A paper published by C. Perreault and S. Mathew in PLOS ONE outlines a new method of dating the origin of language in Africa, and therefore the origin of language of all humans.
The method starts by first estimating a linear (r) and exponential (k) rate at which phonemic diversity increases with time using this formula:
Where t is the date of colonization and PB and PC are the current phonemic diversity of populations B and C, where such populations are hypothesized as follows:
"consider the hypothetical case of two small populations, B and C, that dispersed from the same parent population, A, t years ago (Figure 1). Suppose that B and C are similar in size so that they both experience approximately the same loss in phonemic diversity due to the founder effect. Now, suppose that population B colonizes a large continental territory and subsequently expands and diversifies linguistically [66,67]. In contrast, population C settles on a small island that does not allow for population expansion and language diversification. Because of the differences between the regions colonized by B and C, population B will accumulate phonemes at a faster rate than population C. Furthermore, if population C evolves on a sufficiently small island and remains isolated for most of its history, then the rate of phoneme accumulation in C will be low, and its phonemic diversity will remain approximately stable through time. Consequently, the present-day difference between the phonemic diversity of B and C can be attributed to the new phonemes accumulated within population B. Thus, the current phonemic diversity of population C has remained through time a good approximation of the original phonemic diversity of population B."
They then use the case of Southeast Asia (Pop B) and the Andaman Islands (Pop C) to estimate the linear and exponential phonemic diversity increase rates. The date of colonization, t, of these geographic zones is set between 45 -65 KYA, the phonemic diversity of Pop B and and Pop C were retrieved from the UCLA Phonological Segment Inventory Database (UPSID) and customized with the following assumptions:
"We estimate PB by taking the average phonemic inventory size of the languages in Mainland Southeast Asia. Assuming an eastward, coastal migration route, we have excluded the Asian languages that are located west of Andaman Islands (such as the languages from India and Nepal), as well as those spoken in Myanmar and the Malay Peninsula, because they could have served as departure points for the colonization of Andaman Islands (Figure 2). The 20 languages retained in our sample are thus those spoken in Cambodia, Vietnam, Laos and Southwest China (Table 1). The average phonemic diversity of the resulting sample is 41.21+2.74 (errors represent one standard error). Great Andamanese (ISO 639-2:apq) is the only Andamanese language to appear in UPSID. Its phonemic diversity, 24, serves as our estimate of PC."
Thus, with the above values the linear and exponential rate of phonemic diversity were estimated to be between 0.26-0.38 and 83.17-120.14 respectively for a date of colonization of between 45-65 KYA, where the lower rate of increase in phonemic diversity corresponds to the upper bound of the date of colonization and the higher rate corresponds to the lower bound of the date of colonization.
Next, they proceed to use the rates from above to estimate t0 or “the time it would take for a language to acquire the phonemic diversity observed today in African languages” using the following formula:
Where Pinitial is the number of phonemes that the first human languages started with, and assumed in one case to be 11, or the smallest phonemic inventory ever observed and for another case, 29, or just a median phonemic diversity. PAfrica is the average of the phonemic diversity of click speaking Africans, as they are the populations that are thought to have lost the least amount of phonemes due to founder effect, where as all the remaining macro language groups of Africa; Afroasiatic, Nhilo Sahran and Niger Kordofanian are known to have all undergone serious geographic expansions. The authors substantiate this assumption by stating:
"This idea is consistent with the fact that the average phonemic diversity of Afro-Asiatic, Niger-Congo and Nilo-Saharan languages is 36, 33, and 29 respectively, while the average phonemic diversity of African languages outside these families is 75."
Using a few other criteria they therefore estimate PAfrica to be 71.4
Thus with the above values, the results of t0 for the two different assumed values of Pinitial were calculated for the linear and exponential rate of phonemic diversity increase as follows:
The authors come to the following conclusion from their analysis:
"Our analysis suggests that language appears early in the history of our species. It does not support the idea that language is a recent adaptation that could have sparked the colonization of the globe by our species about 50 kya [1,91]. Rather, our result is consistent with the archaeological evidence suggesting that human behavior became increasingly complex during the Middle Stone Age (MSA) in Africa, sometime between 350– 150 kya [92–100]. However, we cannot rule out the possibility that other linguistic adaptations, that are independent of phonemic evolution, arose later and triggered the out-of-Africa expansion."
More details from the paper, which is open access, can be found here.