As mentioned previously, the classification of languages within the Khoesan language family is contentious given the high diversity within each subclade and extreme divergence among them (Sands, 1998; Ehret, 2000), particularly for the Sandawe and Hadza. A common classification, therefore, groups the three languages spoken in South Africa into a separate branch (SAK) from the more divergent Sandawe and Hadza (Heine and Nurse, 2000). One interpretation of this extreme linguistic diversity is that the last common ancestor of the language family must be extremely ancient, and Ehret (2000) estimates the TMRCA to be at least 20 kya (which approaches the limit in timescale to linguistic reconstruction). The Sandawe and SAK are more similar to each other linguistically than either one is to the Hadza. Geographically, however, the Sandawe and Hadza are extremely close to each other (150 km apart in Tanzania), and both are geographically distant from the SAK populations residing in southern Africa.
A recent study of mtDNA and NRY variation investigates the genetic relationship among the Hadza, Sandawe, and SAK (Tishkoff et al., 2007a). The authors find that in general, the Hadza and Sandawe are more genetically similar to each other than either one is to the SAK. However, the Sandawe and SAK share ancient mtDNA lineages, which may suggest an ancient common ancestry. For example, mtDNA haplogroup L0d is present at high frequency in the SAK and at low frequency in the Sandawe, but is not present in the Hadza samples (Tishkoff et al., 2007a), and the TMRCA estimate of the SAK and Sandawe L0d lineages is ancient (~60 kya) (Tishkoff et al., 2007a). Similarly, the SAK and Sandawe share NRY haplogroup A (M91), which is not present in the Hadza samples (Tishkoff et al., 2007a). On the other hand, haplogroup L4g is common in both the Sandawe and the Hadza and absent from the SAK samples, and the TMRCA for the Sandawe and Hadza L4g is more recent (~25 kya) (Tishkoff et al., 2007a). And all three samples share NRY haplogroup B2b (M112) (Tishkoff et al., 2007a). The authors (Tishkoff et al., 2007a) discuss more than one interpretation of these results. The absence of mtDNA haplogroup L0d and NRY haplogroup A (M91) from the Hadza could reflect loss due to genetic drift because there is evidence of a recent bottleneck in the Hadza (Blurton Jones et al., 1992). Alternatively, the pattern of haplogroup variation could reflect an ancient linguistic and genetic divergence of the Hadza from the SAK. Moreover, the authors (Tishkoff et al., 2007a) performed a likelihood analysis to estimate the time of divergence among the populations and found that the divergence between the Hadza and the Sandawe was >20 kya and the divergence between the Hadza/Sandawe and the SAK was >40 kya. Additional studies of mtDNA and NRY variation have identified ancient shared lineages among the SAK and the Hadza as well as