ~3,000 of the world’s ~7,000 extant languages. In this paper we use these phylogenies to quantify how fast languages expand and diversify through time both within and across language families. The overall diversification rate of languages in our sample is ~0.001 yr-1 (or a doubling time of ~700 yr) over the last 6,000 years with evidence for nonlinear dynamics in language diversification rates over time, where both within and across language families, diversity initially increases rapidly and then slows. The expansion, evolution, and diversification of languages as they spread around the planet was a non-constant process.
As the geographic range of the human species expanded throughout Africa and beyond, eventually including the majority of the world’s terrestrial environments, human populations and their cultures diversified . Today there are over 7,000 languages around the planet [1,2], more than the total number of mammal species . Moreover, most of the world’s current languages belong to agricultural language families , and so the majority of current ethnolinguistic diversity has most likely evolved only in the last ten thousand years since the Neolithic [5–7].
Our results indicate that rates of linguistic diversification are not constant through time, neither as a whole, nor within most of the individual lineages. Overall rates of diversification across all lineages initially increase for the first few thousand years of the Holocene and thus exhibit positive density-dependence where the actual rate of diversification itself increases with each new language. After the inflection point ~5,000 BP, the diversification rate then decreases, thus exhibiting negative density-dependence, likely reflecting increased competition and saturation of populations on landscapes as languages expand. We find strong evidence for similar patterns within individual lineages where diversification rates are most rapid during the earliest stages of diversification (and at the smallest number of languages), indicating that as new lineages are born, they diversify in punctuated bursts. Lezgic exhibits positive density-dependence, where, similar to the initial stages of overall diversity in the early Holocene, the rate of diversification increases through time toward the present and continues to diversify (i.e., diversity begets diversity). Although the slopes of the Semitic and Uralic are negative, and Japonic is positive, they are not significantly different from 0 and so should be viewed as inconclusive sources of evidence of density-dependent diversification. Indo-European shows no time-dependent trend and so diversification seems to be constant over time. A clear implication of these results is that empirically, the process of language diversification is not well captured by the Yule model, a commonly chosen prior in Bayesian phylogenies, which models diversification as a stochastic, constant branching process [49–52]. However, the extent to which tree structures are robust to the choice of priors is a matter of debate [52,53].