ible impact on the language used to discuss it (Ehret, 1967). For example, the proto-Bantu word for milk is related to the proto-Bantu word for breast, but there are several root words for milk (many likely borrowed from other languages) among the Bantu languages. However, there is only one root word for milking (literally to squeeze). This observation is interpreted to support a model in which a Bantu population in Tanzania borrowed the word (possibly from the southern Cushitic speakers) representing milking as well as the actual technology related to cattle milking and subsequently spread the technology to other Bantu-speaking populations (Ehret, 1967).
The shift from food gathering to food producing inferred from African archaeological and linguistic data also resulted in a detectible genetic signal. This relationship between subsistence, culture, and biology due to gene/culture coevolution is one that has been of special interest in human genetics studies. Models of Darwinian (i.e., positive) selection are consistent with subsistence being an environmental factor that can have a profound effect on patterns of genetic variation, and the emergence of agriculture and pastoralism is tied to increased population densities and dietary changes. Thus, genetic variants that conveyed a selective advantage in this shift in diet from foraging to animal and plant products would have persisted and increased in frequency in agricultural and pastoralist communities.
Lactase persistence is one of the better studied examples of gene/culture coevolution (e.g., Durham, 1991; Hollox and Swallow, 2002). In most mammals, once an individual is weaned, it loses the ability to produce the enzyme lactase-phlorizin hydrolase (LPH), which is necessary to digest the sugar lactose present in milk without gastric distress (Ingram et al., 2009). The majority of humans do not express this enzyme as adults (referred to as the “lactase nonpersistence” phenotype) (Swallow, 2003). Several widespread mutations, however, result in the continued production of LPH into adulthood, a trait often referred to as lactase persistence (Tishkoff et al., 2007b). The distribution of the lactase persistence phenotype is intriguing given what is known about subsistence patterns worldwide (Fig. 5.2). Lactase persistence is present at high frequency in Northern European dairying and African pastoralist populations; at moderate frequency in southern European and Middle Eastern populations; and at low frequency in nonpastoral Asian, Pacific, American, and African populations (Ingram et al., 2009). In Europeans, the most common mutation associated with lactase persistence is thought to be a regulatory mutation located upstream of the gene that encodes LPH (a T at position –13910), within intron 13 of the neighboring MCM6 gene (Enattah et al., 2002; Swallow, 2003). Further, this mutation is located within a large linkage disequilibrium block that is thought to have arisen ~20–2 kya, consistent